Repair of the speedometer on a Polaris Sportsman 500 4-wheeler

Last year, a friend of mine had the battery go bad on his 1999 Polaris Sportsman 500 4-wheeler.  Aside from the inconvenience of having to pull-start it, it seemed to work OK.

Sort of.

Soon, it was noticed that the speedometer had died and interestingly, a few other things quit working at the same time such as the ability to put it into four-wheel drive. When the battery was finally replaced the speedometer still did not work so a he dug around in the internet and found that this is quite a common problem with that vintage of Polaris vehicles - and it seems to work out this way.

The charging system's voltage regulator on these vehicles are fairly simple, but they depend strongly on the presence of the battery to moderate the wildly pulsating DC coming out of the alternator/regulator system to maintain the average voltage in the range of 13.5-14.2 volts or so.  If the battery goes completely bad or is removed, the charging system goes haywire and the voltage can (apparently) exceed 20 volts (and is probably higher) and one can risk burning out the various indicator, marker and headlights.

Another fatality under this conditions seems to be the speedometer module itself!

What (probably) happens:

It's probably not the high voltage that actually kills the speedometer:  The voltage regulator circuit in the speedometer seems to be fairly robust, using high voltage (>=300 volt) transistors to withstand the voltage spikes that are endemic to any vehicle electrical system.  What seems to kill these things is heat!

Let me explain.

The job of the voltage regulator circuit inside the speedometer is to assure that the voltage feeding the circuit inside doesn't exceed about 15 volts or so and from there, it is regulated down even lower by other circuitry for the computer that computes the odometer readings and (probably) the speedometer as well as having something to do with the reverse limiter designed to prevent you from accidentally driving backwards at a high speed and the lockout for the all-wheel drive switch.  There are also several small light bulbs inside the speedometer that provide backlighting for the display at night and these, too, are protected from high voltage by the 15 volt regulator.

Under normal conditions the voltage on the vehicle's electrical system is around 14 volts or so and the regulator's job is to suppress spikes and brief excursions above that and in this mode, the regulator itself isn't doing much.  If the voltage rises, however, it has to drop the excessive voltage and and a natural by-product of this is that it develops heat.

Apparently, quite a bit of it!  In testing the speedometer after the repair I applied 20 volts to it and the main regulator transistor soon got too hot to touch:  If this had been a hot, summer day with the transistor crammed inside the waterproof speedometer casing with no free air ventilation, it would have been much hotter.

So, with the bad battery and a subsequently malfunctioning charging system it is easily likely that the speedometer's regulator saw an average of 20-30 volts on its input.  At some point the transistor overheated and eventually failed internally, shorting itself out.  Fortunately, the majority of the circuits in the speedometer seemed to survive this since once the regulator itself had quit, all power feeding the rest of the circuit was lost completely, preventing further damage.

While a new speedometer is available as a replacement part, it will cost you several hundred dollars, new!

Fortunately, it may be that you can fix it!

The obligatory warnings, etc.

• Before you start, read this entire posting so that you'll know what you are in for!

• Repair of the speedometer requires some knowledge of electronics and board-level electrical components.

• Repair also requires good unsoldering and soldering skills and equipment.  If you don't have the proper tools and experience in the replacement and installation of individual, through-hole components, do not even attempt this!

• Assume from the beginning that the speedometer is a total write-off and that you would have to replace it, anyway, to get it working again.  This way, if you can fix it, you will be money ahead - but if you can't, you haven't lost anything!

• The speedometer is part of the electrical system of the vehicle and as such, it is possible that its malfunction - possibly due to a failed repair - could cause additional damage to other components.

• No, I won't repair your speedometer.  I suggest that you find someone versed in electronics to help you out if you need to do so.

• I know ONLY about the speedometers on Polaris Sportsman 500's for the years 1999 and 2000:  If you ask me about speedometers for any other make, year or model, I can't help you!  (They may be the same - they may not - I don't know.) 

• You do this repair at your own risk!  Do not get mad at me if you blow something up, set fire to your four-wheeler or cause all of your dog's hair to fall out!

• You have been warned!

How to do it

Remove the speedometer:
 
The first step is to remove the speedometer from the vehicle.  It's a bit of a pain, but it's not terribly difficult to do as it is the same procedure as would be followed for replacing the headlight.  Inside the housing that covers the headlight you'll find two connectors that snap into the speedometer, held in place with release tabs, as well as two nuts that hold the bracket in place:  Note how these go together before taking them off - make a drawing and/or take a picture before you take everything off if you aren't sure.


Open the speedometer:

Place the speedometer face down on a clean, un-cluttered work area on a surface that you don't mind scratching:  It's recommended that you put a rag or old towel between the face of the speedometer and the work surface.

Now, notice the soft, aluminum ring around its perimeter:  This holds the clear, plastic cover to the body of the speedometer by virtue of crimping between those two pieces a rubber gasket.


Wearing gloves to prevent being stabbed during this step, use a medium-sized blade screwdriver - preferably one that is somewhat worn out with rounded edges - and slide it between the aluminum ring and the plastic body of the speedometer on the back side, prying the ring open as you go along, straightening it out.  With a bit of practice you can firmly slide the screwdriver along the perimeter and straighten out that soft, aluminum ring and you will probably have to go around several times to do the job.

Figure 1: The soft, black, aluminum ring around the perimeter of the speedometer that holds the faceplate to the body.  Carefully pry this straight. (This picture was taken after I'd already opened and repaired it - and partially closed it again.) Click on the image for a larger version.

Once you get 90% of the backside of the ring straightened out, you'll be able to pop the ring off the front:  Set it aside.  With the ring removed you should be able to use your fingernails and pry the front, clear cover from the body of the speedometer.  Be careful with the black plastic rod under the push button, noting carefully how it is installed and taking care that it doesn't fly off somewhere!

Once you have the cover off, set it aside with the black, plastic rod laying inside the cover.

Remove the needle:

This is sort of tricky and it is possible to ruin the speedometer with this step:  Since you have already declared the speedometer to be a total loss, you shouldn't feel too bad if you do.


First, note how far the needle is pushed on to the spindle:  You'll want to remember this when putting it back on.

If you have very strong fingernails, try pulling the needle straight off the speedometer, but whatever you do, apply tensions EVENLY - that is, pull straight out on the needle as you do not want to bend the spindle!  When you pull on the needle make sure that the speedometer is on the workbench with padding on it because if it comes off suddenly, you don't want to slam your hand or the speedometer into the workbench and break something!

If you can't remove the needle with our fingers, you'll need to apply a bit more force.  Cut some pieces of paper or thin cardboard (such as from a cereal box) so that you cover the entire face of the speedometer, but allow access to the needle and its spindle - this being done to prevent you from accidentally marking up the speedometer face.

Now, using two medium-size blade screwdrivers , pry the needle evenly off the spindle using the paper/cardboard to prevent damaging the speedometer face:  You may want to wrap a rag around the body of the speedometer and clamp it gently - but firmly - in a vise.  Hopefully, the needle will come up without breaking anything else!  If you do break something else, save up for a new speedo!

Remove the speedometer module from the body:

Using a small screwdriver blade or, preferably, a similarly-sized and shaped piece of plastic, carefully pry up on the face of the speedometer.  The face is actually printed on a piece of fairly heavy, self-adhesive plastic that is about as thick as a postcard - and it is this, not the actual body of the speedometer itself - that you want to pry up.  It can be a bit tricky to get purchase on the speedometer face and you might bend into a hook a small piece of metal such as a paper clip to act as a tool.

Figure 2: By carefully prying up the speedometer face's plastic, you can access the three screws that hold the module in the body.  The other two screws are located 1/3rd of the way around, at approximately "10" and "40" MPH.  If you can't get the needle off before this step, make sure that you move it out of the way when you pry up on the faceplate so that you don't accidentally break it. Click on the image for a larger version.

Start by prying up on the plastic face below and in the middle of the LCD odometer display, at the bottom of the speedometer and once you lift it a little bit, you will see underneath it a Philips type screw:  Holding the speedometer face up with a small screwdriver, use another screwdriver to remove that screw.

There are two other screws, each located 1/3rd of the way around on either side:  Remove those, too.

Now, the only thing holding the speedometer module inside the case is friction and the silicone used to seal around the wiring connector pins on the back.  Using a small blade screwdriver, work your way around the perimeter of the inside of the speedometer, wedging gently between the outer body of the speedometer and the module itself, reaching down slightly past the face of the speedometer to do it.  After going around several times, applying a bit of twisting and/or prying force, the module will hopefully break loose and gradually come out.

When it does this, the pins from the electrical connectors on the back will be pulled through the case and soon, you'll have the module separated from the case.

IMPORTANT NOTE:

• There are one or two cylinders with granules packed inside them in the case that contain moisture-absorbing compound.  As you  remove the body of the module, they may come out, or they may be (at least temporarily) stuck in place in their own crevice inside the module - but in any case, note where they originally sit.  Take them out and place them in a "Zip-Lock" (tm) bag to protect them from additional moisture while you are working on the speedometer.

Remove the LCD and accessing the back of the circuit board for soldering:

In order to get access to the "solder" side of the circuit board you'll need to remove the portion with the face plate, after you have removed the needle.  In so-doing, you'll also be removing the LCD odometer display.

On the back side of the speedometer module (the "component" side of the circuit board) you'll find four black screws that correspond approximately with the four corners of the LCD display.  Laying the speedometer face-down on a piece of cloth, remove these four screws:  The front faceplate portion will separate from the circuit board.


This front portion also retains the LCD in place and it may fall out.  If it doesn't come out on its own, carefully remove it - noting the markings on the LCD and which way they were oriented with respect to the board.

Figure 3: The repaired board showing the LCD.  Note the orientation of the writing on the LCD with respect to the board.  If you look carefully, you will notice markings on the upper-left edge of the LCD which may be used to indicate which way is up.  Toward the 2-o'clock position of the speedometer board you can see the repair to the damaged trace.  Because the board is coated with a sticky conformal coating compound, it's difficult to avoid discoloring when soldering due to the heat and flux.    Click on the image for a larger version.

The LCD's electrical connections to the circuit board are made via two small strips of pink-ish conductive rubber sandwiching darker rubber (these are often called "Zebra Strips") and these usually stick to the LCD.  If they are stuck to the board, very carefully remove them, but if they are stuck to the LCD, don't worry.  Set the LCD and the rubber strips aside in a clean, dust-free place.  You may notice that the LCD itself has a part number printed on it:  Note its orientation so that you can put it back in the correct orientation.

You now have access to the component side of the circuit board!

Identifying the bad components:

It seems that the one part that is sure to go is a large-ish power transistor, but there can be two components next to it that are also destroyed - and this damage may be evidenced by some burn marks on the circuit board:  See Figure 4, below, for identification of these components.

Figure 4: Location of the likely bad part(s).  When I took this picture I had already replaced the TIP48 and the MPSA42 - but not the Zener diode. Click on the image for a larger version.

Even though only one or two of these components may be bad, I would recommend replacing all three of them.  These are:

• A TIP48 high-voltage NPN power transistor.  This is the most likely component to be damaged and is a transistor with a metal tab.  In Figure 4, above, the leads are, left to right, B-C-E.  (The NTE equivalent is the NTE-197.)

• An MPSA42 high-voltage NPN low-power transistor.  This is a small, black transistor located next to the TIP48.  In Figure 4, the leads are C-B-E from top-to-bottom.  (The NTE equivalent is the NTE-287.

• A 1N5245 15 volt, 1/2 watt Zener diode.  This is located next to the MPSA42 transistor and is a small, (usually) red/orange glass device on axial leads.  In Figure 4, the "banded" end is the lower end.  (The NTE equivalent could be the NTE-5024A or its 1-watt version, the NTE-145A.)

The only critical things about the two transistors are the fact that they are NPN devices rated for at least 300 volts to withstand the normal transients found in a vehicle electrical system.  If you are unfamiliar with electronics I would suggest that you replace these with exact types which are readily available from Mouser Electronics or Digi-Key electronics - or you can get cross-reference equivalents in the NTE family (noted above) from MCM Electronics or even find a local distributor that carries the NTE parts.

If you are familiar with electronic components:

• The TIP48 may be replaced with about any NPN bipolar transistor found on the "mains" side of an AC-powered switching power supply - such as a dead computer supply.   Typically, these transistors are rated for more than 400 volts and are in a TO-220 style case - often with a plastic or insulated tab - which really doesn't matter in this application.  Just make sure that it's rated for at least 300 volts and has the same pinout (B-C-E) as the original TIP48!  Chances are this transistor will have a number that begins with "2SC" (or just "C") followed by 4 digits:  Look up the device's data sheet online and verify that it is, in fact, a high-voltage NPN device.  The one that I happened to used came from a junked VCR, was rated for 500 volts and happened to have a plastic tab rather than the metal tab of the original device.

• The MPSA42 is a high-voltage, low-power NPN transistor of the type typically used in video drivers for cathode-ray tubes on older TVs and suitable equivalents may be found on the small circuit board attached to the end of a CRT on a discarded television.  I happened to have some ZTX458 transistors - devices that had equal or better voltage/current specs than the original - laying around from another project and used one of them.  Just make sure that you take into account any differences in the pinout of the replacement transistor!

• I didn't have a 1N5245 1/2 watt, 15 volt Zener around, but I did have a 1N4744 Zener which is a 1 watt version with the same voltage rating, so I used it.

Replacing the components: 

First:

• Unless you are experienced in component replacement and circuit board repair, I would not suggest you do this procedure at all!

• Use a temperature-controlled soldering iron.  Too little heat, you'll damage the board trying to get the components off.  Too much, you'll damage it that way, as well.

• Do NOT use a soldering gun for this repair work:  If that's all that you have, you really should not try it as you'll likely ruin the board!

• You must have proper desoldering equipment.  A vacuum-operated desoldering tool is the ideal, but "Solder Wick" (tm) or even a spring-loaded "solder sucker" will work.  If you don't have any of these, it will be challenging to get the through-holes cleared to install the replacement components. 

• Both sides of the circuit board and most of the components are covered with a moisture resistant coating.  This coating is slightly rubbery and when soldered, it gets discolored.  Fortunately, you can "solder through" it although extra care should be taken to assure that the solder joints are clean and good.  When you are done soldering, you can clean the flux with alcohol and a cotton swab, but the discoloration will probably remain.  (If your solder uses water-based flux, make certain that you have removed it using clean water as many of those types of fluxes can slowly corrode connections.)

• When removing the old components (the two transistors and the Zener diode) it may be easiest to just clip them from the board first using small, sharp diagonal-cut pliers.  This will remove the body of the device and allow each lead to be removed independently. 

Using "solder-wick" or a vacuum desoldering device, remove the two transistors and the Zener Diode, noting the original orientation of each device.  Make sure that you also clean out the holes through the board!

In my case, the heat of the original TIP48's destruction and its subsequent removal from the board actually damaged a trace so I had to repair it with a short piece of wire (I used #30 wire-wrap wire) on the bottom-side (that's the short green wire visible in Figure 3) but it's likely that your damage won't be that severe.

After inspecting for damaged traces, install the new components.  The notes above indicate which lead is which when replacing the two transistors and the diode.

Figure 5: The newly-repaired speedometer.  It shows a speed because I was injecting a signal on the back to simulate input from a wheel sensor. If you look carefully you might note that the 10th's of a mile digit on the LCD is missing part of its top half:  To fix it I had to take it apart again, clean the rubber pad and LCD with some denatured alcohol and reassemble it to remove the bit of dust that prevented that particular digit from working properly. Click on the image for a larger version.

Putting it back together:

As they say, "Assembly is the reverse of disassembly"!


A few comments:

• When putting the LCD back in place, it may be a good idea to wipe down the top and bottom edges of the rubber strips with an alcohol-wetted cotton swab, and this should also be done for all surfaces of the LCD itself.  Doing this will make sure that there are any dust particles that may cause one or more of the LCD's segments to not work.

• The two rubber strips for the LCD just sit in place, on edge, in the LCD mount and the LCD goes on top of it.  When mating the face of the speedometer back to the circuit board, make sure that you have re-installed the LCD right-side up and oriented properly (remember when you made note of the marks on the LCD?)so that you don't accidentally crush and break it when putting the speedometer face back into place.  Make sure that the four black screws are tightened firmly - but not so tight that you crack the plastic.

• Before sealing everything up, carefully remove obvious fingerprints from the LCD (using a cotton swab) and the inside of the face plate as well as the speedometer. 

• If you have a bench-type power supply capable of 12 volts at about 600 milliamps, you can do some preliminary testing of the speedometer by +12 volts to pin "A" and the ground to pin "B".  If all goes well, the lights will turn on and you'll see the odometer displaying numbers.  See below for a description if the pinout.

• If you have applied power and everything looks OK, gently push the needle partway on the spindle, aligning it with "0".  With the power applied, move the needle up scale (say, to 20-40 miles-per-hour) and watch it go back down to zero:  If it stops slightly off zero (it may take 10-15 seconds to settle) then pull the pointer off and re-align it, re-doing the above steps again until you get it to land on zero.

• If you can't power up the speedometer, line the pointer up with zero and then move it up-scale to 20 or so.  After 5-10 minutes (yes, it may take that long to slowly move back!) look again to see where it is pointing:  If it isn't at zero, re-align the pointer and try it again.

• Once you are satisfied that the pointer is correct, firmly push it in on the spindle.

• In the case you may have noted two small cylinders with granules inside them.  These are moisture absorbers and they sit in a slight recess one one side of the case.  It is recommended that these be retained!

• When you put the speedometer module back into the case, make sure that you move the needle out of the way when you pry up on the faceplate to reinstall the three screws.

•  If you have some silicone grease, apply a thin layer to the gasket that goes between the body of the speedometer and the clear front plate, taking care not to get it on the face plate where you can see it.  If you don't have silicone grease I would suggest the use of Silicone "Plumber's Grease" available at practically any hardware/home improvement store.   (Just wipe the grease off if you do get it on the display.)

• When putting the face plate back on, remember to re-install the black push-rod that operates the button!

• Test-fit the face plate to make sure that you pushed the needle on far enough. 

• Slide the aluminum ring back into place.  With the speedometer face-down in a cloth (preferably a gap to accommodate the button on the front panel) use a piece of wood to re-crimp the aluminum ring to attach the face plate to the body.  You'll probably have to go around the perimeter several times to get it tight.

• Squirt a dab of silicone grease (but not silicone seal!) this in each of the electrical connectors on the back side:  This will help prevent ingress of moisture as well as prevent possible corrosion of the electrical connectors.  "Plumber's Grease" of the sort used to lubricate "O" rings is this same sort of grease and will work fine and this is available anywhere you can buy plumbing supplies and parts - including big-box home-improvement stores like "Lowes-Depot."  It is not recommended that one use "normal" petroleum grease (e.g. axle grease) as this will degrade the plastic and connectors!

Comments about pin-out and testing:

Note:  If you do an internet search, you should be able to locate some online drawings showing the pinouts of the speedometer's connectors.

With the speedometer facing down, orient it so that the 6 pins of the larger connector are toward the top running horizontally and that the 3 pins of the smaller connector (the one for the wheel sensor) is to the right of it with its pins running vertically.

For the large connector, pin "A" is on the left and they are designated on available drawings as A-F, left-to-right.  For the 3-pin connector they are A-C starting from the top and working down.

Applying voltage to these pins using a regulated, current-limited power supply set to 12-15 volts at a maximum current of 600 milliamps to 1 amp, you should be able to power up the speedometer.

• Do NOT power the speedometer being tested directly from a battery as that could supply virtually unlimited current in the event of an accidental short or fault.

• DO NOT connect the polarity backwards - even for an instant.

• If your current-limited power supply "sees" a dead short, remove power immediately and check for solder bridges around the components that were replaced.

If it works, the lights should come on and if you move the speedometer needle with your finger, up-scale, it should reset itself to zero much more quickly than it had with the power off.  An additional test is that if you increase the voltage above 14-15 volts (but not above 20 volts!) the lights will not get any brighter - a sure sign that the regulator is now working properly.  If you do this, now is the time to double-check that the needle points at zero.

If you are curious, you can apply a square wave signal from an audio generator (3-5 volts RMS) between pins B and C of the 3-pin connector and vary it from about 5 Hz to 200 Hz and you should see the speedometer go up with increasing frequency as you simulate, with your audio generator, the input from the wheel sensor.  Pin "C" is ground while pin "B" is the signal input.

Final words:

If all goes well, your speedometer, 4-wheel drive switch, "reverse override" button and odometer will now work properly again!

Note that there's no guarantee that it will be as waterproof as it was before since you probably lack the machine required to properly crimp that aluminum ring down so it's probably best to keep it out of rain as much as possible - a good practice, anyway!


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