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Wiper motor diagnostics and repair.

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Wiper motor diagnostics and repair.

Postby bcadmin » Mon Dec 27, 2004 2:57 am

Wiper motor diagnostics and repair.

Scott Andrews, December, 2004

After fiddling with my wiper motor several times over the past few weeks (it DOES rain, sometimes, in CA). I have developed some theories on what makes the wiper motor go slow, and/or cook.

The key to this is understanding the DC motor. Basically, the DC motor has a big magnet (which is built into the case), and an armature. The armature is a bunch of metal plates wound round with wire. The wires all go to different copper plates on one end of the motor. This is called the commutator. Small spring loaded carbon contacts rub on the commutator and pass current through the different windings on the armature. This current creates magnetic fields in the armature that interact with the permanent magnet that surrounds it in the housing. This causes attractive and repulsive forces on the armature. To balance these forces the armature spins; however, when it turns the commutator moves, and the contacts change the current to flow through different windings, so the motor is always trying to balance it?s forces, and in doing so it creates new it turns.

When the motor turns, the fact that it is turning in the magnetic field of the magnet actually generates a counter current in the windings. This is known as back EMF. The actual power of the motor is given by P=I*E, where P is power (in watts), I is current (in Amps) and E is the back EMF in volts. Back EMF is related to the motor turning speed. If, for some reason the motor is loaded down (bad linkage, bad bearings, etc), then the motor will slow down. This will drop the back EMF and the current will rise to compensate for the load. Under extreme circumstances the back EMF will be zero, and the motor will look like a short wiring and/or blowing fuses. In fact the motor may just simply be running at way too high a current, even if it isn?t stalled, and this will generate a lot of heat, eventually leading to the insulation on the wires cooking. Interestingly, the same sort of thing can also happen with bad wiring contacts (usually from corrosion). In this situation the contacts have a lot of electrical resistance, and so as the current rises in the motor, the voltage drop across the contacts rises. This reduces the voltage across the motor, so it slows down. A slower motor generates less back EMF, and so, to deliver power, the motor current has to rise. With cooked parts just around the next corner.

So, what can make the motor go bad, and why should we care?

We care because a new wiper motor sells for about $300; probably more at the dealer. A used one may be cheaper, but it probably suffers from the same issues yours does?or it soon will.

From what I can tell, the wiper assembly has three main failure modes.
1) Doesn't run at all
2) Runs slowly and starts to smoke (generally shortly thereafter exhibiting #1 symptom)
3) Runs slowly

These failures can come from several places.
The first thing to check is that you have power coming to the motor, and there is not some reason the wipers can?t move. Check these things first.

Assuming you have power properly coming to the motor (that is, that the issue is not in the wiper controller, or wiper switch), and that the wiper linkage operates freely, there are numerous places the motor assembly can fail: These are the primary suspects.

1) The electrical connector is corroded and has high resistance.
2) The motor brushes are shot
3) The wiring is broken and/or has cracked insulation
4) The motor armature windings have been severely overheated, and the insulation has melted.
5) The internal switching has worn out
6) The gears have stripped and jammed

So, how to determine the fault?

a-1) If the motor is totally inoperative: (Problems 2, 3, 4, 6)
-Remove it from the car. Undo the 1mm nut, release the linkage and remove the three screws that hold it in place.
-Using a voltmeter, check the car side for proper signals (ground, high, low, intermittent, park)?just to be sure.
-Using a battery charger, or a battery, ground the case; Turn the motor over. You will find a wiring strip with two wires that come from inside the motor, and several wires from the connector. Apply 12 volts to one of the two wires from the motor. If the motor turns, then try the other wire. If both of these work, then the motor itself is probably OK, and you have a problem in the wiring or in the internal switching (more on this later)
-If the motor will not turn when powered this way, remove the four small screws that hold the plate with the wiring strip. Carefully pull it away, and remove the thin gasket. Inside is a worm and wheel gearset; it will probably be filled with sticky old brown grease.
-Inspect the gears. If the wheel gear is cracked or has missing teeth, try to locate the pieces. This is pretty unlikely, but it might happen. If only one tooth is missing, and has jammed the gears, then remove it and put the cover back on, and try the motor again. It may have a little bump in the cycle, but it might be OK.
-If the gears look OK, then carefully lever out the wheel gear. You should now only have a worm gear on one side of the gear housing. Go back and try the bit with the battery. If the motor STILL doesn?t turn, then it has some internal problems. See the section on internals below.
-If the motor turns freely with the wheel gear removed, inspect the wheel gear shaft.
-Clean off the old grease, and clean out the gear housing. Using some fine emery paper (like 1000 grit black paper), get rid of any corrosion on the wheel gear shaft. Grease everything up with white grease, and put it back together. Loosen the lock nut and unscrew the thrust tensioner (it?s like a screw on the gear box cover. If the motor turned without the wheel, and you cleaned the bearings and shaft and greased it properly, it should turn now. Put it back together, and turn the tensioner until it touches something inside, then back it off about ? turn and tighten the nut.

a-2) If the motor doesn?t turn per any of the steps in a-1, then:
-Undo the two screws that hold the gear case to the main housing (the black cylindrical part). Be very careful, since the internal ?nuts? will come loose, and probably will get sucked into the motor internals by the magnets. It?s OK if they do, just be prepared so they don?t fall out and skitter off under your ancient garage shelving never to be found again?
-Slowly lift the gear casing from the motor housing. You should hear some small ?clicks? as the carbon brushes, disengage the commutator.
-What you should see here is the end of the motor, with the worm gear, and a small cylindrical area that has axial (in the axis of the motor shaft) grooves.
- Clean this with electrical cleaner. If it is not shiny copper color, then take some emery paper, soak it in electrical cleaner and wrap the emery paper around the commutator, and rotate it. After a few turns it should come up bright copper.
- Inspect the brushes. These are small spring loaded carbon bars with a flexible braided copper wire that connects them to their fittings. Part of this fitting is a small brass carrier that holds the spring and allows the brush to move radially in and out. If these brushes are so worn that less than about 1mm is sticking out past the brass holder, then you need new brushes, or you need a new motor. I am, as yet unsure how to replace the brushes. I am certain it can be done, but I don?t have any details. The brushes are not designed to be replaceable, so this will be an adventure.
-Inspect the two wires that come through the housing from the wiring strip. If these are badly cracked, or broken, replace them. You will need a big soldering iron, since these are heavy wires.
- If all of this looks good, then, remove the armature from the housing. It will try to stick to the magnets. Pull it out, and inspect it. It should not be rusty, and the bearing shaft on the other end from the commutator should be clean and smooth. If they aren?t then clean then and sand as necessary with emery. And then clean them again.
-Inspect the armature. If it smells bad and looks cooked, then your motor is probably toast. You can confirm this by using an ohmmeter to test the windings. Put one ohmmeter contact onto one commutator pad, and measure the continuity to each of the other pads. It should have continuity to only one other pad. I am not certain how the two speed winding is done; It is possible that it has continuity to TWO other pads. One of these paths will be higher in resistance. If it has continuity to multiple commutator pads, and/or to the metal plates in the commutator, the motor is toast. Go buy a used motor and then revisit this article. If it seems OK, then continue.
- If you haven?t done so yet, loosen the small tensioner screw on the gear cover. This is held in place with a lock nut (near the wiring strip).
-Lube the bearing end of the amature, and carefully work it back into the bearing down inside the housing.
-Now, the fun part. Place the gear housing back onto the end of the motor so the worm gear engages the wheel gear. It won?t go all the way on. You need to gently press it on with your hand, while carefully pressing the brushes back one at a time away form the commutator using a small screwdriver. Go around it several times, pressing the housing home, and probing inside to pull the brushes back from the armature so it can slip back into place. It will take a few tries, and it will all go together in a rush when the last brush is pulled back.
- Rotate the gear box until the small protrusions engage the slots in the housing, and the whole thing is back in place.
-Using a small needle nose pliers, hold one of the two small thread plates (these are those things that fell out when you undid the first screws on the motor). Carefully slip it into the slot on the side of the motor (your slot may be covered in tape). Once more or less in place, put a screwdriver in the back side of the slot (away from the gearbox) so it is held in place, and can?t fall into the motor. If it does fall into the motor, you have to remove the gear box, fish it out, and do the whole deal with the brushes again (as me how I know this?). Holding the plate in place, put the screw back in from the gear box side. Once it engages the plate threads, you can remove the screwdriver. Do this for the other screw too.
-Tighten up the two screws, and wrap that end of the motor with electrical tape to cover up the thread plate slots.
-Test the motor using the battery/charger as described above. It SHOULD turn. If not, then either you missed something, or there is some other failure process at work that I didn?t describe (open windings, shorted or open commutator, really bad bearings, etc)

b-1) If the motor turns fine when powered directly, but either doesn?t run, or runs slowly (Problems 1, 2, 5)
-Carefully clean the contacts on the car side connector, and on the motor. There are some compounds that are reported to work wonders on electrical contacts. Try those. If this doesn?t help, and you are sure the proper signals are coming from the wiper controller, then press on.
-Open up the gear case by removing the four screws. Inside will probably be a greasy mess. Lever out the wheel gear and clean everything. Use the emery paper to clean up the metal contact stripes on the wheel gear. Re-Grease the gear shaft and the gears, and spread a thin sheet of grease on the contact stripes on the wheel gear. Gently bend the contact tabs on the gear cover (the copper fingers on the inner surface of the gear cover), so they stick down a little farther.
-Reassemble the gear cover and try the motor. It SHOULD work.


Top job Scott! :-)

Postby UKDaveJ » Mon Dec 27, 2004 2:07 pm

A great article, hats off to you Scott! :D :D

Dave 8)

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Postby Chris Wright » Sat Oct 10, 2009 8:27 am

Great write-up, but needs pictures! Here is a blog with photos of the gear case open:



Here is Brucey's write-up of his wiper arm linkage mod:
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Postby Brucey » Sun Oct 11, 2009 6:00 pm

another tip; Drew has posted (I think) about using a VW golf MkII wiper motor on the BMW frame, with a slightly revised wring harness.

Bosch still make very similar wiper motors today, with the three-bolt mounting onto various frames which are more car-specific. I recently overhauled one from an '00 Audi A4 and I reckon the motor was almost a straight swap for the E24, just needed the harness soldering on like the VW one.

This all means that if you raid the breakers, you are almost sure to find a motor to go with your rebuilt linkage!



Postby johnbrnrd » Wed Feb 09, 2011 3:29 am

Brucey wrote:another tip; Drew has posted (I think) about using a VW golf MkII wiper motor on the BMW frame, with a slightly revised wring harness.

Bosch still make very similar wiper motor today, with the three-bolt mounting onto various frames which are more car-specific. I recently overhauled one from an '00 Audi A4 and I reckon the motor was almost a straight swap for the E24, just needed the harness soldering on like the VW one.

This all means that if you raid the breakers, you are almost sure to find a motor to go with your rebuilt linkage!


Wow didn't know that bosch still build them will check them on my area can't wait to seat them in.

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Postby alina » Tue Feb 19, 2013 6:01 am

Chris Wright wrote:.
Great write-up, but needs pictures! Here is a blog with photos of the gear case open:

Rewire the unit so that it takes power directly from the battery. This eliminates the possibility that it is your wiring circuitry at fault, and not the windscreen wiper motor itself.


Here is Brucey's write-up of his wiper arm linkage mod:[/quote]


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