Drivers door window motor - HELP!

For nearly as long as I have owned my 1992 XJS, the windows have been “lazy” Sometimes worse than others. I cleaned the switches and that help a bit. I found new switches and that helped a lot, but still they were “lazy” sometimes. I thought of installing relays as others have done, but recently the drivers door window just refuses to budge. Not even a groan. I tried switching to use the passenger switch which works great - no go. So I pulled the door apart, found the connector to the motor and pulled it apart. Only 2 wires. OK, one must be up and one down with the frame as ground, right? NO. I took a wire directly from the battery and touched it to each of the two pins separately. Nothing happened. Then, just for giggles, I tested the voltage on each of the sockets (that go back to the car). BOTH tested +12v! What makes it even stranger is that with key on and press the window switch up (or down) one pin stays at +12v and the other goes to 0v! Can anyone explain this behavior to me? I have changed nothing so this has to be the way the factory wired it, but I cannot imagine why.
Given this bizarre arrangement, is there any way to test the motor before pulling it out to replace? I guess putting +12v on one pin and leaving the other open was the same as what happens when the switch is pressed. If so, that would mean my motor is dead and needs replacing, but first I’d like to figure out what is going on electrically. (I do not have a wiring diagram, but I’m not sure how much it would help.) Pics show the socket and the probe from the meter in one socket.


Applying 12v to both leads of the motor at the same time stops it instantly when the switch is released. That is normal for your car when the switch is not actuated. The switch disconnects 12v one one side and adds the ground on that side when actuated one way or the other. You can see the switch circuitry on the 92 wiring guide here:
http://jagrepair.com/images/Electrical/Elect-Tech%20All2/92%20XJS%20electrical.pdf

Applying power and ground to the window motor wires directly will tell you if the motor is good or bad.

Jon

Yes, this short circuits the motor via the positive cable. The wiring then acts as a load on the motor which now is a generator (like a dynamo). This stops it quickly. It could also be connected to negative on both leads.

Think of it as regenerative braking minus the battery.

When the switch is pressed one of the two 12V contacts switches to ground.

If your power and switching works then your riser motor is the problem… I had similar trouble on my 88 …changed riser to older style unit… bolts right in . I tried fixing the original motor but wasted money and time…also spraying a little silicone lubricant in the window tracks helped…yes.it can be bench tested .

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Its important to check that you have good power and ground at the motor UNDER LOAD. Just measuring 12v or ground with no load only tells you that you have continuity, not that the switch, wiring, or any connections between the switch and motor can carry sufficient current under the considerable load of the motor to run it. That’s why it’s important to test the motor with an independent power source. If the motor runs with an independent, direct power source, you know something else in the system can’t carry the load.

IF the motor works when powered independently, I’d suspect a bad ground somewhere in the switch circuit that is unable to carry the load. That would explain why it wouldn’t run in either direction.

Once you repair/replace the motor or repair the fault in the switching system, it’s important to lubricate the window mechanism to reduce the load on the motor and wiring. Too much bind in the system can either burn out the motor, or burn contacts in the switch/connectors. Use a good penetrating synthetic lube on the mechanism, and spray the tracks in the door with silicone lubricant. 30+ year old lubricant in the mechanism acts more like glue than lubricant.

Jon

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I checked the wiring and found that indeed it switched from +12v to gnd, but of course that is not under load. I’m trying to figure out how to get good +12v and gnd to the pins in the motor connector. Short of just cutting the wires and then rejoining them with butt connectors do you have any ideas on how to apply the voltage to those two pins?

I did exactly what Scrimbo did. Went to mid 80s version. A used one. They are bigger but fit, and way more reliable.

Small alligator clips and short test leads. You don’t need to test long, just enough to see if the motor operates at all.

Jon

I ended up using 12g wire and just cut it flush and then used an awl to create a little indent so I could make good contact with the pins without shorting out. Motor is dead. ?Good place to get one??

Did you have to disconnect anything to measure that +12 to ground?

If not, measure it again, keep the meter leads on it, now have someone operate the window.

If you get 10V, you have 2V voltage drop (VD).

Typically, VD is indicated as a %, and on electrical circuits (buildings, not cars, but I don’t know why it would be much different) 3% VD is considered the maximum acceptable.

A 2V VD on a 12V circuit would be 17% VD, which is a lot.

Note: First check your +12V as it will likely be greater. Then compare that voltage to the running voltage. I’m just using “+12V” as ‘nominal’ voltage.

That likely indicates a poor connection somewhere, possibly even several connections.

To verify an acceptable VD for that 12V motor, connect +12V to one terminal and ground to the outer, along with the meter leads and see what the voltage is with the motor running

Granted, in the car, the wires are longer, however, the resistance for wiring is given in Ohms per thousand feet, and with probably less than 30 feet of wire between the motor and the battery (I’m not counting the chassis ground return as that likely adds negligible Ohms), the resistance of the wire is not an issue … but each positive connection and each ground connection can be an issue, and each poor connection adds up to a problem.

OK, if I want to go to a mid 80s version, do I just need to buy the motor, or motor and regulator? I thought the earlier ones were 10 teeth and the 92, 8 teeth. If I get a 79-88 motor and regulator will it just bolt right in?

It’s been a few years, i can’t exactly remember. I think i bought it with the motor/regulator together, and it hooked up pretty easily.

Oh wait, you have a 92? I am not sure now. You may not be able to retrofit. Anyone know more about the 90s window motors?

Just for the record… when I changed mine I did motor and regulator… my car is an 1988 and I have read that the 88-9 motors have had problems as they were a different motor…as far as fitment into the later cars I don’t know either. Here’s two pics not necessarily the correct ones but I got rid of the squarish looking one and replaced with the round motor that I got used complete with regulator… bolts right in.
Screenshot_20240610-072932~2
Screenshot_20240610-073217~2
Best to verify with others about this