Opihi59

Up front, I have lurked on the C4 forum and it has been several years since I have posted. I have searched for answers, but do not find a solution. Here is the problem: a friend is babysitting a '78 25th anniv edition Vette for a Marine deployed to Iraq. He brought it to me today with a power window problem that has me somewhat confused. I only have the FSM for my 94 so I am digging into this on the basis of experience only. He tells me that several days ago the power window on the driver's side stopped working, at least it was in the "up" position. Then several days later, the passenger window stopped working, about 5" from the top.
The fuse is fine. I pulled both door panels and found that the driver's side motor has been replaced before, the passenger side is original. I checked power to the terminals by disconnecting from the window motor, and toggled the switch. I get 12V at both leads (Up, and down leads) on both driver's side and passenger side. Suspecting the motors--but why both would fail within days of eachother wouldn't make sense--I left them disconnected from the wiring, and hooked them up to an independent 12V power source. Each one works, and will cycle up and down without apparent straining.
So--with each motor showing me it works, and power at the terminals where the wiring connects into the motors showing appropriate voltage through the switches, it would seem the windows should be working. I reconnected the terminals, and made sure they were connected, but the power windows will not work. I am baffled, and this makes no sense.
If anyone has any thoughts on this, I would appreciate it, and so would the babysitter and deployed Marine.

Thank you for your input.

...Roger...

Are you using a 12v testlight or voltmeter to check for voltage at the term?

...Roger...

When you hooked up your independent power source where did you attach the ground?
When you took your voltage readings at the terminals where did you attach your ground lead?

Opihi59

DWncchs--thanks for replying, I saw you had a few power window posts in my search. I used a Voltmeter for my testing, so I got actual numbers, 12V. The ground for the independent power source was connected to the body of the electric motor, and then was hooked to the inner door frame. It gave me the same results in both positiions.

When I took the voltage readings at the terminals, the ground lead was touched to the motor housing, and then to the door frame. It gave the same results in both positions.

...Roger...

I'm not saying grounding is your problem but my next move would be to retry the independent 12v power source test but moving the "ground" from the door or motor to the negative post on the battery in your car.

I would also recheck the Voltmeter test by moving the ground lead from the tester from the door or motor to the negative term on the battery in your car.

This will provide the exact same ground path that is used normally.If the results are the same (motors work) then we will know the ground path is not at fault.

(i do have to say as usual i'm not fond of voltmeters for testing-i would prefer a standard 12v testlight)

speedreed8

i had same problem once, (79)both windows quit, but i had voltage at both plugs when switches were pressed, i found the ground wires for the window circuit had broken off , its inside the passenger door attached to a screw..

never mind, i think that was the doorlocks i'm thinking about, (50 yr. old brain malfunction)!!!!!!!

Opihi59

Thank you, I will do some more testing. It doesn't make any sense really. The fact that they failed not at the same time, but driver's side first, followed in several days by passenger side would tend to imply a more "central" common issue such as ground, relay, controller rather than a "peripheral" issue such as switch, motor.
At any rate, I will pull it all down again as soon as I get the chance and will resume this thread. I had put it back together because the more pressing issue was a partially open window, Vette not being driven, but that doesn't fix the real problem.
Will get back to you all shortly.

...Roger...

with this kind of logic you WILL find it.
Keep in mind that the voltmeter is very sensitive to voltage,you can have 12v with virtually "no" amperage behind it.For instance in the relay-it can be tripped by ignition "on" and have the 12 v going to the switches but if the points are corroded there wont be enough amperage to move the motors.
One step at a time and you"ll get it.

Opihi59

Okay folks, problem solved (more or less) Just remember that your dog can simultaneously get fleas, AND lice at the same time, as well as having worms. Problems can be simultaneous and there was no single unifying answer for this problem.
I tested both motors' power input terminals grounded to the neg battery terminal, and toggled switches and got 12V for up/down at the terminals for both sides. So here is the fun part. Yesterday I tested the passenger side for function with the motor OUT of the regulator and it ran fine with an external power source and it worked. I for some reason DID NOT test it while out connected to the native wiring and dangling free from the regulator. Today I did that and it worked. It turns out the window regulator was the problem, too much friction from rust and lack of oil that it couldn't operate. I lubed everything and pulled the window up and down by hand while working in the lube. I then reassembled and the passenger side is fixed. So--on the passenger side it was a friction problem in the regulator, possibly combined with a tired motor. I was 20 when this car was new, so I understand getting tired after a few years.
With the Driver's side, I pulled the motor out of the door for the first time, hooked it to the native wiring circuit and it also went up and down. This motor had been replaced before and had a certain amount of duct tape holding a non-OEM connector to the motor, the wires here weren't making adequate contact to operate. In addition, once I re-installed it, it only worked a few times and quit again. The torquing of the motor at end of cycle was enough to wiggle the wires loose, so in this case I just soldered the wires directly to the motor. With the door open or ajar, the window spools up and down nicely, but drags badly with the door shut as the top of the window engages the foam molding/weather strip. I believe here that as well as having an inadequate connection, the motor was tired. I did of course thoroughly lube the regulator on the driver's side as this was the apparent problem on the passenger side.
So it seems simple enough now that it is done, and with the 20/20 vision of retrospective analysis, but WHY would both sides fail within days of each other with separate, independant and apparently unrelated problems??!! Conspiracy theorists would enjoy this one. It probably is related to the mechanism whereby I am unable to find a particular tool that I just had in my hand while underneath the vehicle, only later to discover it was laying on my chest all the time. I understand this with my Jeep (Just Expect Every Problem), but this is a Corvette.
I honestly believe that the motors should likely be replaced, at least in the Driver's side since it seems tired. I will do that if requested by the Marine out there defending our frontiers of freedom, but I may very well be out there again for my 4th tour before he gets back.
At any rate, when your dog has fleas, he may also have lice, worms, AND ticks as well.

Thank you all, I'm having a beer now.