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UPDATE: Power window 30 amp fuse blowing

[argghh, I just spent 20 minutes typing all this and all of a sudden it all went poof. Tried it again and it did the same thing, so I'm now pasting this from an offline composition. Hope I remember it all]

Hi again Randy,

I hear you about being in recovery mode. A bunch of us here are now getting older and it seems some days our old Volvos may be in better shape than us.

Your new observations very much narrow down the possibilities.

First off, if the rear passenger windows operate without blowing the fuse and only the driver door window is affected then that says there is likely no broken or shorted wire at the hinge area, also that it's not at the relay, not at the fuse panel and not at another door. Unless a window at one of the other doors is being operated then the only active power in the harness through the driver hinge is +12V from the relay (on the yellow-red wire) and +12V back to the rear door window switches (on the pink wire, attached to the child lockout switch).

If what you say is correct then there's one more test you can do to verify this. You should be able to operate the other windows from the driver door switches as well as from their own door switches. Only the driver door window being operated by the driver door switch should blow a fuse. If that's not the case then I think you need to repeat your diagnosis.

I now only see three possibilities:

[A] There could be an internal short in the switch. These switches are known to be troublesome, collecting dirt inside and the contacts wearing to the point they don't work or work intermittently. I haven't seen it myself, but if one of the spring loaded prongs inside wore out then the spring could drop down and cause a short if it got down into the contacts.

Power windows switches are momentary DPDT switches that act as a flip-flop providing power down one of two wires to the motor with the other wire providing ground to the motor (switching power and ground on those wires makes the motor operate forward/reverse). In the neutral position there is continuity through both sides of the switch to ground. Pressing the switch one way interrupts ground on one side of the switch and instead provides +12V on the wire to the motor. Pressing the switch the other way interrupts ground on the other side of the switch and provides +12V on that wire to operate the motor in the other direction. If the two sides of the switch ever short together when +12V is being applied then the fuse blows. In theory, if the short was weak then the motor might flinch for a moment before the fuse blows, but I think that's highly unlikely, it's either shorted or it isn't.

To see if it's the switch, you need to remove the connector from the back of the switch (remove the pocket frame and try to lift the switch pod up, else remove the door panel for proper access). In the neutral position, you should see near zero ohms between pin 1 (for reference that had the black wire on it) & pin 2. In the neutral position, note the resistance between pins 4 & 5 as X. For a switch with an indicator light, X will be low ohms, for a switch without an indicator light X will be infinite ohms (open). With your meter on pins 1 & 2, depress the switch one way then the other. If either way shows near zero ohms (meaning significantly less than X) then the switch is shorting inside. A simpler test (at the risk of needing another fuse) is to simply put the right (passenger) switch in the left (driver) position and if the driver window now works normally then the switch was obviously faulty. For me, I've repaired those switches so many times I'd also open it up (carefully) for a cleaning and inspect the inside contacts for wear and damage. BTW, if it is the switch then post back as the driver door switches are a little different than the other door switches. In a pinch, you can use a rear or passenger door switch at the driver door, but will need to add a pigtail jumper wire to one of the pins.

If it's not the switch then that leaves two possibilities:

[B] There's an overload blowing the fuse. If the motor gets worn and/or the window frame starts to stick then the window will work a lot slower and harder. That's common for our old RWDs, so well worth considering. When the load gets high enough, the internal thermistor will trip and the motor stop as if the window was at end of travel (the thermistor resets after you release the switch). The thermistor will normally trip before the fuse blows, but if the thermistor is faulty then it may not. If there are other active loads on the fuse then the fuse may trip before the motor. Fuse 8 only has the power windows on it, so as long as the other windows aren't being operated at the same time then the motor should always trip first unless the motor is faulty.

[C] If it's not the switch or the motor then it's most likely as I originally suggested and a short in the wiring between the driver door switch and its motor (the red-black and black wires). That branch of the wire harness may have been pinched or chafing against the window or door lock mechanism and worn through so that it's now touching metal. Also be careful of sharp speaker screws damaging the wiring. Remove the door panel for inspection and it should soon be obvious if that's the case and easily repaired.

Good luck, keep us posted and hope you're back in full gear soon.

For your further interest I'll relate a good power window story. Many years ago my wife's 940 got rear ended sitting at a stop light. No body damage, but the air bags and seat belt tensioners detonated, blowing out the windshield and injuring her slightly (the SRS system is not supposed to detonate in a rear ender, but in this case she bounced back off a truck trailer hitch in front of her to make the controller think it was a second hit from the front). It took three months to get the car back from the body shop (an ugly long story of SRS red tape and the insurance company, they wanted to write off the car). Everytime she operated the rear window fuse 8 would blow. My wife turned around and took it back to the shop. They were busy, so said to be patient and they'd get to it as soon as they could. One more month of excuses went by before they finally admitted they couldn't find the problem and I'd have to take it to dealer sevice to figure it out. When we finally got the car home I knew what they'd likely done and within 5 minutes I had the B-pillar seatbelt panel off to reveal that they'd pinched the right side wiring harness under the seat belt bolt. I loosened the bolt to reveal one of the power window wires was badly crushed when they torqued the bolt and braid was touching the back of the washer, so the fuse would blow every time power was applied. Five more minutes to tape it up and re-torque it to spec and all was back to normal. I gave that body shop hell and never went back again (it was actually owned by the brother of the main local Volvo dealer, which is why the insurance company sent it there in the first place having access to the SRS components at a discounted price).
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now






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New Power window 30 amp fuse blowing
posted by  rstarkie subscriber  on Mon Oct 21 09:53 CST 2024 >


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