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Old 04-06-2021, 09:46 PM   #1
vishmutzy
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Default Power window fuse 10 high current

Had the power windows stop working while driving my '91 245 today with the kids in the back. Rolled the windows up on the freeway and once we were off we couldn't roll them back down (got hot without A/C!). I pulled over and found fuse 10 (15A) had blown, replaced and all windows worked again.

Stopped at HF and picked up a fuse current tester. Found that when either front window reached full closed and the motor stalled (holding switch for a second), current peaked at 15A. When more than one window was closed at same time the current exceeded 25A. Three windows and the fuse popped (over 30A).

So obviously I can't roll up more than one window at a time. But it seems like a really poor design to be able to max out a circuit fuse with a common operation (especially with kids in the car that can't remember the "one window at a time" rule). Is this a normal, just-how-it-is, deal-with-it thing on these cars, or are my window motors drawing abnormally high current?
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Old 04-07-2021, 02:22 AM   #2
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Abnormally high current is pretty likely, when the grease has been collecting dirt and drying out for several decades. If they were manual windows, you'd be noticing how hard it is to wind them up and down, and you'd be astonished how much better they work after you disassemble everything inside the door, clean the regulator, rollers, sliders... re-grease it, and holy cow it behaves like new again.

That poor little motor has a heck of a job to do. Not only that, I wouldn't be surprised if the motor armature, itself, is sluggish as well. I've not (yet) disassembled one from a Volvo window, but I've seen phenomenal improvement on every other old motor I've cleaned, so I have no reason to believe this would be any different.

I'd bet your current measurements drop in half, after doing this.
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Old 04-10-2021, 10:24 AM   #3
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Abnormally high current is pretty likely, when the grease has been collecting dirt and drying out for several decades. If they were manual windows, you'd be noticing how hard it is to wind them up and down, and you'd be astonished how much better they work after you disassemble everything inside the door, clean the regulator, rollers, sliders... re-grease it, and holy cow it behaves like new again.

That poor little motor has a heck of a job to do. Not only that, I wouldn't be surprised if the motor armature, itself, is sluggish as well. I've not (yet) disassembled one from a Volvo window, but I've seen phenomenal improvement on every other old motor I've cleaned, so I have no reason to believe this would be any different.

I'd bet your current measurements drop in half, after doing this.
+1 on this.

Blew that fuse recently on a 240 that had been wasting away in a vacant lot for years. Took it as a reminder to clean and lubricate the power windows. Huge improvement. Also cleaned out the switches with CRC contact cleaner while I was at it (be careful around plastics if you go this route). Windows are now working flawlessly.
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Old 04-10-2021, 11:32 PM   #4
vishmutzy
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Good advice on cleaning & greasing everything. I did an in-door clean & lube on both front doors and noticed improvement in the closing operation, but I suppose next time I have the doors apart I'll remove the regulator & glass runs to properly inspect & clean everything.

However, the current peaks that overload the fuse occur after the glass stops moving and the motor stalls (zero rpm = highest torque & current draw for electric motors). So regardless of the condition of the regulator & glass sliding mechanisms, the fuse is being overloaded once the motor(s) stalls at full closed position. Might be that the 30 year old motors have shorted windings that are causing high current, maybe corroded switch contacts are causing higher resistance.

Furthermore, the driver door motor had the highest stall current at 15A, the passenger door was second at 13A, and both rear doors measured around 10.5A. So that rules out a single "bad" motor, meaning that the design of this circuit simply doesn't allow raising more than one window at a time without exceeding the fuse rating.

We know that fuses are designed to sustain over 100% of their rated current capacity; that maximum point of failure will occur suddenly with a high load or gradually with several lesser loads. So I will probably be able to raise both front windows several dozen times before the fuse blows, or both rear windows several hundred times, or three windows maybe once or twice.

Last edited by vishmutzy; 04-10-2021 at 11:55 PM..
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Old 04-11-2021, 06:09 PM   #5
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Quote:
Originally Posted by vishmutzy View Post
the design of this circuit simply doesn't allow raising more than one window at a time without exceeding the fuse rating.
Actually, your assertions point to not the raising of multiple windows, but the allowing of multiple stall situations concurrently.

I have met people who would happily keep their finger on a button for long periods of time even if (or especially if) nothing appeared to be happening... but with your excellent grasp of motors and stall current, I wouldn't expect you to succumb to that urge. Not sure what I'm missing, but the classic doctor response comes to mind:

"Doc, it hurts when I do this."
"Well, don't do that."
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Old 05-10-2021, 09:42 AM   #6
vishmutzy
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Actually, your assertions point to not the raising of multiple windows, but the allowing of multiple stall situations concurrently.

I have met people who would happily keep their finger on a button for long periods of time even if (or especially if) nothing appeared to be happening... but with your excellent grasp of motors and stall current, I wouldn't expect you to succumb to that urge. Not sure what I'm missing, but the classic doctor response comes to mind:

"Doc, it hurts when I do this."
"Well, don't do that."
Thanks Doc. You're not missing anything, you nailed the diagnosis in that concurrent stall conditions are causing circuit overload and this can be avoided by not doing that. I have the self-control not to blow the fuse, but it's my 3 kids that don't understand Ohm's Law and will require me to carry spare fuses.

Fortunately 240's have a window lock switch
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Old 05-11-2021, 02:22 PM   #7
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I find this an interesting thread from the first post. If anything, I would expect the stall current in a window motor to have decreased after 30 years rather than become larger. Also I would expect a new car (design or at dealer) to be rejected if the owner couldn't hold down all four switches until the motor sound stopped and expect to repeat this without changing a fuse.

My personal habit is to do two at a time, but from here I'm gonna try all four at every opportunity to see if I can bust a fuse, return to this thread and compare stall currents. Now, as to fuse aging (or manufacturing tolerance in new fuses for old cars), that's another subject.
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