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rear window heater - voltage at terminals? 900 1993

posted by darmstrong on Fri Dec 29 06:15 CST 2006
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last visit: Sat Sep 13 12:21 CST 2014

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Can anyone tell me what they're getting as a voltage reading betweeen the rear window heater terminals? Mine doesn't seem to be heating that well, and the voltage between the terminals at each end is only 9.8 volts. All my voltage readings were taken with the ignition on and the engine not running, but this still seems a bit low.

I've made the following additional voltage measurements (I used the metal ring on the ignition key cylinder as a convenient ground; that may have impacted on the voltage values):
- 11.5 volts at dash switch/relay input with heater off
- 10.8 volts at dash switch/relay input and output with heater on (this seems to indicate there is no loss at the relay)

I'm surprised that there would be a .7 volt drop on such a short run to the switch/relay. It suggests that there's a relatively high resistance connection somewhere from switch/relay to fuse panel to battery. Does anyone have wire routing/connection details for the rear window heater on a '93 940 so I can trace it through?
--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

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rear window heater - voltage at terminals? 900 1993

posted by Ken C on Fri Dec 29 07:00 CST 2006
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last visit: Fri Oct 24 11:39 CST 2014

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I didn't directly measure my voltage across the window heater's terminals -- they're covered by a nice plastic wrap of insultion that I don't want to disturb -- but I can't imagine why, if your circuits are in tip-top shape, you're not getting full battery voltage of (under those circumstances you mentioned) around 12.6 volts.

But those voltage figures are way too low. There clearly does seem to be a voltage drop somewhere (also consider the ground near the window's grid), but don't forget to also check the battery. The battery, even with the engine off, shouldn't drop that low with the heater on -- after all, this isn't a starter motor! Maybe the battery is on its last legs?
At the battery's terminals, you should have 12.6 with a fully charged battery, and all the way down to 12.3 or so with an almost fully discharged battery. When you're engine is running, the battery should first (cold) go up to about 14.1 or so, and then level off at about 13.9 -- all these figures are without using the rear window heater, of course.

Good luck.

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rear window heater - voltage at terminals? 900 1993

posted by darmstrong on Fri Dec 29 13:36 CST 2006
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last visit: Sat Sep 13 12:21 CST 2014

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I could repeat the tests using a more definitive ground point, and with the engine running, but I'm guessing I'll still be dealing with voltage drops in the window heater circuit. Would be great to know what someone else reads right across the heater element terminals, along with the battery terminal voltages at the time of the test, so I know if this is relative to nominal battery voltage or charging voltage, giving me the typical losses. There are no other electrical issues with the car, so I'm doubting I have any alternator or battery issues.

I guess I'll just try some rudimentary tests, by turning the heater on and measuring the voltage drop between the ground side of the heating grid and battery ground, and between the high side of the heating grid and battery positive. The original tests suggest the voltage drops are on the positive side. (Need those wire routing/connection details so I can work through any voltage drops sequentially - still hoping!)
--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

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I've got the wiring diagram book -- I'm sending you the page .... 900 1993

posted by Ken C on Fri Dec 29 16:41 CST 2006
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last visit: Fri Oct 24 11:39 CST 2014

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Okay. I've got the '93 (Volvo's own green) wiring diagram book. I'm e-mailing you the page (pg 103).
Hope it helps.

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I've got the wiring diagram book -- I'm sending you the page .... 900 1993

posted by darmstrong on Sat Dec 30 06:02 CST 2006
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last visit: Sat Sep 13 12:21 CST 2014

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Got the drawing, thanks, but it's for the '93 240, not the '93 940. Conceptually, much is likely the same, but sometimes, the devil is in the details. For example, I think I saw somewhere that the wiring for the window heater might loop off the cigarette lighter?

At any rate, if you have the equivalent drawing for the 940, that'd be great.

And if anyone can do the voltage check across the heater element terminals + the equivalent battery terminal voltages at the same time (so I can know typically expected losses), that'd be very helpful too.

--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

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