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Re: Heated rear window
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Wed, 2 Mar 2005 06:04:55 Share Post by Email
In Reply to: Re: Heated rear window, goodwrenchldj [Profile/Gallery] , Tue, 1 Mar 2005 19:55:59
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The typical resistance of a heater element is about 2-3 ohms. Most digital meters aren't very accurate down that low, so a reading of 2.6 maybe be 2.0 or 3.2 ohms. What counts is that you have continuity across the heating element.

I would concentrate on the ground side. 1.6 ohms is pretty high. Look at it this way; if the total resistance to ground is 3.6, and 1.6 is from the female connector to ground, then the heating element is 2 ohms.

Also, measuring the open circuit voltage is good (the 13.6), but not sufficient. If there is even a slightly bad connection in the line feeding the 13.6 volts, it will read fine with no load. But put any load on it, and the voltage drop across the bad connection will result in little or no voltage at the heating element. The 13.6 indicates that the relay is closing, not much more. But a good start.

Since the resistances are so low, without a special meter to measure low resistance, use voltage.

With the heating element CONNECTED, set the meter to volts, and measure voltages with respect to chassis ground. You may have to probe into the back of the connector, or make up a thin wire with a paper clip or the like. What is the voltage on the driver's side with the heating element connected? If it's above 12 volts or so, that's good. If it drops way down, then the problem is a poor connection on the source side. If the dash light isn't coming on when the heating element is connected, but comes on with it disconnected, I would suspect a bad contact inside the relay, or at the relay pins. If the Dash light works fine with the heating element connected, but you see a low voltage (well under 12v) at the driver's side of the heating element, I'd suspect the wiring between the relay and the heating element.

That's only half the story - say the voltage on the driver's side is just fine with the element connected. Now measure the voltage on the passenger side (ground). It should be low - less than 0.5 volts, hopefully lower. If it's higher, it indicates that the ground connection is poor. For example, with a bad ground, you could measure 13.6 on the driver's side, and 13.5 on the 'ground' side. 0.1 volts across the heating element isn't going to do much. If the ground side isn't very close to 0 volts, trace the ground wire and clean up the ground connection - remove the bolt where it meets the chassis, and clean the surfaces with some fine sandpaper or Scotchbrite.





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