Mechanics

REAR DEFROSTER

1997 Plymouth Breeze • 4 cylinder FWD Automatic • 130,000 miles

The rear defroster on my car will turn itself on and off, with a bias on keeping the defroster turned on. If I turn it off, it will instantly turn itself back on again. I can't confirm whether or not the defroster is actually turning on, or just saying it is, but my instinct says it is, What would cause this?
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On3pointking
May 5, 2011.



Update, as I drove home I noticed a few things, first, the odo was flickering. Two, just before I arrived home the battery light turned on.

Tiny
On3pointking
May 5, 2011.
I can help with the battery light. If the charging system is not recharging the battery while driving, the low system voltage is going to affect other computers of which your car has many. Start by using an inexpensive digital voltmeter to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is low, the problem can be further narrowed down by measuring the voltage on the two smaller wires on the back of the alternator, but those readings MUST be taken while the engine is running.

Caradiodoc
May 6, 2011.
Thanks. I pulled the fuse for the rear defogger and everything seems working normally (though, I don't have a defogger).

I have the new relay, but I'm not entirely sure where the old one is located. It wasn't in the fuse box, does anyone know off hand?

Tiny
On3pointking
May 9, 2011.
Sorry for the delay. High winds, trees down, power out, 'puter dead until a few minutes ago.

What relay are you referring to? There's two fuse boxes. One inside on the left side of the dash and one under the hood. I don't think you have a relay problem. After all, it appears the defroster is trying to work. Lets handle the charging system problem first, then see if that solves the intermittent defroster. The system is run by the Body computer and it's going to get confused if system voltage is low.

If the problem persists, suspect the Body Computer. There isn't much inside the switch that can cause a problem.

Caradiodoc
May 10, 2011.
Hey, I'm happy to get a reply, don't worry about any delay.

The issue with the defroster is it turns on by itself, rather than me turning it on and it not functioning. Another individual said it could be the relay.

I think the issue with the charging system might be secondary to this one. I think maybe the defogger staying on effectively the entire trip might have drawn more electricity than could be provided, which led to the drop in voltage, if that makes any sense.

As an update, I removed the fuse for the rear defogger and was able to drive around for a bit without the battery light coming on.

Tiny
On3pointking
May 10, 2011.
I think you might be due for a load test on the charging system. Besides voltage, (electrical pressure), which is what is detected to determine when to turn on the warning light, the alternator must be able to provide the necessary current, (volume or flow). There have been a lot of problem with wear of the brushes inside the alternator that leads to it being dead and the warning light turns on. That always starts out being intermittent and can stay that way for weeks. The part is very inexpensive and fairly easy to replace, but most people just buy a rebuilt alternator.

A less-common problem is one of the six internal diodes shorts. That will show up on a load test as it can only deliver very close to one third of the rated output. A 90 amp alternator is one of the common sizes. If it has one bad diode, all it will be able to produce is around 30 amps. It takes around 10 amps to run the fuel pump, 10 amps to run a pair of head lights, then you still have the tail lights, radio, heater fan, fuel injection and ignition systems, so there isn't any spare capacity left over. The rear defroster also only draws around 10 amps so that is insignificant, ... IF the alternator is working properly. You might have stumbled onto a clue that there is a charging problem before something worse happens.

Caradiodoc
May 10, 2011.
Follow up, replacing the relay solved the initial problem for around $10. It was a hard part to find, though.

Tiny
On3pointking
Jun 6, 2011.
Very happy to hear it's working. Gonna have to add that to the memory banks. Where is that relay located?

Caradiodoc
Jun 6, 2011.
It is located inside, in the driver side of the dashboard.

Tiny
On3pointking
Jun 6, 2011.