fberus

I need some help understanding the combined heated mirror and rear window defogger system on my '96 convertible. When I press the defog button on the HVAC control unit, the heated mirror fuse blows every time. The indicator light below the defog button stays on for about 2 seconds then turns off. If I repeatedly press the defog button (with the fuse now blown), it does the same thing: i.e. the light goes on for about 2 seconds, then turns off. I get no heat to the mirrors or rear window defogger. The rear window defogger fuse does not blow.

I would suspect a short in the mirror heater circuit but why would that prevent the rear window defogger from working once the mirror fuse has blow? The service manual does not have a diagnosis procedure for this symptom so I don't know where to begin.

Thanx

65Z01

My '88 has a Defogger Relay applies power two separate circuits, one powers the outside mirror heaters through the Heated Mirror Fuse and the other Rear Window Defogger and is not individually fused. The Defogger Relay receives power through the Defogger Circuit Breaker.

I would check for a short in the mirror heater circuit(s).

It may be that the Defogger Circuit Breaker (that powers the relay) is being tripped by an additional short.

Is your circuit different or similar to the one I've described?

fberus

Thank you gentlemen,

The engine is running during all my tests. It sounds like a short in the heated mirror circuit is the problem. I'm no electrical engineer but the '88 and '96 circuitry seem to be somewhat different - I only have a '96 service manual. The '96 has two separate circuits, each protected with it's own fuse - 5A in the I/P block for the mirrors and 40A in underhood fuse block #2 for the rear defogger. However, I don't think it matters in this case because the fundamental problem is the same. The service manual says to check for DTC 22. I haven't seen any of these when plugged into the OBD port but I haven't been plugged into the port while specifically cycling the defogger button. I'll give that a try and attempt to isolate a short in the mirror wiring.

If I have additional questions, I'll post them here.

-Floyd-

fberus

Figured it out so I'm posting the results in case someone else encounters this problem.

Since a short in one of the heated mirror circuits was suspected, I decided to unplug the each mirror, one at a time and try the defog with a fresh heated mirror fuse. This requires removing the interior door panel. There are two connectors inside the forward cavity of the door. There is a three-wire connector for the mirror angle adjuster and a two-wire connector with an orange wire and a black wire on the supply side (the mirror wiring uses different color wires). The two wire connector is the one that needs to be disconnected,

Since I had recently repaired damage to the driver's side mirror, it was the most likely to be the source of the short. Once the drivers-side heated mirror was disconnected and a fresh fuse plugged into the fuse panel, I tested the rear defogger. The rear defogger and passenger side mirror now worked fine and no fuses blew. The driver's side mirror was definitely the problem.

I pulled the mirror out of the housing and turned it over to view the electrical connections that go into the backside of the mirror. It didn't take long to find the problem. The two connections (one hot, one ground) enter the rear of the mirror unit at the same point and are very close together. By gently pushing on the wires I see the exposed connectors of the wires touching each other - an obvious short.




To correct the problem, I gently spread the two connectors apart with my finger. I then used a small paint brush and some liquid electrical tape.



I worked the liquid electrical tape between the connectors and used two coats to make sure I fully isolated the exposed metal connectors from each other.




It looks a little messy but I don't want to have to come back here again. After letting the electrical tape dry for about 15 minutes I put the mirror back in the housing and reconnected it inside the door and tested the defogger and heated mirrors. End of job.