1981 Rear window defogger

[Vette79]

Sounds like you are as bad as me. I will probably never ever use the defogger but it is the fact that it is there and does not work.

 

[Nobody Cares]

Exactly.

 

[Mr. Chuck]

81 Rear Defogger Info

My mistake. No relay. Just a connector in the rear left quarter panel.

Diagram shows 30 amp fuse for the defrost on circuit with orange and black wire. Guage fuse also has something to do with it on the pink/black wire. Coming out of the switch is a black to ground. a purple that goes to the connector in the rear left quarter and switches to a black which terminates at the grid.

Check your grid lines on the window a worn one could cause a problem.

Jim

Shop Manual shows that Defogger Switch & Timer Relay are all part of the same switch unit. There is actually one 35 amp circuit breaker & one 20 amp fuse that powers the rear window defogger (both located on fuse panel). The breaker is hot all the time and fuse is hot when ignition switch is in start or run position.

See attachment for The entire diagram:

 

[Vette79]

Diagram is pretty hard to read. If you are correct, this would make things easier eliminating a seperate relay under drivers side dash. Is the CB normally closed? If so, I could check for continuity accross the terminals to verify it is good right? Same with the fuse. Otherwise, it would be a bad connection or ground elsewhere. I thought for sure there was a seperate relay. Can anyone say for sure?

 

[Mr. Chuck]

Relay Position Confirmed

Diagram is pretty hard to read. If you are correct, this would make things easier eliminating a seperate relay under drivers side dash. Is the CB normally closed? If so, I could check for continuity accross the terminals to verify it is good right? Same with the fuse. Otherwise, it would be a bad connection or ground elsewhere. I thought for sure there was a seperate relay. Can anyone say for sure?

Shop Manual flat statesment is that relay is part of swwitch assembly! If needed I can attach a higher resolution picture so you can expand it more, let me know. The following tips may help in trouble shooting efforts.

One caution regarding use of volt meter to confirm a good / reliable electrical circuit; Volt meter tell you that there is a connection there, but not if the connection / circuit will function properly (carry a load current). The use of a test light (12 V lamp connect to two test leads) connect between the power source (fuse, relay contact or circuit breaker) to ground / frame yielding a bright illumination of the lamp tells positive that there are no high resistance connections between that point and the power source. Note: The test lamp needs to be of the same voltrage rating as the power source to give a reliable estimate of circuit ability to function correctly, also using 6 V lamp in a 12 V circuit will result in lamp failure (burn-out).

 

[Vette79]

The 79 shop manual suggests that the timer relay is located behind the emergency brake at the very rear of the center console. So there is a 10amp fuse, 35amp circuit breaker, the defogger switch, the timer relay and the various connections along with the grid. Not to mention the ground points. A test light is the best tool to use to figure this problem out. CB is probably good because it only trips when there is an overload in the circuit and then resets. It appears from the multiple responses that the 78 and 81,82's are different. Will keep you all posted with what I find later. Thanks for all the help...Jeff.

 

[Vette79]

NobodyCares, any luck with this fix yet? I have not had time but I will this weekend.

 

[Nobody Cares]

Vette79, not yet kinda in the same boat. Still have about 15 inches of snow in the yard. Been nice so far this week , but back down to the high of 30's and snow again by Friday. Will keep you updated when i get too it. Dave












:bar

 

[Vette79]

Well, I got the Rear Defogger working!!! Using a low voltage dc test light I found that I had good power to the switch so I decided to start from the rear of the car and work forward. I took off the passenger side upper panel and re-connected the black ground wire. Did a quick check by turning the ignition to the start position and then turning on the rear defogger switch. IT WORKS!!! Green light and all. Also fixed the drivers side courtesy light, cigarette lighter and installed original Delco 8 track stereo. Everything works in my Vette now...

 

[Nobody Cares]

Vette79 i'm jealous. Thanks for letting me know what you found. Time for me too get the test light out. I'll check the ground out also. Thnx again Dave

 

[Vette79]

Well, damn thing is on the blink again. Was working and now it does not. I have power to the switch and the connections are good as well as the rear ground at the left passenger pillar. May be fuse box or timer relay. I have no need to use this accessory but now that I know it works and does not is bothering the hell out of me...

 

[Nobody Cares]

Vette79, i know what you mean. Mine was doing the same thing. Would work , then not work. Drives me crazy :bash . Our weather is finally get better , so hopfully soon i can tackle the defoogger again. Dave