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1986 coupe rear defrost troubleshooting

Liam86VetteOwner

New member
Joined
Nov 23, 2022
Messages
2
Location
IOWA
Corvette
Black 1986 C4 coupe with a glass top
Hi all! I am a new owner of a 1986 black coupe with glass top. 140k miles. I am reasonably good with electronics if I have diagrams and part locations, but have found it hard to locate this info for my specific vehicle. The rear and mirror defrost is inoperable currently. This is in addition to a myriad of minor electrical problems that are pretty typical of an older car that has been sitting. Most of the switches have been fixed with a little contact cleaner, a bit of patience, and working them to break up internal surface corrosion.

The defrost on the other hand has not started working. The original switch does not light up when turned to on, and i dont hear any click of a relay or any other i dication of operation. I have checked and no heat at the mirror or rear grid.

I tested all fuses, and circuit breakers in the passenger side fuse box. All test fine.

I have tested continuity of the rear grid, all gridlines have continuity to ground, and show correct resistance, so the ground, struts, grid connections and grid lines are good. I have 0 volts at the rear regardless of switch position.

So first thought was to replace the switch since most of the other switches have been intermittent/corroded/or inoperable when I got the car.

The new switch also does not light, and still no defrost.


I plan to test the switch incoming and outgoing legs tomorrow morning, but since I dont have a pin out, and also don't know if the relay is built into the switch, or if there is an external relay, not sure which wires/connecting pins should have battery voltage, which are switched, etc. This makes troubleshooting a hit or miss proposition. Any advice, diagrams, etc would be helpful.

This vehicle has the manual climate controls with the turning toggle style defrost switch/timer control that twists left for off, right for on, and rests in the center between both positions.

I appreciate any assistance the collective group can bring. I really want to restore this car to its former glory, and make it respectable to see in car shows again.


I have several other gremlins I am working through, but with freezing Temps in my area, and this being a semi daily driver out of necessity at the moment, this one is highest need for me.
 
Last edited:

Liam86VetteOwner

New member
Joined
Nov 23, 2022
Messages
2
Location
IOWA
Corvette
Black 1986 C4 coupe with a glass top
More info. Prior owners have done some additions to the electrical system through adding a 3 outlet add on, a permanently wired radar detector cable that is run up through the a pillar and front roof trim to just behind the rear view mirror. Since the additional wiring may or may not be done correctly, tracking these as part of my troubleshooting will take place tomorrow when I have adequate lighting and time to trace them.
 

JK93Vette

New member
Joined
Sep 13, 2022
Messages
1
Location
Tucson
Corvette
!993 C4 Sport Coupe White
Hi all! I am a new owner of a 1986 black coupe with glass top. 140k miles. I am reasonably good with electronics if I have diagrams and part locations, but have found it hard to locate this info for my specific vehicle. The rear and mirror defrost is inoperable currently. This is in addition to a myriad of minor electrical problems that are pretty typical of an older car that has been sitting. Most of the switches have been fixed with a little contact cleaner, a bit of patience, and working them to break up internal surface corrosion.

The defrost on the other hand has not started working. The original switch does not light up when turned to on, and i dont hear any click of a relay or any other i dication of operation. I have checked and no heat at the mirror or rear grid.

I tested all fuses, and circuit breakers in the passenger side fuse box. All test fine.

I have tested continuity of the rear grid, all gridlines have continuity to ground, and show correct resistance, so the ground, struts, grid connections and grid lines are good. I have 0 volts at the rear regardless of switch position.

So first thought was to replace the switch since most of the other switches have been intermittent/corroded/or inoperable when I got the car.

The new switch also does not light, and still no defrost.


I plan to test the switch incoming and outgoing legs tomorrow morning, but since I dont have a pin out, and also don't know if the relay is built into the switch, or if there is an external relay, not sure which wires/connecting pins should have battery voltage, which are switched, etc. This makes troubleshooting a hit or miss proposition. Any advice, diagrams, etc would be helpful.

This vehicle has the manual climate controls with the turning toggle style defrost switch/timer control that twists left for off, right for on, and rests in the center between both positions.

I appreciate any assistance the collective group can bring. I really want to restore this car to its former glory, and make it respectable to see in car shows again.


I have several other gremlins I am working through, but with freezing Temps in my area, and this being a semi daily driver out of necessity at the moment, this one is highest need for me.
Regarding your rear defroster, the SM notes that engine needs to be running when shooting this circuit. I had a similar issue with my ‘93. It turned out to be power thru the gas springs. Take care to clean the connection lugs very well.
 

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