Ok need some advice on a weird issue. When I purchased the car I noticed that the power door locks didn’t work, upon further inspection there was a blown 20amp fuse in the fuse box location. I tried a new fuse, worked briefly. Gave up for a bit and recently decided to investigate further as I have time to work on the issue. First thing I found was this should be a fuse but a 30amp thermal circuit breaker, ok sourced one and replaced it. Power locks seem to work (could be smoother operation) but I noticed after I came back to the car after about 20min that the rear defroster light was on. Checked the rear glass - defroster definitely on. Tried to turn off via the switch, no joy still on. First shouldn’t this not work at all with the car off? Could it be the switch? Something else? Can’t figure out why this would be working this way.

Ben
1987 Fiero GT

Maybe the switch is broke but Defog "relay" is likely bad.
10mm Head Bolt to column/pedal frame just above brake pedal.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Hey Ben,
The 'PWR AC Circuit Breaker' feeds the Power Door Lock Relay AND Rear Defogger Timer Relay Contacts Only which feeds the rear defogger element when the Defogger Timer Relay Coil says 'go'!

When you replaced the 30-amp circuit breaker.....

quote FieroGT1987: I noticed after I came back to the car after about 20min that the rear defroster light was on. Checked the rear glass - defroster definitely on. Tried to turn off via the switch, no joy still on.

The Rear Defogger Uses Two Voltage Sources:
'The 'PWR AC Circuit Breaker' as above is HOT @ All Times
'Gages' fuse feeds the Defogger Control and the Timer Relay Coil which are only HOT when the Ignition is in RUN/START

FYI From my '88 Factory Manual:

quote With the Ignition in RUN, voltage is applied (from the 'Gages' fuse) to the Defogger Control. When the Defogger Control Switch is pressed ON, voltage is then applied (from 'Gages' fuse) to the Defogger Timer-Relay. The relay contact closes, which provides voltage (from PWR AC Circuit Breaker) to the ON Indicator and the Rear Defogger (element). The contact in the Defogger Timer-Relay will stay close until the Defogger Control Switch is OFF, or until the timer cycle is complete.

If the Ignition is OFF and the Rear Defogger Indicator/Element are ON; I concur with Ogre that the solid-state timer relay contacts are welded together or 'sticking'.

BTW: Most people don't use the rear window defogger so you can simply remove the timer relay and be done OR replace it which is GM 10026005 for '84-'87.

The diagram below doesn't show the brake switches which are situated on top of the relay.


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Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

[This message has been edited by Vintage-Nut (edited 12-07-2023).]

quote Originally posted by Vintage-Nut: BTW: Most people don't use the rear window defogger so you can simply remove the timer relay and be done OR replace it which is GM 10026005 for '84-'87.

That is a true story.
I don't think anyone has ever needed it.
remove the relay and call it a day.

I've use my rear defog to drive when car is cold. More so when have ice storm because Fiero rear glass is harder to clean let alone get ice remove by hand.

Note: While many doc's say 10/5 minutes (first use / any after w/o I-key turn Off), Many have 5/2.5 minute "relay" from the factory.
Is marked on the relay w/ time data & think GM PN but most times no longer can get as new.

5 minutes & very small glass warms up fast & heavy ice is easily removed. often way faster then waiting for engine heat to make heater box to work & that doesn't take long w/ coolant system actually works right in most cars.

I've never push the switch for rear defog during driving on any car.

quote Originally posted by theogre: I've never push the switch for rear defog during driving on any car.

My Formula (originally from Knoxville, TN) doesn't have a rear defogger (and I don't miss it)... but I use the rear defogger all the time in my Subie wagon (except in the midst of summer). There's a slight water leak in the back (which I've tried to find but failed) and a small amount of water eventually collects in the spare tire tub. Condensation then forms on the rear hatchback window... and the rear defogger is therefore absolutely invaluable!