78 Trans Am Rear Defrost Wiring - Second Generation Pontiac Firebird (1970 - 1981) - Firebird Nation

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78 Trans Am Rear Defrost Wiring

rear defrost wiring

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7 replies to this topic

#1 78TA403guy

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Posted 27 March 2020 - 08:20 AM

Hey TA owners,

 

Does anyone have any info and pics on the rear defroster wiring? 

 

thanks



#2 sea dog

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Posted 27 March 2020 - 10:50 AM

I consulted firebird wiring diagrams in 78 service manual. They did not show rear defrost wiring.

 

However, let me know what parts you have to work with and I can make a simple wiring diagram for your rear defrost. The standard set up would include the switch, a relay, and the defroster grid.

 

From what I understand, standard factory system for 78 included a timer to automatically shut off the grid after a certain time period. I can't help you with the timer part, but I can make a wiring diagram of the type system that employs switch, relay, & grid. Let me know if you wish this.



#3 78TA403guy

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Posted 27 March 2020 - 12:28 PM

Thanks for the reply. I have the rear defroster switch which is powered (green light comes on). I have the relay.



#4 sea dog

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Posted 27 March 2020 - 01:20 PM

OK, I'll get that wiring diagram made for you in the next couple of days.

 

By the way, when I design a system with a relay, on-off switch will have no power wire going to it. Instead, it will be the ground for the coil part of relay. When switch is turned on, the needed ground to relay's coil part is provided. This causes magnetic field that will close relay points. And provide power to grid.

 

I will draw a complete diagram, but basically, 1 contact of switch is wired to good ground. 2nd contact of switch goes to 1st contact of relay coil. 2nd contact of relay coil gets either live or key on power.

 

1st contact of relay points gets either live or key on power. 2nd contact of relay points goes to 1st contact of defrost grid. 2nd contact of defrost grid goes to ground.

 

You will need a Bosch type relay  with standard relay terminal code contact 85 & 86 are for coil part of relay. Contact 30 & 87 are the relay points.

 

Bosch type 30 amp relays are sold at most every auto parts store. For the grounds in system, except for defrost grid ground I suggest black wire of 16 gauge. For ground at defrost grid,14 gauge wire.For power wires to relay , I suggest red wire of 14 gauge. For power wire from relay to defrost grid, red wire again. But because of the length of wire from relay points to defrost grid, I suggest 12 gauge wire.



#5 sea dog

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Posted 29 March 2020 - 06:07 AM

Here's your defrost diagram.

 

The switch is SPST = single pole single throw. Switch will have only 2 terminals and only off- on positions.

 

Terminal 85 of relay goes to a key on only power source. One that's able to supply around 5 amps.

 

Terminal 30 of relaygoes to a live all the time power source. Battery will work just fine. Also this wire needs an inline fuse. Start with a 20 amp fuse. Because the relay coil is only powered wuith key on, defroster will shut down anytime key is turned off. Even if switch is left on. This is for safety.

 

When you wire up and test defrost try not to leave on for more than 5 minutes. This is because as wiring & defrost grid heat up, more power gets drawn than normal. That's why factory systems have a timer in the circuit.

 

If in the future, you want a timer, you would install it between key on power source and terminal 85 of rely.

 

Since switch & relay will be a distance from defrost grid, run wire to grid under kick panels & carpet.

 

Any questions, feel free to ask.

Attached Files



#6 78TA403guy

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Posted 03 April 2020 - 08:55 AM

Great! Thanks



#7 78TA403guy

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Posted 03 April 2020 - 09:08 AM

All,

 

Please post pics of what the rear defrost cable looks like if ya like for others. Any details here on wiring is welcome. Thanks



#8 Aus78Formula

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Posted 04 April 2020 - 06:17 AM

49735056847_d3a029710a_c.jpgFirebird 1978 Accessories Wiring Defogger demister by Aus78Formula, on Flickr






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