1985 gmc radio questions
 

Hi,
I have a 1985 Sierra with the stock radio. It didn't work at first, turns out the fuse was blown so I replaced it and it worked. But when I drive the truck the fuse blows every time even if the radio is off. I am using a 15 amp fuse. Is this the correct size? Thanks

Re: 1985 gmc radio questions
 

Sounds like its grounding out somewhere. Most basic radios don't need more than 7 amp fuses.

Re: 1985 gmc radio questions
 

The weird thing is the fuse will only blow when driving in overdrive (700r4). Regular drive is fine. Any ideas?

Re: 1985 gmc radio questions
 

Yes, 15 amp is the correct size fuse for this circuit because it powers more than just the radio. This circuit also powers the 3 "ACC" (accessory) receptacles in the fuse box. Those are used to supply power to various options such as the 700R4 lock-up torque converter solenoid, 4 wheel drive indicator light, and auxiliary battery relay.

I can't find many details on the 700R4 wiring in the 1985 wiring diagrams but the 1986 diagrams have some info. It appears as though many of them have a switch inside the transmission that closes when the 4th clutch (overdrive) is engaged. That feeds power to the torque converter clutch solenoid which should have a resistance of 23.5Ω. So a short to ground in the wiring (internal to the trans) between that switch & the solenoid or a shorted solenoid are a couple possibilities that could cause the radio fuse to blow only when the trans is operated in overdrive.