I'll try keep this short, but want to include as much detail as possible. I've been fighting this for 2 days straight.

Background: Car is an '86, just swapped in '92 engine/dual fans/wiring harness firewall forward. I did redo the C100 connector. The secondary fan always runs with key on, I removed the A/C a while ago and since I Autocross, I just left it always on. A few weeks ago at the second autocross after the swap, the fuse blew. It had a 10amp fuse since the car previously only had a single fan. We guessed that when the temp got up to kick on the primary fan and since the secondary fan was already running, the fuse was overloaded. Replaced with proper 20amp fuse. Last event, same thing happened, I'ld replace the fuse, secondary fan would run, I'ld make a lap and the fuse would be blown. This weekend I'm trying to solve the problem. Yesterday, I replaced the 20amp fuse, turned power on, fuse immediatly blows. Repeat with 30amp, same thing. Now I'm thinking there is a short.

Now to find the location of the short. The wiring as best as I can follow is like this... From fuse, splits to C100 (Firewall) and heater relay. From C100 splits to both fan relays and generator.

Power on, I use a test light across the fuse. Glows as I'ld expect as the secondary fan relay is grounded. I disconnect all loads I can follow... Both fan relays, generator, heater relay. Test light across fuse, still glows. Disconnect C100. Test light across fuse. No glow. Ok, so the short is forward of the firewall, right? I then connect test light under hood to power, probe generator connector, primary and secondary fan relay connectors, no glow. Probe G8 on C100 engine side, no glow. Probe G8 on C100 on firewall, no glow. This says to me the short is inside the car. I'm at a loss of what I can do now.

Ideas?

Thanks

 

The 10A fuse should be more than sufficient for the fan circuit. The fuse only protects the primary power (tan/white wire) for the fan relays, not the actual fan motor power. The fan motors are protected by fuse links direct from the battery positive side. One thing to consider is that many relays have a diode across the relay coil to kill inductive "kick" when the relay is de-energized, and if the polarity of the relay is not observed, the diode can conduct to ground when power is applied.