Overheating. No High Speed Fan

[Travis Belair]

This car had the fuse box/wires melting like many have had. It was fixed with two relays outside the fuse box on the inner fender. It has had issues still with no high speed fan. I replaced the fan with a GT500 as I plan to autocross and race it. I thought maybe the computer isn't sending the signal to ground the relay and close it to run the fan. So I temporarily shorted the #85 wire to ground and the relay clicked close and fan started.

Well yesterday it got hot. I pulled over and confirmed the fan wasn't on. I unplugged the low speed relay and shorted the #85 wire to ground to close the relay and start the fan. I could hear and feel the relay closing and clicking. But the fan still would not turn on. I even swapped the low and high speed relays and tried again. Same thing, I hear it click and can feel click but still no fan. So the only thing would be the wiring between the relays and the plug that the fan harness plugged in.

Any thoughts? Im at a loss.

 

[Pentalab]

This car had the fuse box/wires melting like many have had. It was fixed with two relays outside the fuse box on the inner fender. It has had issues still with no high speed fan. I replaced the fan with a GT500 as I plan to autocross and race it. I thought maybe the computer isn't sending the signal to ground the relay and close it to run the fan. So I temporarily shorted the #85 wire to ground and the relay clicked close and fan started.

Well yesterday it got hot. I pulled over and confirmed the fan wasn't on. I unplugged the low speed relay and shorted the #85 wire to ground to close the relay and start the fan. I could hear and feel the relay closing and clicking. But the fan still would not turn on. I even swapped the low and high speed relays and tried again. Same thing, I hear it click and can feel click but still no fan. So the only thing would be the wiring between the relays and the plug that the fan harness plugged in.

Any thoughts? I'm at a loss.

They use a resistor to reduce the voltage to the fan...in low speed mode. On high speed mode, a relay shorts out the resistor, applying the full 14.75 vdc to the fan.

IE: relay #1 applies 14.75vdc to the fan. Fan is now in low speed mode, with reduced voltage.
Relay #2 shorts out the resistor, and full voltage is applied to the fan..... " high speed".

IF the resistor has gone OPEN, your low speed setting on the fan will not work. BUT the high speed function will still work, since relay #2 is now just shorting out the bad ( open) resistor.

You have either an open resistor, bad relay coils, or bad or shorted, or grnded wiring. You can see if 14.75vdc ( or less) is present with a digital volt meter. You could also have a bad fan, with an open winding. You could also temp apply 14.75 vdc directly to the fan itself, to see if the fan works on high speed. Although one or both relays operate, one or both contacts on the relay may be bad. There is only so much to go wrong in that fan circuit.

 

[Travis Belair]

Here's the thing I can plug my OBD scanner in and manually turn the high or low speed on and they come on prospectively, so the circuit is complete. Sometimes when the car gets hot it wont turn on whether the AC is on or not. I guess I left out some details. It there anything in the circuit, like that resister, that would get hot and not work. The fan is new from FRPP.

 

[86GT351]

What year and engine?

 

[SVT Rider]

Likely the resistor inside the fan plug is melted loose. Happened on my Low Fan circuit. The plug is modular on the fan so you can pull it out and inspect it.

 

[DieHarder]

Troubleshoot per Pentalab's instructions. However, the reason the wires are melting is high current. The reason for high current is higher resistance than normal. Higher resistance can only come from an old fan motor; corrosion/resistance in the wiring/connectors or poor/bad grounds/relays.

Since you just added a GT500 fan (new?) it's unlikely that's the issue. So if I was troubleshooting I'd trace the circuit and perform continuity checks on all wiring noting resistance on each of them. Clean all wiring terminals/grounds best you can and look for signs of any corrosion and again clean those connections and measure continuity both end-to-end and ground. Test again. Any improvement?

Since you can command the fans on with your OBD scanner (would like to know what model?) the circuit is working but not under normal conditions. (Once the engine is warm and the fan isn't working can you still command it on? If so, maybe PCM/heat related). If you have a VOM with current capability measure the lines to the fan noting current readings both cold and warm as it seems part of your problem is heat related. Note: the wiring in our cars is pretty old now and at least in my case I've seen lower voltages out of my alternator; not due to the alternator but due to higher resistance in the cabling/grounds. For instance I just replaced my main battery (to starter) cables and my alternator output went from 13.3v to 14.0v meaning that cabling/grounds were a problem for me. ($50 well spent).

If above doesn't help... If you can identify where the lines are heating up/have highest current you can try sistering or adding a wire/cable of the same size (solder them) to the primary positive and negative lines (one for positive; one for negative). This will spread the current/heat load across (what the circuit sees as) larger conductors and help negate any corrosion issues. Kind of a last resort fix but in my own case adding a separate ground for my alternator increased my voltage by a volt meaning I had some bad grounds (which I'm still working on). For temp checks pick up one of those point/shoot IR models at harbor freight. Have a couple of them; work great.

 

[Travis Belair]

Sorry for delay. Life and work and stuff.

The fan is brand new. The scanner is a bosch and only like a $500 model. I tried to command it on when it was hot recently with no response, High or low. As I stated I just got the car. The previous owner said it was the PCM and I verified that with the mechanic who rewired the relays for High and Low.

I think that resistor could be the issue or the PCM. Only two things I can think of left.

Since you just added a GT500 fan (new?) it's unlikely that's the issue. So if I was troubleshooting I'd trace the circuit and perform continuity checks on all wiring noting resistance on each of them. Clean all wiring terminals/grounds best you can and look for signs of any corrosion and again clean those connections and measure continuity both end-to-end and ground. Test again. Any improvement?

Since you can command the fans on with your OBD scanner (would like to know what model?) the circuit is working but not under normal conditions. (Once the engine is warm and the fan isn't working can you still command it on? If so, maybe PCM/heat related). If you have a VOM with current capability measure the lines to the fan noting current readings both cold and warm as it seems part of your problem is heat related. Note: the wiring in our cars is pretty old now and at least in my case I've seen lower voltages out of my alternator; not due to the alternator but due to higher resistance in the cabling/grounds. For instance I just replaced my main battery (to starter) cables and my alternator output went from 13.3v to 14.0v meaning that cabling/grounds were a problem for me. ($50 well spent).

 

[TexasBlownV8]

What relays were put in? They must be able to handle high current. I've melted 30A external relays just on low speed. Just have to figure out how you want to run/trigger the relays. And as you've seen, the relay-side is wired-hot, and the pcm grounds the other side to trigger the relay.
It's a very simple circuit to figure out.
Good choice on the gt500 fan.