I have an '86 305 TPI and I'm having problems with the cooling fans. They are always on when the engine is running. I know the car used to have A/C, but all that's left of the A/C system is the compressor, I don't have the pressure switch or tubing, etc. I have read that the fan request wire on the ECM can be grounded out if the pressure switch is not present, but won't that cause a short and damage the ECM or battery? The ECM is putting out 12 volts on the fan request wire that normally would go to the pressure switch (pin D11 on the ECM, blue wire) but I'm afraid if I ground it I'll damage something. Does there need to be some resistance between that wire and ground, or can I just put a wire across both ends of the pressure switch plug?

thanks for the help!

 

I do not much about the internal workings of ECM's, but if you ground out a hot wire coming from the ECM, it could cause a short circuit, and damage the ECM. If your fans are coming on with the ignition switch, and staying on, it sounds like something may be shorted out somewhere. I would check the fan switch wire, as that is a common problem area. Check the green wire, with a black stripe, to the fan switch usually located in the passenger side head, just above the starter, to see if it is burned off and touching metal. Also, a former owner may have grounded the fan switch wire, which will make the fans run with the ignition switch. The fact that your A/C is not in place should not have any effect, unless you jump a wire into the A/C pressure switch, then your fans will run all the time from that circuit. My knowledge is limited, however these are the first places to check for problems. Maybe someone else will add some helpfull information. Charles

 

The fans don't actually come on with the ignition switch, I can have the key in run without the fans running, they are on continuously while the engine is running, which leads me to believe it is the lack of an A/C pressure switch that is causing them to run. I looked for the fan switch wire you mentioned, but I didn't see anything other than the knock sensor near the starter on the passenger side, however I do have a single green wire going to a sensor on the drivers side right next to the exhaust manifold and the number 3 cylinder, is that my manifold air temp sensor? Is that what causes the fans to come on when it gets to a certain temp?

Thanks.

 

The wire on the driver's side in the exhaust manifold is the O2 sensor wire. It has nothing to do with the fans. The green wire on the drivers side head, in between the first and second spark plugs, is the temp gauge. Most cars have a fan switch located in the passenger side head, between the last two spark plugs and is usually green wire with a black stripe. Since that type of switch works by grounding, and it is so close to the exhaust manifold, it is common for the wire to burn off and touch the engine or chassis providing a ground to the ECM and causing the fans to run all of the time when the key is on. A few TPI cars have a fan switch located in the front of the intake manifold, just above the water pump. That type of switch has two wires instead of the usual one wire set-up. It works by providing a feedback to the ECM. Not sure how the A/C switch operates. It may be a feedback loop, or it may provide a ground. Based on the information you provided, 12 volts coming from the wire from the ECM to the A/C pressure switch, I am afraid if you ground it you could short out the ECM and damage it. Maybe someone else with more knowledge of the operation of the A/C pressure switch can shed some light on it. Charles

 

Does your A/C pressure switch have one wire or two on the plug? If it has only one wire, it probably works by grounding the circuit. If it has two, it is a feedback circuit. If it has two wires, you might try a jumper wire from one side to the other, and see it your fans operate correctly. Charles

 

The A/C pressure switch has two wires, dark blue and black, and the black is a ground. I checked the blue wire, it goes to pin D11 on the ECM, so I know it is the A/C pressure switch, I'm just not sure how the switch functions to make the ECM think the A/C is off? Also, I do have a couple of sensors on the front of the motor, above the water pump, one is a Cold-start sensor to power the 9th injector, and the other is a coolant temp sensor. I just replaced it recently but I'm not sure if it has anything to do with the fans. I've been using chevythunder.com as a reference and the coolant temp signal wire at the ECM is supposed to have 1.6 volts on it at normal operating temperature, but I noticed mine was a little above 2 volts, could this be part of the problem? Does a higher voltage there make the ECM think the engine is hotter and turn on the fans? If so, how would I lower the voltage, or is the CTS bad?

Thanks again for all the help!

 

I did an experiement with my car, as it is an 86 TPI also, but everything is intact and working. The engine was cold, so the fans were not getting power from the fan tempature sensor in the manifold. The fans run with the AC engaged. With the blue and black wires plugged into the AC pressure switch and the AC off, my fans are off. When the plug is removed from the switch, the fans turn on, even with the AC off. I placed a jumper wire between the blue and black wire, and the fans turned off. I think your original theory about the AC pressure switch being the problem is probably correct. Apparently the switch triggers the ECM to turn on the fans by interupting the power flow through the blue and black wires. When there is a connecton between the blue and black wire, the ECM calls for the fans be off. Hope this helps.

Charles

 

i had the same problem on 86 camaro when i took out my ac.. one fan would run after stat up. i put in a jumper wire over near bat. were the ac pressure switch was and all set.. one fan runs off the ecm and the other runs off the fan switch in the pass head. hope that helps you..

 

Don't want to confuse you, I forgot to mention that my car came from the factory with a single fan, allthough I do believe that the AC pressure switch works the same if it is a single fan car or a dual fan car. If you connect the blue and black wires going to the AC pressure switch, it should lock out the signal from the AC pressure switch to run the fans, and then it will rely on the cooling fan tempature sensor/switch/thermostate on the engine to control them.Charles

 

Fantastic! that seems to be the problem then. Just to be clear, you didn't have a resistor or anything between the blue and black wires, just a piece of wire, correct? Is your black wire on the pressure switch grounded? I guess I'll do that then, the ECM must sense a connection there and discontinue putting 12 volts on the blue wire, so it doesn't short out? Thank you for all the help with this!

 

I don't know if the black wire is a gound or not. I may be, then it may also be a feedback loop, or an isolated ground. I would not feel safe running the blue wire to a chassis ground. The only connection I made was from blue to black which is the same function that the pressure switch does when there is low pressure in the high pressure freon line.

Charles