I have an '89 Formula 305 with dual electric fans. The fans have recently been coming on at various times they shouldn't. For example when I start it in the morning they kick on and when it is running at normal temp they will kick on. They will turn off so they aren't stuck on all of the time. Could it be a bad temp sensor? This is with the AC off.

 

The fans are controllled by the ECM (or AC pressure) using a connection to earth or chassis common, if the action is random it sounds like you may have the earth control wire faulting to the chassis. This could be hard to find but check for loose or wire damage near the the fan control relays and AC pressure switch.

 

Could be the temp sensor. Mine have always come on during cold start. If you have a scan tool you can see what the Coolant temp sensor says. Yours should have dual fans I think. If the left one comes on at wierd times, it could be the Coolant temp sensor in the intake. If the pass side one comes on it could be the sensor in the pass side head. Or a grounding wire on the exhaust manifold.

 

where are the fan relays and the ac pressure switch located? I noticed an electrical connector on the top of the compressor and the wires coming out of it are taped up.
And both fans are coming on so I guess it is not the temp sensor? And is there more than one temp sensor? One for the temp gauge and one for the ecm? Or do they both use one?

 

I only know about my 88 IROC...the AC pressure switch is mounted on the filter-accumulator (silver football) opposite #6 cylinder. The (2) relays are front-left close to the battery when facing front of car. I think that there is only the one temp sensor used by both the gauge and ECM so if the gauge is steady..?
Wiring, have a look at Http://www.thirdgen.org/techbb2/show...readid=161229.
I know my car had been reworked with one of the relay wedged close with a bit of plastic and the AC pressure switch had been shorted across the switch. A temporary fault is the worse kind, when searching for an open or short wire I normally disconnect the battery and attach a ohm meter to the suspect circuit and move the wires about trying to isolate the damage area. Hope this helps.

 

The ECM gets its temperature readings from the sensor in the front of the intake. The temp gauge gets its readings from a sensor in the drivers' side cylinder head, between cylinders 1 and 3. The sensor for the gauge has nothing to do with the fans turning on or off.

 

The temp gauge is working fine, but it sure acts like the ECM is getting a false temp reading. I think I will try replacing the temp sensor in the intake. Thanks

 

IIRC, the ECM only controls the primary cooling fan. There is an additional temperature switch in the LH head to control the secondary cooling fan.

Additionally, if you suspect the CTS, don't simply replace it (unless you need to drain/flush the cooling system anyway), but test the sensor resistance at a given temperature. No sense in replacing good parts.

 

So at what time should both fans be running? When it is hotter than 220 and the ac is on? Because when they come on, it is both of them.

 

The Chilton manual shows the standard fan is controlled only by the ECU which reads temp from engine resistive sensor. The Aux. fan is controlled by (3) a temp switch, ECU and AC pressure switch. Normally the standard fan would come on first unless you have the AC on in which case they both could be on....this depends on the AC pressure cycle. The Aux. fan temp switch and ECU setting would I believe both only come on at high temps. In other words the only time both fans would be on together under normal conditions would be with the AC on and even then....they would not necessary start (switched) at the same time.
Note, when or if you ever change to the newer AC refrigerate the AC pressure switch will need to be reset as the new stuff runs at a higher pressure. Have you had any work done recently to the AC system?