Ex-Ford guy

Hello again im back, not that you noticed I was gone, but anyways...'87 2.8L I cannot diagnose the freakin fan problem. The fan does come on EVENTUALLY but at about 240-250 deg. It used to kick on at about 210-220, then shut off at about 200. So I went and bought a 160 thermostat, still nothing, replaced sensor, still nothing. So I hooked up a pain in the rear manual switch. Relay tests out fine. Now with the switch, I either forget to turn it on and stuff starts boiling or I leave it on all the time and I dont get anymore than 140-150 degrees out of it. An electrician, but car electrical still befuddles me . HM. Thanks

Rustydawg

That does sound mysterious.

I have a theory that might be worth checking out.

The temperature sensor that the ECM uses for fuel/spark mapping goes lower in resistance as it heats up. If you had a poor connection on this sender the ECM would mistake the higher resistance for a cooler engine. Perhaps try cleaning up the contacts thoroughly and checking for corrosion.

It isn't likely that the sender has gone bad, a crummy connection is more likely.

It is also possible that the wiring between the ECM and the coolant temp sensor has been damaged, any signs of this?

Best of luck...

Ex-Ford guy

I highly doubt the wires are damaged I havent found a bad wire on this car yet, but hey anything is possible. Where is this sensor at? I probably looked at it a million times but just couldnt put a name to it. Thanks, Mike

Rustydawg

Well, you know I pulled an '86 MPI V6 apart and kept all the sensors from it (I believe they are standard for V8 TPI etc) but I don't remember where they were. I seem to recall they were in the intake manifold somewhere, not in the cylinder heads.

One of them was marked with a temperature like 230 F and it was quite a large, beefy thing. I'll take a look through the parts I kept from that experience.

I thought that the computer's ECM turned the fan on and off depending on the coolant termperature that it 'saw' from the sensor, but the fan may in fact have it's own switch - anyone know that? I'm starting to think that the fan does have it's own. Maybe the Haynes electrical diagrams show it. I'll take a look.

Which sensor did you replace then and where was it located?

Danno

According to my diagrams the 2.8L fan is controlled strictly by the ECM. All GM cars with electric fans typically turn around 225/230 degrees. Unless the ECM has altered it's Prom setting(impossible) you have another issue. The CTS does not directly control your fan turn on temp so replacing it was a waste of time and your scratch. I would suggest you have a coolant system pressure problem. My first thought was a guage acting erratically but since you mention it DOES boil over then forget that. However it should not boil over at 240 with a 50/50 coolant-water mix. I would pressure test the system first. If in fact you have something as silly as a bad radiator cap the boiling point of the coolant drops dramatically. Also without pressure the coolant will develop hot-spots so the CTS may see one temp and the rest of the system is at another. Any coolant in the oil? 2.8's love head gaskets. When you bypassed the relay control with a manual sw did you control the coil or jump the high current side? Based on your backround I am betting you did do the coil side of the relay. Just a joke but I know you guy's just LOVE switchgears. One question, the temp sensor you replaced, did it have one wire or 2? I am assuming MR. Chilton is correct with his print and it had 2 wires. All GM fan switches have only one wire, ground to turn on the relay is through the block. I would do some more looking, you gotta have something else going on.

Ex-Ford guy

Well things aint bad around here now that the temp is down to about 20. And lots of snow so the old Ford trucks are gettin broken out. But I have been driving the car all week letting the fan run its course on its own and everything went fine. As for my switch, I tapped into the ground part of the relay dunno what color, made sure not to run hot all the way into the car and back me not dat stoopid. LoL. But still my fan hasnt been kicking on til 240, but no boiling or anything, plenty of pressure too. Dunno what the hell to say. The senson I changed was in fact the fan sensor, the left of the 2 sensors on the intake manifold, just to the right of the TB. Tested temp sensor on the head to see if it was making my gauge dumb, but that checked out ok. Head gaskets are fine, all thats in the oil is oil, made sure of that. I'll just let it go and see what happens. Thanks for your help.

sl2_99

hey what did you ever come up with i also have the same problem with my 88 2.8, i went to get a fan switch no luck the 2.8 doesnt have one i is controled by the ecm from the coolant temp sensor or a/c pressure switch. havent changed anything yet and dont want to take control of it myself i want to fix it.