2.8 MFI weird cooling fan behavior

[AmericanLocomotive]

Hey guys, working on an '89 Olds Cutlas Ciera for a friend. Yeah, I know it's not a W-body, but I figure the PCM and other engine controls must be similar enough, right? It's a 2.8 MFI engine with a single, single-speed cooling fan controlled by the PCM.

 

Anyways, my buddy calls me up and tells me his car is overheating and puking coolant. He gets it to my house, I carefully removed the radiator cap once it cooled, stuck a digital thermocouple down into the tank and started the engine. After a couple minutes the thermostat opens and coolant starts flowing, and the temperature starts going up. 190 - 200 - 210 - 220 - 230. By the time it hit 230 the coolant was starting to boil pretty aggressively and still no cooling fan. We let the engine cool again and I started doing some diagnostics.

 

I unplugged the fan relay, and jumped the heavy gauge wires directly and the fan started and ran fine. I then jumpered A and B together on the ALDL port, turned the key on and the cooling fan turned on as it should. So the wiring to the computer and the relay is good, the relay is good and the PCM can control the fan.

 

Next I started the car and turned the A/C on. The cooling fan did not start - which it should whenever the compressor is running. I know the computer is receiving the "A/C request" signal, because in this car the A/C compressor is controlled by the PCM, and the compressor was running. We tried replacing the coolant temperature sensor as well, but it yielded no change in operation.

 

However something very interesting happened as I was poking around checking things. After a couple of minutes of running with the A/C on - the cooling fan started, and then turned off shortly after. I stuck my thermocouple into a radiator fin and watched. The radiator would hit around ~200-204*F, and the cooling fan would switch on. Once it dropped to around 190*F the fan would switch off, like it should normally. These temperatures will be a little lower than what is seen at the engine since I just had the thermocouple lightly wedged in a radiator fin.

 

I turned the A/C off, and sure enough the fan stopped cycling and the coolant temperature swung way up to 220+ again. Turned the A/C back on and the fan started cycling normally again.

 

So to sum up:

All wiring to fan, relay, pcm checks out

Temp sensor replaced

Jumping A/B on ALDL causes fan to activate as it should

Fan does not come on at all with A/C off

Fan activates to normally to regulate engine temperature when you turn A/C on, but does not run constantly with A/C on as it should

 

So at this point I'm thinking something has failed in the PCM, and it needs to be replaced? Any other ideas?

[Galaxie500XL]

Are you absolutely certain there's no air bubbles in the engine, preventing the coolant sensor from being in contact with the coolant, or an issue with the wiring associated with the coolant temperature sensor itself? That could cause the sort of behavior you're describing. A scantool would be very useful, as you could then read what the ECM thinks the coolant temperature is.

[AmericanLocomotive]

We bled the system, and as long as you have the AC on, the fan cycles and nothing overheats at all.

 

I unfortunately don't have an ALDL scan tool or know anyone with one

[RobertISaar]

fan on-temp for no a/c is 226, off at 219.

with a/c, drops down to 223, 215.

there is a minimum fan-on time of 25.4 seconds.

 

the way the fan is controlled for a/c usage is kind of odd. there are 3 separate switches that effect a/c operation:

 

a low-pressure cutoff(prevents compressor operation below 8PSI)

a high-pressure cutoff(disables compressor at 430PSI, doesn't allow it to turn back on until 200PSI)

a fan-request switch(informs the ECM that a/c pressure is above 320PSI)

 

so, to get the fan to come on from a/c usage alone, you need to have a/c high-side pressure above 320PSI. the ECM manages the operation, so it would appear that the ECM is capable of controlling the fan(diagnostic mode also confirmed this).

 

your ECM is either not getting a good coolant temp signal(potential wiring problem) or it is failing internally.

 

you may find this helpful.

 

http://imgur.com/dHdgH6q

[AmericanLocomotive]

Thanks for the information. So this car does have two different fan engagement temperatures depending on fan on/fan off, which is interesting.

 

However in my case, with A/C on, the fan comes on at around 202*F in a radiator fin (actually coolant temp will be much higher of course). With A/C off, the temp in a fin hits around 230*F with no sign of the fan coming on, and at that point it starts pushing a lot of coolant into the overflow bottle. I'd wager the actually coolant temp must be around 240-245* at that point.

 

Do you know if there is any way to get a generic ELM327 OBD-II reader to communicate with the ALDL port with some creative wiring, or do I need to find some garage that happens to have some old GM diagnostics hardware laying around?

[RobertISaar]

OBD2 equipment will be of no use here. a scanner/ALDL cable needs to be built with GM OBD1 in mind for it to work. the communication protocol for ALDL is a bit out of the ordinary, so there aren't a lot of solutions that work. I've built a lot of cables, but I have a bit of a supply issue for the moment.

 

if you have a multimeter, I can suggest a few tests to try and rule out some causes.

[AmericanLocomotive]

I do have DMM that I can use

[RobertISaar]

unplug temp sensor and ECM, check for resistance from the temp sensor's yellow wire pin to the ECM's E16(yellow connector, should have small characters cast into the end of it for location) harness connector pin. should be at/near 0 ohms.

 

same procedure, but now with the temp sensor's purple wire pin and the ECM's B6(black 12-pin) harness connector pin. should also be at/near 0 ohms.

 

if either of those fail, you have a harness issue and you're down to the ECM. you can test it if you want, but it should test bad at this point.

 

on the ECM itself, the B6(black 12-pin) pin should be at/near 0 ohms with system ground pins(such as A12(black 12-pin)).

 

on the ECM itself, the E16(yellow) pin should be at/near 4000 ohms with pin A4(black 12-pin) if I'm looking at my diagrams correctly.

[AmericanLocomotive]

Checked everything and it measured out fine. I took the computer apart, cleaned the connections, removed the ROM and cleaned its connections. It *seems* to be working now with the fan coming on right at 226*F. We also installed a new overflow hose, put in fresh coolant and changed the radiator cap with a new one.

 

We'll monitor it over the next few days and see if it still pukes coolant or not

[RobertISaar]

hmm.... maybe it was just a semi-bad connection somewhere along the way then.

[Psych0matt]

Maybe somebody Peed on it causing corrosion

[RobertISaar]

eh, rodent perhaps. with the A-bodies, the ECM is located in-cabin rather than underhood and they're not weatherproofed the way W-body PCMs are.