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Cooling fans wont come on

58211 Views 19 Replies 6 Participants Last post by  ryan22soy
I'm working on my son's Malibu. It's a 97, 3.1L, auto, 125,000 miles.

The thermostat is opening when it should, the water pump is pushing water, the coolant temp sensor was just replaced and has proper Ohm reading at cold and hot temps. All fuses are good and all relays test good too. The fans come on if I unplug the coolant temp sensor (although the car runs like crap with it unplugged) If I jump the Fan 1(location #12 on the fuse box) relay with a fused jumper wire from pin 30 to 87 both fans come on (seems like low speed). If I jump fan 2 (Location 14) from pin 30 to 87 passenger fan comes on (seems like high speed).

So the temp sensor works, all the fuses and all the relays are good, and the fans themselves work as well.

When I turn AC on and blower on high the fans do not come on. But that may be because previous owner removed compressor and put in an AC bypass pulley. Not sure about that...

Haynes says where the relays plug in the fuse box I should have constant power to pin 30. And I should have power to pin 85 w/key on. I do have power at both pin 30's, but don't get power to pin 85 with the key on, with engine running, or with engine running above normal op temp.

I do however have power to pin 86 for fan 1 with car on or off, and I do have power to pin 86 and 87 on fan 2 with car on or off. Which doesn't seem right.

Extra info, it has a new expansion tank cap, system is 50/50 coolant water, all air has been bleed out through the bleeder valve. Has good vacuum, good fuel pressure, full oil (not milky) do not suspect head gasket or lower intake manifold gasket. No coolant leaks either.

From the beginning of the overheating story: the car was overheating and overflowing from under the cap, my son kept adding water/coolant but never bled the air out. It kept getting hot, (never got to red because I told him these cars blow gaskets easy and to just pull over) The cap was bad so we bought a new one, refilled and bled the air out. Then the car held temp right at 1/2 line or just above at red lights. Indicating a fan issue. Since fans came on with sensor unplugged and relays and fuses tested good, we put in a new temp sensor. But still no fans. That's when I started rechecking relays and power at the pin locations in the fuse box and got some odd reading that didn't add up.

I'm really hoping there is something else that would cause this besides the PCM, is there anything else I can test, try, replace, etc besides the PCM? Is there another sensor besides the coolant temp, fuse, or relay besides what's in the under hood fuse box that might be keeping these fans from switching on?

Thanks in advance, I've already check everything the haynes manual says to check, but it doesn't say what to do if pin 85 doesn't have power with key on. I've read lots of overheating threads online and seems like most or from loosing coolant, not bleeding air out, bad relays or bad fans. I have none of that and am kinda stuck. Thanks in advance for any help/ideas!
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I'm not real familiar with the BCM and all it controls. I know it is the source of some no start issues and anti theft or remote key issues. I'll have to do a little more research I guess.
Good point though, just because the coolant temp sensor is producing the right resistance for various temps, it doesn't mean the BCM is reading it right. I wish I had the sftw and ALDL-USB connector. It would have come it handy so many times. I guess I'll just have to take it to a shop with with the right equipment to get live feedback.
I know the OBDII scan doesn't tell much, but I did take it up to Autozone and get the engine codes read, and only got the one for the temp sensor, but that was because he was driving it for a day with the (new) sensor unplugged. He was trying to keep the fans on, but realized it ran bad like that. I left the battery unplugged overnight which erased the code and it has not come back, and the car runs really well, accelerates great, shifts smooth, again.
Anyway, good to know the PCM may be OK, looks like a BCM is a little cheaper. Can it be reprogrammed in order to fix it, or if it needs to be replaced does it need to be programed like a PCM? Sorry, just not sure on the function or process of fixing a BCM malfunction. I'll have to get it scanned and see what temp is actually being read and see if it's actually sending the signal to turn the fans on or not before I get ahead of myself here.
Thanks for the reply, let me know if there is anything I forgot too.
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Well my sons car has continued to overheat. The fans do come on sometimes, sometimes they don't. He's still adding water and bleeding air out daily. He also got to check engine code pop up, P0300 random cylinder misfire, and P1651 PCM fault (quad driver 2) or Cooling fan relay 1. So we took it up to a shop and put it on a diagnostic computer. Results were PCM was reading the right temp, fans came on when they were supposed to when in the shop( but don't always), and tech said, it's probably the intake gasket and/or the head gasket. He suspected problems with the heads or head gaskets because of the random cylinder misfire.
So I put a radiator pressure tester on it. He had a good steady drip coming off the transmission pan from underneath. All related hoses and coolant connections above that area were not leaking. I finally found the leak coming from the side of the block between the throttle body and EGR area. It's kind of dirty and hard to see, it could be just the intake on the side there where they just used silicone (most likely) or could be the head gasket too. He's ran it hot several times and could be either or both really. At this point the repairs are more extensive than the value of the car, and he's looking for something else, and probably just sell this one for parts. Which is sad, because he's put 8-900 in new parts in it over the past year. It's got dents and faded paint, a rough interior and no AC. So he doesn't want to waste any more money or time into it.
Not the greatest news, but figured I'd do a follow up for anyone else having the same type of problems. Thanks everyone.
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