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  1. #1

    Cooling fans won't come on

    Cooling fans won’t come on

    96 Cutlass Supreme SL 3.1 auto (only 37K miles)

    With the car in motion, the temp needle would point to about 1/3 of the way up, just above the “C” mark. But if idling in traffic or at a stoplight, the temp would creep upwards. The highest I let it get to was just below the “red” zone, right at the letter ”H”.
    I checked and the radiator cooling fans never came on. If I got the car moving again the temp would come back down to where it should be. If I turned on the A/C then ONE of the radiator fans would come on briefly. This would cool the engine a little but not down to a point where I was comfortable with it. Also turning on the heat full blast would cool the engine somewhat but still not much. The only thing that would work every time to get the needle back down to 1/3 was to get the car moving again. I found a 3-wire switch/sender/sensor located near the thermostat, and if I unplugged that unit with the engine running, BOTH cooling fans will come on and the temp gage will go dead. Soon as I plug it back in, back to same problem.
    So, on the advice of numerous threads on this forum, two A&P (aircraft) mechanics, and two parts stores, I replaced the coolant temp switch/sensor, the one located right by the T-stat. Did not fix problem. Cooling fans still will NOT come on when the engine starts to overheat. In fact, the condition is worse! Now, when I drive the car at any speed above a crawl, the temp gage will creep up to about ¾ of the way to the “H”. If I slow down, coast or idle for a while it will cool off to about 2/3 of the way to hot. If I turn on the A/C, the ONE cooling fan will run briefly but not enough to cool the engine very much. I can drive around town if I run the heat full blast and the mode switch on Defrost (which runs the compressor and so therefore will engage the ONE cooling fan briefly), and the needle will stay at 2/3 to ¾ of the way to hot. I live in a small town far from any shops so I’m afraid to drive out on the highway for fear the engine will overheat and do some serious damage. I took the cooling fan relays out and had them tested by AutoZone and the A&P mechanic. He said they are good. As a test I swapped them around, and swapped them each with the IGN relay which has same part number on it, no change. With the new coolant switch, I can still get BOTH fans to run by unplugging it, same as before. I have opened the bleed screw several times with the engine running and heat on, to make sure there is no air trapped in the cooling system but this has not helped at any time either. I can not find any fuses or circuit breakers that are blown or tripped. Put car back together and carefully drove to Autozone, because their answer was “there must be another sensor somewhere”. Three of their guys looked under hood and could not find “another sensor”. There is a switch or sensor located on the side of the radiator, but it is for coolant LEVEL, not temperature.
    Various posts on this forum and others say the switch/sender/sensor I changed out is the only one that sends signal to the ECM to turn the cooling fans on. RockAuto has a listing for both a cooling temp “sender” and cooling temp “sensor” for this vehicle. Under the listing for “sensor” it shows the 3-wire unit that I have already replaced. Under “sender” it shows a choice of either the same 3-wire unit OR a 1-wire unit. Does not tell where that 1-wire unit is located. Autozone says there is no 1-wire cooling system sender or sensor applicable to this vehicle.
    Please someone help me! The car is really nice but if it is not going to be reliable then I will have to sell it as-is or for scrap.
    Thank You!

  2. #2
    Night Fury's Avatar
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    Re: Cooling fans won't come on

    Wish I could help, the car sounds NICE! I claim first dibs!

    EDIT: You said you have seen them both run, correct? As in the fans themselves are good?
    -Buck W.
    ****1996 Chevrolet Silverado Z71, 4x4 5.7 V8, auto, 167,000 miles****

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    ^^^Bi-polar bitch. The epitome of the phrase "A GM will run poorly longer than most cars run at all"

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    Re: Cooling fans won't come on

    There is a 1-wire coolant temp sensor in the rear cylinder head, on the driver's side of the engine. I'm not sure what it would be on your car, but if I remember correctly, on my GPs, it is a green wire. Hope that helps.
    2008 Ford Ranger XLT- 2.3 DOHC I-4 5-speed manual
    2010 Nissan Altima SR Coupe- 3.5 V6 6-speed manual

  4. #4

    Re: Cooling fans won't come on

    Quote Originally Posted by TGP Nick
    There is a 1-wire coolant temp sensor in the rear cylinder head, on the driver's side of the engine. I'm not sure what it would be on your car, but if I remember correctly, on my GPs, it is a green wire.  Hope that helps.
    with his being OBD2 (96) he would only have the one sensor (hence the three wires).

    Have you had this problem since you owned the car, or just recently did this start?

    VenomousVert
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    Re: Cooling fans won't come on

    Nice catch, I didn't look at the year.
    2008 Ford Ranger XLT- 2.3 DOHC I-4 5-speed manual
    2010 Nissan Altima SR Coupe- 3.5 V6 6-speed manual

  6. #6

    Re: Cooling fans won't come on

    Thanks for all the replies! It's a fairly recent problem. The car obviously doesn't get driven much. But I drove it quite a bit last summer, and never had any issues with the engine temperature...it always ran cool, even at idle with the A/C on full blast on 100 degree days. The first time I had any hint of overheating was in the McDonald's drive-up on a fairly mild day a few weeks ago. I have checked both before and after changing that coolant temp sender, BOTH fans WILL run if you unplug it. Thanks again for helping!


  7. #7

    Re: Cooling fans won't come on

    I'm still driving the car, but only for short trips and when it's cold outside. I never let the temp needle get above 3/4 of the way to Hot. But I did notice that even when the car is running hot, the radiator cap is only warm, not burning hot like you'd expect it to be. Same with the upper radiator hose, you can keep your hand on it while the engine is running. Could it be that the t-stat is not opening? Today I started the engine cold with the radiator cap off. Then fast-idled the engine till the gage showed about 3/4 to Hot. Should'nt I have seen the coolant moving in the radiator at some time? Back in the old days when cars were simple (yes I am a very old guy), if I even thought a t-stat was bad I'd just replace it because it was easy and cheap. Well on this car it looks REALLY hard...all that throttle body and crap has to come off and then be put back the right way! Is there any other way to check the t-stat operation? The heat works fine. Thanks!

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    Re: Cooling fans won't come on

    If your upper hose is only warm, and not hot, then it sounds like you have a plug somewhere. Most likely, the sensor that sends the signal to the PCM for the fans is probably seeing a cooler temperature, since there is a plug.

    I'd venture to say it is the T-Stat. It's cheap enough, and easy enough, to fix that it is worth a shot. That, or pull out the current one and toss it in boiling water, to see if it ever opens.
    Chris D.-
    '02 Buick Century- Wife's
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    Re: Cooling fans won't come on

    Did you ever get this fixed?? My 1997 Grand Prix 3.4 is doing the same thing.
    2007 GMC 2500 HD duramax (LBZ)
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    Re: Cooling fans won't come on

    A sender with the single green wire is for the guage and has no bearing on this. The coolant temp sensor should have 2 wires, I believe yellow and black. The black wire is a signal ground to the PCM, and the yellow is signal to the PCM. Some vehicles had both senders in one 3 wire unit, and yours probably does from what you said about the gauge going dead when you unplug the ECT. But it may have the single green wire guy to drive an idiot light. The PCM takes the data and decides via its program when to turn on the fans. The "primary" fan relay coil is grounded by the PCM through the green wire that runs to it at 195 degrees, the same temp as the factory t-stat. The "secondary" fan relay coil is grounded by the PCM through the blue wire that runs to it at I think 210 degrees, or when the A/C is engaged. If the PCM turns the fans on when the ECT is unplugged then the fans are good, and the relays are good. Do a good visual on the plug to that sender and make sure it's not corroded in the pins, then nick the yellow and black wires and test continuity to the plug to make sure the crimp in the plug isn't getting crappy. The ECT is just a variable resistance and if corrosion is adding resistance the PCM thinks it's running cooler than it is. If that's good you can test continuity all the way back to the PCM or it could be a bad PCM, but I'd hot wire those relays to ground and ignition power and call it a day before trying a new PCM. In fact I did, mine is running now that way until I get a new tune in my PCM to lower the fan temps since I ran a cooler t-stat. You'll get a code for each fan relay when you cut the wire but if you solder a 1K resistor from power the the control wire the PCM will still think it sees the relay.
    "She may not look like much, but she's got it where it counts, kid. " - Han Solo, Star Wars (1977)

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