Coolant Temp Light = Sensor?
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HT4100 4.1, 4.5, 4.9 Discussion, Coolant Temp Light = Sensor? in Cadillac Engine Technical Discussion; I have a 1985 Fleetwood Brougham FWD 4.1 Gas V8, and have been having coolant issues for a little while ...
  1. #1
    Dangerman is offline Cadillac Owners Member
    Automobile(s): 1985 Fleetwood Brougham FWD
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    Exclamation Coolant Temp Light = Sensor?

    I have a 1985 Fleetwood Brougham FWD 4.1 Gas V8, and have been having coolant issues for a little while now. I burst the top coolant hose from the radiator to the engine block and replaced it, after which I had no problems for a week or two. Now my Coolant Temp Light is flashing after driving 10-15 minutes. My coolant seems to be at normal levels as shown in the overflow tank, and I do not see/smell any leaks after driving or while parked. A friend suggested that my thermostat or temperature sensor may be bad, and not switching the flow of coolant correctly. I am not very mechanically inclined, but I am able to tackle most simple maintenance/repair tasks. I bought a thermostat and gasket, and a temp sensor, today, and plan on replacing these ASAP to see if that makes a difference.

    My question is, what else should I be looking for? Wiring issues? I was told that the wiring harness for the sensor is completely tied into the rest of the engine wiring and could not be replaced independently. Could a loose connection be to blame? Water Pump (I hope not...)? Am I barking up the right tree?

    I joined here when I bought the car, but have had no other iffy questions, despite some clear cut repairs I have had done (brakes, exhaust, vacuum pump and hoses). I am glad there is a forum to help me with this car, because I would love to hold on to it for a while, it is in good running order and the interior is to die for, all creamy white leather and great upholstery.

    Any help would be appreciated.

    Ben Provost

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  3. #2
    noahsdad's Avatar
    noahsdad is offline Cadillac Owners Connoisseur
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    Re: Coolant Temp Light = Sensor?

    Check your codes, watch for an E41. If you're getting a light but have no overheating or coolant loss, you may have a coolant fans problem, which will turn on the C-T-F telltale. If you have the code, it's usually a failed coolant fans module. On an '85, you'll find it behind the left headlight. About the size of a deck of cards, with finned aluminum heat sink on top.

  4. #3
    carnut is offline Cadillac Owners Master
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    Re: Coolant Temp Light = Sensor?

    It also could be simply not filled completely. If the radiator level is too low, it will not pull from the recovery tank. An air pocket will cause the coolant temp light to flash. Remove the cap with engine stone cold, have a buddy hold the idle at about 1500 rpm, then when the level drops because the thermostat opens, fill the radiator to the top and while holding the hi idle quickly reinstall the cap. Also have the climate control set to 90 degrees to help purge the air when you are filling.

  5. #4
    Dangerman is offline Cadillac Owners Member
    Automobile(s): 1985 Fleetwood Brougham FWD
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    Re: Coolant Temp Light = Sensor?

    I changed the thermostat and the temp sensor, which caused me some concern because it sat a little proud out of the block, but the new sensor had a 1/4 inch more threads than the old one, so maybe it isn't a problem. My coolant was a little low, but I am keeping a good eye on it, and considering a full flush anyhow. Is this something I can do myself, or should I go to my mechanic? What is too much to pay?

    My Idiot Light has turned off, so I hope it was a bad thermostat.

    Thanks to those who responded. I ran my codes and got what looked like 4-8-8. What's that mean?

    I wasn't able to locate the coolant fans module, maybe I wasn't looking hard enough. Are these units prone to failure, especially after 20-something years?

    Thanks for the input guys, and sorry for not posting in the correct forum. Thanks Mods for moving this thread.

    DM

  6. #5
    noahsdad's Avatar
    noahsdad is offline Cadillac Owners Connoisseur
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    Re: Coolant Temp Light = Sensor?

    Quote Originally Posted by Dangerman View Post

    Thanks to those who responded. I ran my codes and got what looked like 4-8-8. What's that mean?

    I wasn't able to locate the coolant fans module, maybe I wasn't looking hard enough. Are these units prone to failure, especially after 20-something years?

    DM
    Never seen that code sequence. Are you sure you hit the entry buttons correctly?

    Yes, the Coolant Fans module is a common failure item. If you trace the wires from the fans (easier from below) it will lead you right to it. But don't change it if you're not getting the code.

    You can do a search here for "Coolant Fans Override" for instructions on how to test the module and fans, but one simple way is to start the car and turn on the A/C. Both fans should run continuously with A/C on if all is well.

  7. #6
    Dangerman is offline Cadillac Owners Member
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    Re: Coolant Temp Light = Sensor?

    No, I'm not sure that I ran the codes right. I followed the instructions I found elsewhere on this forum, but the numbers (code) didn't stay up on the panel for more than a second or two. A new development is that my heater fan has stopped working. I am beginning to fear there is a bigger problem lurking in my heater/coolant system somewhere. I am driving it as little as possible until I figure this out. Its a good thing I have a nice road bike and it isn't too far to work.

    How much is a fan module, and can I replace it myself? Would this fix the fan failure?

    Could it be a problem with my heater core that is affecting the rest of my engine cooling system?

    This is my first Caddy, and I really love it, but if it is too much of a problem to upkeep myself, I am going to have to get rid of it. I don't have the space near my house to be constantly working on my car, and in a month or two we will have some snow here (NH), so driveway work becomes unpleasant. I will try and get a better reading for the codes, and bring that back to the table here later.

    Thanks for the help.

    DM

  8. #7
    noahsdad's Avatar
    noahsdad is offline Cadillac Owners Connoisseur
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    Re: Coolant Temp Light = Sensor?

    The code procedure is: Key on, hold OFF and WARMER simultaneously until all LEDs light. Release buttons. Fuel Data Center displays codes, starting with the E.XX series, followed by the F.XX series. Each code displays for about 3 seconds, so write them down. When the FDC shows .7.0, the code check is complete. Turn the key off or hit AUTO to exit diagnostics.

    Yes, you can do the fan module yourself. It's two bolts and an electrical connector. You do have to remove part of the headlight assembly to get to it, but it's fairly easy. I replaced mine with a junkyard part and it's working fine, but if you go that route, buy two in case you get a bad one - unless the junkyard will allow returns on electrical parts. BUT AGAIN, DONT REPLACE ANYTHING UNTIL YOU KNOW THE MODULE IS BAD.

    The heater core can cause troubles, but only if it's leaking or plugged. If you have heat, the core is probably okay.

    The HVAC fan is on a completely separate circuit from the cooling system. It also is controlled by a module that is also a fairly common failure item. Again, a junkyard part will often suffice. Check the blower fuse first.

    As far as how reliable of a car you own - it's a coin toss. A lot depends on miles and the maintenance it had. An '85 Cadillac is built of pretty good stuff, but it's still a 27 year old car and things will break. If you choose to keep it, do yourself a favor and buy a Factory Service Manual from eBay. Mine has paid for itself 100 times over.

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