Some of you may have seen a few of my threads concerning a fan problem I have had since picking up a 91 RS w/ 305. Discovered the first day I drove it home that the fan was not coming on when the car got hot enough. Not a problem. The car is 18 years old and has 144k on the clock. Not knowing what is going on and how the cooling system works I began asking questions here at thirdgen. People point me to the fan, the relay, the switch, fusable link etc.... While under the car I find the connector to the switch is broken (basically missing the plastic) and only the terminal is present. Until I can get a new connector I ground the terminal to the chassis and discover the fan comes on. That rules out a bad, fan or relay wiring, etc... Put the new connector on and plug it into the fan switch between the #6 and #8 cylinders. Car warms up approaches the red zone and the fan never starts. Shut down and come here for more help. Change the coolant fan switch out since it is the likely culprit and test again. Guess what doesn't work. Maybe i have a bad switch. Buy another one this time for a 87 GN (Lower temp range) and a throw it in along with a new lower temp stat. Guess what...still doesn't work. At this point I'm pretty much out of ideas. Everything related to the cooling system is good (unless I bought two bad coolant fan switches in a row), but the fan will not kick on. In the mean time I order a Hayden fan switch as suggested by the tech article here on thirdgen. It's a work around and should get the fans going. Up until now I have been told that the cooling system for a single fan setup is NOT ECM controlled. Since the switch and fan turning on is NOT ECM controlled I haven't considering other possible issues. Well yesterday I figured what the hell. Everything on this car is old and going bad here and there so why not change out the Coolant temp sensor in the intake manifold. It looks worse for wear. Buy a new one swap it out in 3 minutes and retest the cooling system. Run the car in the driveway. Temps pass 100, then, 160, then 220, damn no change then around 230 per the dash guage the fan kicks on!!! Wooohooo!!!!!! It would seem on a single fan setup for a 91 RS with the 305 the fan IS ECM controlled. Now that, that is resolved I would like to verify how far the dash gauge is off by. I read on here that there is a way to tap into the readings from the coolant temp sensor but I do not know how to do it. Can someone explain it please? Thanks.

you can read the coolant sensor with an ohm meter. 230 is too hot in my opinion, did you install a lower temp fan switch (195*)?

I installed the fan switch for an 87 GN. From what I read here it comes on 204 and cuts off at around 190. I want to check the actual coolant temp via the temp sensor because I don't trust the gauge. Maybe it's really coming on at around 200 and not 230. Ideally I would like the car to run right around 170 to 180 at all times. While moving and air passes over the rad the car sits right at the second white mark on the gauge.

I read your originally very long thread and something does not make since.

You said in your thread that you grounded out your green wire that connected to the temperature switch on the passenger side of the car, and your fan turned on.
If that is true, then the switch on that side of the car, if good, and if properly grounded and getting heated correctly should turn on the fan for sure.
It is a simple grounding switch, nothing complicated there.

To test if its getting good ground you just get an Ohm meter and go from the side of the sensor to the block, which will show you if you have a good ground.

If you have switched 2 sensors already good chances your switch is good, but it isn’t known for sure.

The other possibility is that coolant isn’t flowing correctly through the block, due to blockage or something, or you have an air pocket near the sensor.

So why would the change of your ECM temperature sensor fix the issue if it was not the problem. I can only guess, but I know that in some of the years the ECM had a temperature fail over mode, that would turn on the fan as well, incase your main temperature switch failed. It is possible that your computer is now turning on your fan but your temperature switch is still not working correctly.

Its just a thought, based on what I thought your original thread said.

i would just cut the wires to the fan and run one to the ignition then have the other go to the switch and ground. that way you bypass the ecm

Talk about dangerous and asking for trouble!!!!!

This has been gone over many times "don't do that".....

The wires are too small to handle the load of the fan circuit.

oh yeah cos thats what the relays for. Thats how my buddy has his cutlass wired and it works fine though. I ran one of my fans power wire to the ignition and it works fine I used speaker amp wire and it works fine. But i had to change the gauges fuse from a 10A to a 20A though.

That's correct I did ground the green wire that goes to the switch to the chassis and the fan ran. When I said terminal in this thread I meant the terminal at the end of the green wire that normally has a plastic connector around it that plugs into the switch.

I'll have to come up with an Ohm meter to make sure I'm getting a good ground but I agree the odds of two bad switches is slim and I can't see why they wouldn't be grounding when they are tightly screwed into the block.

Blockage near the switch maybe I guess it is a possibility but I've bled all the air out of the system so I can rule this out.

I agree with your last paragraph if you are correct. If in fact I still have a problem with the switch and it's still not triggering the fan and the coolant temp sensor does act as a fail safe and turn the fan on via the ECM in the event the switch fails then in changing out to the new sensor yesterday would suggest that indeed the original sensor I replaced was bad and not able to tell the ECM the coolants temp and trip the fan. Either way it's now running properly although be it hotter than I would like. I'm going to try to verify the actual coolant temp versus the temp displayed on the gauge and go from there.

The only time the car get over the 200 range is when it's sits for more than 4-5 minutes stationary. For the most part it runs in a good temp range.

Not going to bother with this but thanks for making a suggestion. For now I have little chance of overheating and if the fan is being turned on by the ECM I don't want to bypass it.

Coming on at 230 is too hot and it is not fixed. I think you will find that the temp guage, though maybe a little off, is not that far off. But it may be.

The only thing controlling the engine temp is supposed to be the thermostat. Keep that in mind. Don't lose sight of that. If you have a 195 or 205 deg thermo and your temp fluctuates up to 210 deg you don't have something right in the system. Everything else in the system is designed to have the water cool enuf to allow the thermostat to control the temp.

If the fan comes on when you ground the green wire to the engine, then I'd say the switch in the pass side head near the exhaust manifold is not working properly or your temp gauge above is way off.

That is the critical piece of information right there. That narrows down the possible causes of this problem to only one thing. The temperature switch (or bad connection at temperature switch).

I am 95% certain TBI cars never had ecm controlled fans. I just looked up the wiring diagram, for TBI, for 89 (year I have factory manual for). The ecm is not capable of activating the fan. Only the AC system, or the fan switch, can do it...Test that one out. Just turning on the ac (whether it works, or not), should activate the fan...nice to have as a backup method.

edit:
BTW, according to the book, the OEM switch turns on the fan at 110C (238F)

the oem switch may turn the fan on at 238, but that is asinine if the thermostat is trying to regulate at 195 or whatever.

When my thermostat is opening to regulate the engine at 195, I want the cooling to be available to help out.

I don't see the reason for an engine to fluctuate from thermostat temp up to whatever temp the fan comes on. And have this happen constantly.

You're thinking backwards on the thermostat. It regulates minimum temperature, not maximum temperature.

I agree, 238 is a ridiculous temperature to turn on a fan. I run a 180 stat, with a 203 degree fan switch.

Thought I was thinking the right way...

But, there should be enough capacity in the system to control at the thermostat temp. The upper temp is not meant to be controlled nor is it meant to be allowed to heat up to whatever.

Once my engine gets to operating temp, it is stable right there. It gets no hotter or colder regardless if the A/C is on, sitting idling, or ambient temp. That is how the cooling systen is meant to operate. This idea that it should fluctuate by 43 deg (195 to 238 for example) is not reasonable even though many think it is normal.

I would have my fans come on as late as possible though to avoid running them when not necessary. Even if it meant a little rise in temp. Like 5 deg.

The A/C does turn on the fan.

So the latest switch I put in is supposed to be the GN switch that kicks on right above 200. If in fact I have just installed a direct replacement switch and my dash gauge is relatively accurate then yes it sounds like switch is turning the fan on and not the ECM via the coolant fan sensor. If they gave me the wrong switch after specifically ordering the GN one I will be pissed. I might just get one more (third) switch from a different vendor throw it in an see what happens. Just need to make darn sure it's GM part number 3053190. I went to Napaonline but could not find where to do a part number cross reference.

I'm running a 180 degree stat and the car takes forever to warm up. Per the gauge again not necessarily accurate the car sits around 130-140 for more than 15 minutes before slowing hitting 170-180 once I stop moving. it will then sit around there while I'm moving and will only slowly creep past that when I come to a stop and sit for more than 3-5 minutes. In ordr to get the car ht enough to get the fan to turn on I let it sit and idle for 20+ minutes.

well it wouldn't have anything to do with it being a 180 stat since its not going to turn on until the car warms up to 180 anyway

Click "Parts PRO SE Catalog" link. It takes you to a page with a search blank, and below the search blank, you can select to search by keyword, part number, or interchange. Select interchange, and put in the GM PN.

Thanks.