Fan Switch Circuit (Page 1/2)
mkoski SEP 06, 09:15 PM
1985 2M4

I think the build date was May 1985

Issue: Radiator fan operation.

So I thought the 1985 had a two speed fan and we are unsure if the dash temperature gauge is the problem or some unknown ground for the circuit. I can let this car idle for 30 minutes The fan never comes on gauge seems to operate normally rising from room temp to hanging mostly at 220 but going beyond to the 230or so after awhile no fan. If I turn on the A/C fan comes in as expected. We went looking for a resistor up front but found none. We knew the temp switch needed replacement ordered the single speed kit from Rodney Dickman for 195 thermostat

If with the car at 220 I ground the wire going to the switch bingo the fan comes on.
theogre SEP 06, 10:15 PM
Only 84 Fiero w/ AC have 2 speed rad fan.
Engine at 220-230°F is not an "over heat" condition and why GM used 235° engine switch.
AC turn on rad fan is for added air flow because AC Condenser heats air above ambient and could make problems for engine cooling.
See my Cave, Rad Fan

Quickly climbs at Idle may not just no air flow.
Change T-stat to Stant SuperStat. That works a little different to normal even often tho looks same to you.
See my Cave, Thermostat

Check for crush pipes etc in rest of section.

May still have block rad tubes and worse from weak/no "antifreeze" in 30+ years.
You might see block rad tubes but often need to empty, pull left hose, a try to snake a bore scope to at bottom of tank and tubes.
Right Tank often looks perfect w/ same problem.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

mkoski SEP 08, 12:53 PM
Suspect we have a failing temp gauge. The radiator core is flowing, system has pressure car does not boil over. That fan never comes on though, sounds like this motor is just happy running at a warmer temps. I'll leave it alone for the time being and move on
to the next issue.
mkoski SEP 19, 03:29 PM
[img]http://images.fiero

We do have some kind of issue in this circuit fan never comes on will overheat eventually.

My infrared thermometer indicated top of head gauge sender read 266, thermostat flange at the head read about the same fan switch was tougher to hit but was 255. No fan. If I ground the wire going to the fan switch in the head fan comes on if we turn on the A/C fan comes on.

We jumped across the connector at the fan relay upfront the red to black fan comes on. Replaced the fan relay no change.

Back to the engine fan switch connected meter positive to the switch negative to the strut mount stud. Run the engine up to 240 ish meter goes from open to about 7 ohms resistance so I’m guessing the switch is good

This seems like an issue on the ground side of the circuit control.

We Teflon taped the switch threads on the fan switch is there any chance that could foul the ground enough.

I’m confident the ECM is good.

Someone has spliced in a wire in the ECM harness I’ll guess it was from a fuel pump issue but who’s to say I’ll try and post a picture.

Suggestions appreciated.

[This message has been edited by mkoski (edited 09-19-2020).]

Blacktree SEP 20, 10:40 AM
The fan switch activates the fan relay, by providing a ground for the relay coil. And the switch grounds through the threads on the outside (to the intake manifold). So yeah, Teflon tape can mess with that. Also, make sure the engine-to-chassis ground strap is intact. The ground path of that circuit goes through the engine to the ground strap, then through the chassis to the battery.

Also, 7 Ohms resistance on the ground circuit is too much. Ideally, it would be less than 1 Ohm. If you remove the Teflon tape and use paste instead, that should help.

Best of luck!
mkoski SEP 22, 03:42 PM
Thanks for the reply, The fact that I can ground the green/white (I think), wire going to the fan switch in the head and the fan turns on, makes me think all the wiring between the switch and the fan motor is good. Bad switch right? I ordered a new factory switch and installed it without any Teflon tape same problem as with the Dickman part after awhile temp gauge pegs idiot on dash illuminates overheat, overflow tank starts to make noise. Ground that wire, fan comes on motor is back to a happy 220 or so in short order. Now I'm under the impression that the ECM takes information from the temp sensor in the thermostat neck in order to manage fuel trims etc. it does not really care a whole lot about what the fan switch is doing at least in the 1985 model and thus I'm not worried about a damaged driver in the ECM.

The guy I bought this car from boasted a bit at the time that his kid drove it all the way through high school and "dad" had mostly kept it going with junkyard parts. So really I doubt this fan was working when I dragged the car home. I have an occasional code 35 but that's it and even the 35 has started to disappear on its own.

I am concerned though that I may have an assortment of components that don't belong together. In the engine compartment I have two wire connector red in color. unused a green/blk maybe and a green one of the wires is keyed power and guess what if I ground it with the ignition on yep, the fan comes on. these are not added wires they go into the factory harness.

Am I perhaps missing a component here?
Blacktree SEP 22, 07:15 PM
You are correct about the temperature sensor for the ECM. It is completely separate from the fan switch. And the fan switch does not interface with the ECM.

They didn't start doing ECM fan control until the '87 model year.

Sounds like the previous owner did some wiring hackery. Can you take a photo of that 2-wire connector?
mkoski SEP 22, 09:09 PM
Sure, here you go see what you think I guess next step is to open up the loom it’s in and look for spliced wires. Still though if I ground the wires at the switch connector fan comes on. ,
Patrick SEP 22, 10:51 PM

quote
Originally posted by mkoski:

If with the car at 220 I ground the wire going to the switch bingo the fan comes on.



Just to clarify this statement from the first post... The coolant temperature should make absolutely no difference. At any temperature, grounding the wire that goes to the fan switch should turn the rad fan on.

It's not making much sense that two different fan switches are not turning on the fan, if indeed the coolant is reaching the required temperature. It would help to know what temperature your coolant actually is. I don't suppose your have a scanner (or appropriate cable/WinALDL) to use with this car?
mkoski SEP 23, 09:30 AM
Correct, if I ground the wire the fan comes on regardless of coolant temperature.

Actual coolant temperature at a given moment during testing: I can only estimate based on the dash gauge that clearly is wrong but does operate, My infrared thermometer which provide me with more of an estimate based on my experience with infrared.
However the dash idiot light will come on signaling over temp that one I trust.

Regret I do not have the connector reader that you referenced. I did consider ordering it but it comes with a CD. I no longer have anything with a CD drive.

It strongly points to a bad switch in the head or the switch is not submersed in coolant since when I ground the wire the fan turns on. The AC belt is gone the compressor is there, the P.O. indicated when I bought the car that it used to work but he thought the evaporator had a leak I think he gave me a used evaporator with a collection of other parts. I have not checked the wiring over to the compressor because like grounding the wire if I turn on the AC at the dash the fan comes on.

Yesterday I ordered AutoMeter Cobalt Digital Series Gauge 6937 and a Standard Motor Products TS85 Temp Sender/Sensor which I think will fit in the switch position and will activate at a much lower threshold. I know it may foul the spirit of the ECM engineers that put the parameters into the system but I can order another factory switch once I confirm that is the issue.

Did you have any guess about what that red connector was for ?