coolant fan Low speed resistors

[Nick-Ricky]

1986 - 951 - every day driving car - putting back together

low speed fan resistors - this is the only thing so far that's got me stumped.

the hockey puck resistors were in the "milk crate" with a few other things when i got the car, found the wiring of a mad scientist under the dash, so I took ALL interior out & traced down & cleaned up as much as I could - the good news is the car fired right up after wiring work!

when i connect the battery & the key NOT in ignition, passenger fan runs(coolant fan #2) & 1 resistor gets very hot, disconnect 1 wire on hot resistor & fan stops

does it matter how they are wired?

mine only have the 2 white puck resistors, do i need the other outer ones as seen on some internet pictures? (see 3 pictures from internet)

do i even need these or can I just tie those together somehow & run both at high speed whenever temp sensor determines? - thanks, nick

[bleu951]

The two big resistors are all my car has. They provide low speed operation to each fan. Just be sure you have one resistor in each fan circuit. This is a test procedure I wrote a few years ago and it may help you sort out the wiring mess. Before you drive the car, make sure the fans are working properly.

During normal operation, and you have the factory rated thermostat, the coolant temperature gauge should read at or near the lower white line (80 degrees C) and will creep up to the mid-point between the upper and lower white lines when sitting still in traffic. The upper white line represents 100 degrees C. If the temperature normally runs above the mid-point, you need to investigate immediately.
There are several temperature ratings of the thermo switches, so it is hard to say at what point they should turn on for your car. If they are not both running at high speed when the temperature is above 90, you might have a problem.
The fans are controlled by the cooling fan relay which is in turn controlled by the thermo switch, and the a/c system. Whenever these switches are closed, both fans should run at the same speed at the same time. If only one is working or you only get high speed operation, there is a problem.

You should always have power available to the relay, so the fans can run at any time. Remove the cooling fan relay. It is the large rectangular silver relay in the back of the panel. Check the diagram on the fuse panel cover for the exact location. All of the following checks can be made at the fuse panel. You will need a bright light to see the markings on the fuse panel.

1. In the Bosch world, a terminal designation of “30” means it is un-switched battery voltage. Check for voltage at positions 30 M1 and 30 M2. You should get 12v + at both positions. If no voltage, check the wiring from the battery, the connections to the fuse panel, and the fuse panel itself. You will have to pull the fuse panel up, but disconnect the battery before messing around under the fuse panel.

2. Check for continuity between ground and the temp switch positions "TS" and "TF" on the fuse panel. If you have continuity, the thermo switch is calling for fan operation. If the car is cold, the thermo switch is bad.

3. Fabricate a jumper wire with flat blade connectors on each end, and a 20a fuse in the middle.

4. Connect the jumper between M1 30 to M1, the drivers fan should run at high speed. If the fan does not run, check for voltage in the connector at the fan motor, if you have voltage but the fan does not run, replace the fan motor.

5. Check the passenger fan by connecting the jumper between M2 30 and M2, the passenger fan should run at high speed. If not, the same drill.

6. Next connect the jumper between M1 30 and V1, the drivers fan should run at low speed. If not the resistor or wiring is bad. Check for voltage at the radiator fan connector and then check the resistance across the resistor.

7. Next connect the jumper between M2 30 and V2, the passenger fan should run at low speed. If it does not run, check the wiring and resistor.

If the car is still running hot and the fans are working properly, you have other issues, like a clogged radiator, water pump failing, air bubbles, bad thermostat, or a bad head gasket.

The other positions on the fan relay are:

X = battery power to the logic board in the relay
31 = Ground for the logic board in the relay
AC = AC freeze protection switch

[Nick-Ricky]

This is how I wired it (also, if thermo switch bad or stuck, could cause fan to want to run - even when thermo switch wire is unplugged?)


(G-10) RT/GN > tab-a > on either r e s i s t o r > tab-b > SW/RT
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>>> Fan 2
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(G-10) .....................<<<<<<<<<<<<<................ .................. SW/RT




(G-10) RT/WS > tab-a > on other r e s i s t o r > tab-b > RT/SW
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>>> Fan 1
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(G-10) .....................<<<<<<<<<<<<<................ ................. RT/SW


any advice, diagrams or pictures regarding above would be welcomed.


Thank you,

Nick Ricky

[AaronM]

Originally Posted by bleu951

During normal operation, and you have the factory rated thermostat, the coolant temperature gauge should read at or near the lower white line (80 degrees C) and will creep up to the mid-point between the upper and lower white lines when sitting still in traffic. The upper white line represents 100 degrees C. If the temperature normally runs above the mid-point, you need to investigate immediately.

The official factory shop manuals list the thermostat as "beginning to open" at 83 C and as "fully open" at 90 C. This means that the engine should never be colder than 83 C and should actually be running in the 85 C to 87 C range under most conditions.

Under normal conditions at speed, the temperature should be slightly above the first white line up to the mid-point between the two lines.

Sitting in traffic is highly dependent on the weather. In most mild to moderate climate situations, the temperature should fluctuate around the low-speed fan set point (stock, 92 degrees, so a bit above the mid-point between the two white lines).

On very hot days when sitting still in traffic, temperature may creep up to the upper white line (100 C) before the high-speed fans come on.

Originally Posted by bleu951

There are several temperature ratings of the thermo switches, so it is hard to say at what point they should turn on for your car. If they are not both running at high speed when the temperature is above 90, you might have a problem.

The stock fan switches turn on low-speed operation at 92 centigrade, and don't turn on high speed until 102 centigrade. According to the factory shop manual, the stock low-speed switch comes on at 92 C and continues to run the fans, at low-speed, until the coolant temp has dropped to 87 C.

Assuming a stock fan switch, the fans shouldn't be on at all when the coolant temp is 90 C (though coming on to low-speed at 90 C is probably acceptably within range for the stock 92 C switch). They definitely shouldn't be on high unless the A/C is also on (the A/C will force the fans into high-speed operation if A/C high-side system pressures exceed about 200 PSI, regardless of coolant temp).

If the fans are coming on to high-speed operation as low as 90 C without the A/C on, something is very wrong, or someone has installed a lower-temp fan switch.

Note: Installing a fan switch that is lower than the rated thermostat is pointless (since there's no coolant flow through the radiator until the thermostat opens).

What you want is for the low-speed fan temperature to be a few degrees higher than the thermostat's "fully open" temperature. This way, when the thermostat opens at speed, the fans don't come on immediately, instead allowing the car's motion to force air over the radiator and provide the cooling without needing to run the fans.

You don't want the fans to come on until slightly after the thermostat is fully open. With the stock, 83 C thermostat, you don't want the low-speed fans coming on any earlier than they do with the stock, 92 C fan switch.

[Tbone425]

Nick-

Ill past the diagram I made later. I wend and got a resistor unit off a donor 86 turbo and cut the branch all the way back to the main harness (car was going to scrapper).

Chris

[Tbone425]

Sorry its a bit quick and dirty but should have everything you need. Let me know if not...

[dougs951s]

Can anyone tell me the function of the two small switches circled in red? They are only present on 86 cars which has the resistor packs inside the car, vs 87+ cars which had them under the hood. I'm repairing damaged wiring on my 86 that caused me to have to slow speed fans, but those small switches broke off the silver tray that holds the resistors. I'm wondering what their purpose is and if I can safely bypass them. I've been told they are thermal switches that will shutdown current to the resistor if they get too hot, but I'm trying to confirm that. Since my low speeds fans have never ever shut down on any of the 86 cars I've owned and later cars did away with them despite the resistors being in a location that i dont think would provide any additional cooling; it seems to me like if they are indeed thermal shutdown switches that it would be ok to bypass them. Can anyone comment further?

[BobCT]

I have the exact same situation on my '86 Turbo (old post I know). The mounting studs for each of the (switches?) in each corner broke off. They are shown in the diagram above with each having two spade terminals.

Can anyone verify what these do? I have them JB welded to the aluminum bracket which should be fine as long as there isn't a need for this to be grounded to the bracket and then to the body of the car.

thanks
Bob

[kdjones2000]

My guess is that they are fusible links, since it's inside the car.

I think that these wires (or some of them) are not even fused at the panel, so they would be there for safety.

Should be no need to ground them...

[BobCT]

then I should be ok, the JB weld is a solid attachment now. Thanks