Update to Adjustable fan controller article printed in October 2022 club magazine.

[Steven Smith]

I am currently in the process of reinstating the air-conditioning in my 1986 C4 and on the electrical side, one of the things I need to achieve is the ability for the ECM to turn on the radiator fan when the AC is turned on.
Because I bypassed the original fan control relay to fit an adjustable thermostat that didn't happen.
After studying the wiring schematics I have a simple solution that also has added benefits.
I reinstated the original fan control relay and tested its function, the fan came on at 225°F as it's designed to and also when the AC is turned on, so far so good.
The fan control relay has four wires going to it, red is a permanent live via a fusible link from the battery, black/pink is the feed to the fan, dark blue is a fused, switched positive from the ignition and dark green/white is a switched negative from the ECM.

The ECM controls the fan by applying a negative to the dark green/white wire, whenever it wants to turn the fan on, this energises the coil in the relay which then supplies the positive feed for the fan to turn on.
It is the dark green/white wire we need to modify as this controls the whole circuit.
I cut this wire and added a "piggyback" connector then reconnected it back to the relay.
The adjustable thermostat has two connections, you run one to a negative chassis point and the other to the "piggyback" connector.
With this wiring configuration the adjustable thermostat will supply a negative feed to the fan control relay at the temperature you set it, and the fan will turn on.
When the AC is switched on, the ECM will supply a negative to the fan control relay and the fan will turn on.
This has two benefits, the fan will switch off when the ignition is turned off because the relay coil will lose its positive feed, and most importantly of all, if the adjustable thermostat should fail the ECM will still turn the fan on at 225°F as it was originally designed too.

 

[teamzr1]

FYI
Looking at the ECM calibration for a 1986

Not commanding fan on till 224 F, 107 C

That is too hot
I would lower those commands down to at least 190 F deg or lower it Tstat was replaced like with a 170 or 180
as C4s ran too hot, reason GM trying to lean the piss out of AFR for only smog purposes

As you see, I could command fan rates for with and without A/C on

 

[teamzr1]

I am not clear in what you're trying to do, but the ECM is the monitor of coolant temps from the engine ECT sensor
if the fan is on or off is 12 volts to one side of the coil and the ECM using reference to ground is the switch to control the coil to be energized or not
When it is, that forces the normally open contacts of the relay to close the 12 volts circuit supplied to the fan and is than functional

So not knowing what you're trying to do, best is some wiring of your 3rd party controller, and if a second relay added would work for stock and
override of ECM fan control

 

[Steven Smith]

I am not clear in what you're trying to do, but the ECM is the monitor of coolant temps from the engine ECT sensor
if the fan is on or off is 12 volts to one side of the coil and the ECM using reference to ground is the switch to control the coil to be energized or not
When it is, that forces the normally open contacts of the relay to close the 12 volts circuit supplied to the fan and is than functional

So not knowing what you're trying to do, best is some wiring of your 3rd party controller, and if a second relay added would work for stock and
override of ECM fan control

As I haven't got access to the ECM to change the fan settings there, I wanted the car to run cooler.
I have previously fitted a 180° thermostat but wanted the fan to come on earlier than the factory setting.
The modification above allows that but also keeps the ECM control in parallel to allow it to switch the fan on if AC is turned on and the engine is not up to temperature. It also has a "fail safe" in that if the additional fan controller fails the ECM will turn the fan on at the factory setting.

 

[teamzr1]

I need to see the wiring for this product and what it does

As I showed the tune in your ECM, the A/C on and fan setting temps are the same as with A/C off
The C4s the ECM had little control and fan command is the 224F degree switch point

If you know someone who could burn an E-prom than I can do a full custom tune I email to you, and they burn the tune into the E-Prom
This also allows correcting how GM used little timing for crap smog reasons and I redo the timing tables to walk that engine up
and lowering the 225 fan switch points to 180 F deg

You could cut the 12 volt wire that is the feed for the fan when relay switches
Now take a 2nd relay, take that 12 volts
Use a 2 pole switch and when if is off the normally open contact of 2nd relay feeds the 12 volts to the 12 volts of feed to GM relay, and now it is normal as GM designed but when switch on then relay switches and now 12 volts is diverted to your controller, leaving stock setup in as GM designed to control fan

Your new relay you're supplying it ground, 12 volts for coil
and that relay switching 12 volts either to your controller or the feed of 12 volts to stock relay
You controller than needs a ground from frame.

 

[Steven Smith]

I need to see the wiring for this product and what it does

As I showed the tune in your ECM, the A/C on and fan setting temps are the same as with A/C off
The C4s the ECM had little control and fan command is the 224F degree switch point

If you know someone who could burn an E-prom than I can do a full custom tune I email to you, and they burn the tune into the E-Prom
This also allows correcting how GM used little timing for crap smog reasons and I redo the timing tables to walk that engine up
and lowering the 225 fan switch points to 180 F deg

You could cut the 12 volt wire that is the feed for the fan when relay switches
Now take a 2nd relay, take that 12 volts
Use a 2 pole switch and when if is off the normally open contact of 2nd relay feeds the 12 volts to the 12 volts of feed to GM relay, and now it is normal as GM designed but when switch on then relay switches and now 12 volts is diverted to your controller, leaving stock setup in as GM designed to control fan

Your new relay you're supplying it ground, 12 volts for coil
and that relay switching 12 volts either to your controller or the feed of 12 volts to stock relay
You controller than needs a ground from frame.

Adjustable Fan Controller Thermostat

Complete installation kit with full instructions. Can be fitted into the top hose. Just pull off the hose, insert the probe and re-fit the hose with a little PTFE tape around the capillary beneath the hose clip. The probe can even be clipped directly to your radiator fins. Alternatively you can...

www.carbuilder.com

It's a simple fan controller, I have it set to approximately 194F.
Wiring it into the switched negative negates the use of multiple relays and keeps it simple.
The ECM uses the temperature sensor at 224F as you say but also the AC fan controller, both send a negative to the fan control relay.
The above thermostat, instead of being in series with the positive fan supply, I have wired it so when the temperature is reached it supply's a negative to the original fan control relay, hence switching the fan on.
If I can find someone to burn an E-Prom I will take you up on that.

 

[Steven Smith]

Wiring diagram showing how I have added the adjustable thermostat into the existing fan control wiring.

 

[teamzr1]

If what I lined in red is your adjustable thermal unit, I am missing something as
this only shows 1 wire goes to the reference to ground going to C1 of ECM, is your wire in series or parallel to C1,
other wire to ground when vendor drawing shows going to 12 volts and their thermal probe

 

[Steven Smith]

The adjustable thermostat doesn't need a 12v supply to work so I have wired it so when set temperature is reached it just supplies a negative to the fan control relay coil.
Fitted it and tested it yesterday and worked perfectly.