1G Radiator Fan Cooling Issue

[Nochobrine]

My bone stock '91 is having a strange issue where the fan for the engine on the radiator will not come on, but the motor still works and has power going to it. I've checked the relay , the TCS (still reads and completes the circuit) and the wiring harness to the TCS and motor. However, when the TCS comes up to temp, the relay clicks but the fan doesn't come on. I have been able to force the fan to run by jumping it at the relay block, but it will not turn on by itself.

I have also noticed that when I have the AC radiator fan running, turning it off with the button sends power to the engine fan for a split second, but it comes to a stop right after.

Anyone have any tips or ideas on where to go from here. Considering that everything was working fine up until last week when this started, I would like to get it working properly again. Thanks for any help in advance.

 

[petrosian]

Welcome to the forum! You should fill out your vehicle profile .

From what you wrote above, only the condenser fan runs when the AC is on? Both fans (radiator cooling fan and condenser fan) should run when the AC is on. But, one thing at a time…

For 1G DSMs, the radiator cooling fan is normally controlled by a temperature switch that is screwed into the radiator itself (See pink lines on the attached diagram). It provides a path to ground for the radiator cooling fan motor relay. I’m not sure if that was part of your check process, but that sensor and the relay both are worth checking (and their connections/wiring).

When AC is on, things are a little different. The other diagrams show where current flows when AC is on. The AC controller (green lines) chooses either a low or high fan speed by controlling the Hi-Lo Changeover Relay (I can’t remember how the AC controller chooses, but I think it’s based on the cabin switch position [Amber/Green] and evap air inlet temp).

If low, both fans are driven at low speed via a resistor (the radiator cooling fan relay is bypassed). If high, the resistor is bypassed, and the radiator fan relay is energized by the control unit (instead of depending on the radiator temp sensor). This gives full voltage to both fans for high speed operation. It is worth mentioning that the condenser fan relay is energized for both high and low speed.

The movement in the radiator cooling fan you noticed when cycling the AC may be from the AC controller briefly energizing one of the relays, or from the discharging of that capacitor A-20X (labeled old-school as a “condenser” in the diagram) . Ultimately, though, BOTH fans should also be running whenever the AC is on.

Hope this helps. Keep us posted!

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[Nochobrine]

Welcome to the forum! You should fill out your vehicle profile .

From what you wrote above, only the condenser fan runs when the AC is on? Both fans (radiator fan and condenser fan) should run when the AC is on. But, one thing at a time…

For 1G DSMs, the radiator fan is normally controlled by a temperature switch that is screwed into the radiator itself (See pink lines on the attached diagram). It provides a path to ground for the radiator fan motor relay. I’m not sure if that was part of your check process, but that sensor and the relay both are worth checking (and their connections/wiring).

When AC is on, things are a little different. The other diagrams show where current flows when AC is on. The AC controller (green lines) chooses either a low or high fan speed by controlling the Hi-Lo Changeover Relay (I can’t remember how the AC controller chooses, but I think it’s based on the cabin switch position [Amber/Green] and evap air inlet temp).

If low, both fans are driven at low speed via a resistor (the radiator fan relay is bypassed). If high, the resistor is bypassed, and the radiator fan relay is energized by the control unit (instead of depending on the radiator temp sensor), giving full voltage to both fans for high speed operation. It is worth mentioning that the condenser fan relay is energized for both high and low speed.

The movement in the radiator cooling fan you noticed when cycling the AC may be from the AC controller briefly energizing one of the relays . Ultimately though, it should also be running whenever the AC is on.

Hope this helps. Keep us posted!

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Thanks for the full diagrams and explanation! This is perfect! I had talked with a friend of mine in between starting the thread and now that suggested that I check ground wires, but here I was chasing the wrong ground! I was checking Ground 2 thinking it was the one connected to the radiator fan motor and not 3. Hopefully, that'll be the smoking gun and I can have the radiator fan working properly when I get back from work tomorrow. And I'll probably end up using this when I revamp my AC setup not far from now. Thanks a bunch, petrosian! I'll update once I get started tomorrow, and I'll setup my vehicle profile too.

 

[Nochobrine]

Update : After a good cleaning and adding dielectric grease to the ground for the motor, nothing seemed to change. Considering I could eliminate everything else in the system by jumping and nothing happened, I ended up taking out the motor itself and trying to run power directly through it. It only jerked a couple of times, then refused to do anything else afterward. I assumed that it had finally given up after 30 years of unwavering service, so I have an OE replacement fan motor on order that should arrive Monday. I'll update again once I have it and solder it onto the stock harness.

 

[Nochobrine]

After going through 2 different fan motors (1st one might have been a dud because it refused to spin when directly powered), they both refused to spin. Jumping the TCS would still power the relay and close the circuit, but both refused to spin. I'm going to have to check every single wire in that harness because I can't think of why else the motor wouldn't be spinning unless Unimotor has horrible QC.