740 & 760 Cooling fan set up on a 1990 740GLE

OldnDecrepitO&D @OldnDecrepit GoldCoast
edited April 11 in Troubleshooting

I need some help with my 740. The car already has an electric fan that is supposed to activate when the car becomes too hot. There is a sensor in the radiator that is supposed to trigger this, however, it has never been triggered. The sensor could be faulty.

The fan does work but when I connect one side of the switch to the other (with the engine running) it does not trigger the fan.

Does anyone know where the relay for the electric fan is?

I want to set up the fan to activate when the AC compressor turns on (it's hot up here in QLD). There is a wire that runs above the compressor that I could use as a trigger wire but I'm not sure if this is a suitable source. Does anyone know?

I was intending to connect this wire to one side of the switch on the radiator so that I could use the original relay setup to trigger the fan.


  • The cooling fan relay will be on the right side strut tower near the coolant tank, there are two relays in this location that are identical, one is the cooling fan relay and the other is the radio suppression relay. While we're there, if you ever have stalling or non starting issues with your car, one of the most likely causes are a lack of connection from the wires going to the radio suppression relay to the relay itself, the four wires pull out of the plug connector causing a disconnection, the reason you need this relay is because the fuel injectors ground through it. No ground, no injector pulse but it will trick you because you will have spark, and you will have fuel just no injectors working so no fuel will be injecting if the wires to the relay are slightly pulled out.

  • Thanks ramrod.

    Any idea which relay is which?

  • The one that causes the car to not start when removed is the radio suppression relay

  • I've established that it's the relay nearest the engine. So now I need to test the relay. Tomorrow.

  • Many thanks for your help ramrod.

    Still need some advice from someone about the compressor wire.

  • edited April 10

    The fan switch on the radiator can be tested. If faulty the cooling fan will never work

    I would start there, no extra wiring or piggybacking needed

    The cooling fan relay can also be faulty

  • OldnDecrepitO&D @OldnDecrepit GoldCoast
    edited April 10

    I will have to test the relay and the switch.

    But I want the fan is be triggered by the compressor to aid in the AC cooling. If I can get it to be triggered by either that would be good.

  • The clutch fan is efficient enough for the AC to work well. There might be something else up with your AC system if the performance is not satisfactory. By using the electric fan to help, you are trying to use a band-aid fix

    If I were you, it's almost winter now, so that's the perfect time to get the AC system checked and serviced if need be, then find out why the fan isn't working. That electric fan is not an AC fan but an overheat protection

  • No, the AC is working fine. I'm just fussy about the set-up. There is some additional heat load from the large transmission cooler but the the AC is very cold in the 740. There's no reason the electric fan can't be triggered by either the compressor or the overheat switch.

    I have an electric fan in my 164 triggered by the AC and I want to do the same here. I intend doing it in my 960 too.

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