I have the fairly common it seems issue of cooling fans not working. Coolant temp sensor is working fine. Both fuses are fine. I suspect the Fan Control Relay. Is there a simple way I can test if this is bad? I have a suitable multimeter but I'm not massively au-fait with electrics.

 

There is no simple way to test the fan control relay. There are actually three relays in the module that, for low speed, connect the fans in series and, for high speed, connect them in parallel.

You should check fuses 12 & 14 (30A) in the larger engine compartment fuse box and fuse 14 (10A) in the smaller engine compartment fuse box. If all fuses are good, unplug and test each fan separately using an external battery. If one of your fans is bad (open circuit) the other fan will not run either when they are switched to low speed. If then the engine temperature gets to the point that triggers the high fan speed, the good fan will run.

If both fans test good, you can do a direct test of the module if you tap into the two wires (white & white/blue, thin wires) that go from the module to the ECM and ground them which would get the fans running if the module is good.

The problem is that there is another thin white/blue wire (at the right of the above diagram) which should not be touched so it would be a bit difficult to determine which white/blue is to be used for the test.

Alternatively, you can try testing the fan operation via the connector to the AC pressure switch. This is an indirect test of the fan relay module, via the ECM.

Unplug the harness from the switch and insert some thin wires as probes into the connector sockets for the red/white and white/blue wires. At ignition "on", if you connect the red/white to the ground, the low fan speed should come on. If, while keeping the red/white grounded, you also connect the white/blue to the ground, the high fan speed should come on.

 

Ok so testing it via the AC pressure switch reveals the fans working as expected. So I know the fans work.

but with the pressure switch plugged in the fans don’t run with the ac on or off.

where to next?? The AC definitely needs regassing, but is it true to say that irrespective of the a/c being inop the fans should come on if the water temp gets high enough?

 

Sounds like the pressure switch may be bad

 

If the refrigerant pressure is below 2 Bar at startup, the AC compressor will not run so the AC system will be disabled. Even if the compressor runs (pressure more than 2 Bar), with low refrigerant the system pressure may not rise to the point (12 Bar) which triggers the Low Speed of the fans.

Yes, when the coolant temperature reaches 90 C, the fans should come on at Low Speed. At 97.5 C, the High Speed of the fans is triggered. If the fans are not coming on at about 90 C, replace the coolant temperature sensor and see if this solves the problem.

 

Many thanks for that. I'm using a bluetooth OBDII reader and monitoring coolant temp as I drive, and it hovvers around 86-88. I've not let it get above 92 whilst sat idling before shutting down the engine just in case. However this does suggest to me the coolant temp sensor is operable, as it isn't erratic and does follow what I'd consider a normal path through warming up. I've not seen the fans start up though, except when grounding out the two pins on the AC pressure sensor.

It's cold here currently, < 10 degrees C, so maybe I've just not let it sit long enough.

Is there a way of testing whether the pressure switch is bad? I could get it re-gassed, but then that's £75 that will need to be repeated if it turns out the pressure sensor is inop, as I presume replacing that sensor will cause the system to depressurise?

 

With a working water pump, 92 isn't high so why be so worried...

 

OBD readers (especially the cheap ones) may give you somewhat erratic reading so, when it shows 92 C, the actual temp might be, for example, 89 C. In so cold weather, the fans may not come on at all if you are moving. You can try idling the engine for a bit longer while watching the temp on the OBD. Don't be afraid to let the temp go to some 97-98 C (if the cap on the header tank is good, i.e. if it keeps the system pressure).

You can replace the pressure switch at any time as it will not cause any refrigerant loss (there is a Schrader valve behind it). If there are no signs of the compressor oil leak inside the pressure switch connector, it is probably good. You should first re-charge the AC system and, when the charge is completed, ask the mechanic to keep the AC pressure gauges still connected for a while so that you can check operation of the pressure switch. When the refrigerant pressure at the High Side exceeds 12 Bar (174 psi), the fans should come on at the Low Speed. This will cause the pressure to drop and, at 8 Bar (116 psi), the fans should stop. Then, pressure goes up again and so on. However, at 10 C ambient the test will not work as the refrigerant pressure will not reach 12 Bar at all. You will need at least ~ 23-25 C ambient to perform this test.

Looks like that, for both tests, you will have to wait until summer.

 

That's really helpful info, thanks very much. I'll go ahead and get it regassed and take it from there. Cheers!