Fans cause engine to die

[Simon_P]

Just recently and about 1000 miles after an engine rebuild.
Symptoms:
When the fans come on either from engine temperature or A/C the engine dies, the fan relay drops out and the engine pics up relay cuts in again engine dies etc.

Thankfully it has only happened when I've been stationary and I have have been able to switch it off.

I've looked at the fan wiring diagram and can't see how the fan would interfere with the engine. I'm going to check for loose earth's over the weekend.

So does anyone have any similar experience or better knowledge of the wiring that can explain what might be causing this.?

[Saltire]

Simon, I haven’t got the wiring diagrams in front of me at present, but I seem to recall that the connection from the cooling fan relay to the switch (and thereby to Earth) passes through the ECMHC connector, which also carries wires from the ECU. Is it perhaps possible that a fault at that connector is causing the ECU to short to Earth when the fan switch triggers?

Just a vague memory from an old codger, so could be well wide of the mark, but that’s where I might start.

[GeoffSmith]

Possibly an earthing issue causing sensor readings to go awol. I would check the fan earth wrt battery negative or connect a jump lead from the battery to the fan earth.

[lotusflasherman]

Idle speed is controlled by the ECM and should be aware when the A/C is switched on and adjusts the Idle Air Control Valve to increase idle before energising the a/c compressor. It does the same for power steering and should react to demands on the alternator.
For the SE the voltages that should be supplied to the ECM are shown on EMK4 Page 6 so you can check with a digital voltmeter - for the S2 I haven't a clue what page but the principle is the same .. Over to you Geoff

(I'd put my money on a multi-pin connector problem)

[HJ2]

Yeah, Check all the main bond straps between engine components, alternator, startermotor and chassis first. They can make just enough contact, but when the current goes up when the fans switch on, it all breaks down

[Simon_P]

Working through it logically....
A/C request causes revs to rise
Grounded fan temp switch to test function
Check main cable connections OK but condemned battery as 11.6 v
Checked and cleaned ECMHC plug/socket.
Check other plug/ sockets.
Checked earth bonding straps all OK
Checked front member bonding strap to fan wiring sockets - high raeistance
Traced fan earth wire to connection on front member (as per diagram).... Loose screw!

Thanks for all the suggestions.
Geoff 10/10.

[GeoffSmith]

I know I talk BOLLECKS most of the time, but sometimes, very occasionally…

[Giniw]

[Rambo]

I know I talk BOLLECKS all of the time, but sometimes, very occasionally I speak some sense…

[HJ2]

I hope i’m still here to whitness it