Idle drops when radiators fans come on

[dpathurley]

The car idles fine and increases under any electrical load as normal but when the radiator fans come on it’s drops to nearly stalling

[jimi]

Sounds like a problem with the idle up circuit, it should increase the revs by about 200 rpm when the fan runs. If the revs increase with other electrical loads (lights, brake lights, heater blower, rear defogger) then the problem is confined to the rad fan part of the circuit. It's going to be a pain to track down, how are you with electrics ?

[jimi]

I'd be concerned that the rad fan is cutting in, under what circumstances does it cut in ?

[dpathurley]

Hi thanks for the replies the fan only kicks in in heavy standstill traffic situations and the valve that increases the revs whenever you turn stuff on like headlights etc is working fine

[jimi]

Sounds to me like the idle up circuit is working but the radiator fan doesn't appear to be triggering it, which in turn points to a wiring ( or possibly a relay fault) on the rad fan side of the idle up circuit. As I said " it's going to be a pain to track down, how are you with electrics?"
My fan doesn't cut in (at least not on full speed) even in heavy traffic on a hot day. The MK1 cooling system in good condition should easily cope with that situation without the fan running at full speed.

[dpathurley]

Not great with electrics although managed to diagnose replace and set up the TPS when that failed. The cooling system is good had the head gasket replaced after it was pressurising the coolant then dumping it out the expansion tank and I’d flushed and bled it numerous times before doing the gasket in the hope it was an airlock. The fans have always come on though when at the temperature they’re supposed to only for maybe 10 seconds at a time and the temp gauge always sits just below halfway.

[dpathurley]

Is there an idle up valve on the fan side of the system ?

[jimi]

There's only 1 idle up vsv, it's controlled by inputs from:-
Lights
Brake lights
Heated Rear Screen
Heater Blower (or A/C if fitted)
Radiator Fan
The inputs are fed to the vsv via diodes, a signal on any one or more will turn the vsv on.

[dpathurley]

Literally every item turns on the vsv apart from the fans lol In fact if I’m in queueing traffic I’ll turn the heated rear screen on to activate the vsv to compensate for when the fans come on

[jimi]

As I said

Sounds to me like the idle up circuit is working but the radiator fan doesn't appear to be triggering it, which in turn points to a wiring ( or possibly a relay fault) on the rad fan side of the idle up circuit. As I said " it's going to be a pain to track down, how are you with electrics?"
My fan doesn't cut in (at least not on full speed) even in heavy traffic on a hot day. The MK1 cooling system in good condition should easily cope with that situation without the fan running at full speed.

When I said "a pain to track down" I meant for someone who is not electrically based, it would be fairly straightforward for me if I was there, but I'm not.
Let me have a look at the diagrams and see if I can find some easy places to check so we can narrow it down. Do you have a meter ?

Still suspicious of your fans cutting in, the only time in nearly 20 years I've heard my fans running is when I made them run to prove to myself that they did work

[dpathurley]

Yes I have a meter I did some readings on the battery which read with the car of 11.9v with the engine running was 14.2v and when the fans came on while running it was 12.4v.

[dpathurley]

Have you ever just left it to idle and see if the fans come on pretty sure they should that’s what they’re there for lol

[jimi]

Have you ever just left it to idle and see if the fans come on pretty sure they should that’s what they’re there for lol

Fans are there to keep the engine at it's optimum operating temperature I'm not in the habit of leaving my car idling for long periods of time, however I have had it idling for around the 10 minute mark ... no fans (at least not on full speed, you can't hear the low speed) Once my engine is up to temp, the gauge sits 1 mark below half, summer, winter, in traffic . I even thrashed it around a sprint track for 20 mins and the gauge didn't move. The only time I've seen it move was when my thermostat was faulty. I also now only have 1 fan fitted, still no change. Bear in mind that the MK1 cooling system holds 12.4 litres of coolant (over twice the capacity of your average front engined car) by the time the water travels round the system it's lost a lot of heat

My original standard setup


new setup


Anyway it's your car, if your happy with the way your cooling system is operating that's fine, I was only offering my opinion based on my experience

Regarding your other problem
The signal from the rad fans to activate the idle up vsv comes from the feed to the fan motor, so since your fan is operating it proves part of the circuit is OK (relay and part of the wiring) The first place I would check for the signal is at the diode (your looking for +12v between a good body earth and the wire (Blue/Red) As far as I can tell from my diagrams the diode for the rad fan signal is located behind the dashboard on the passengers side, I think you can get to it by removing the glove box interior (or possibly through the speaker hole). If you get 12v on the incoming side of the diode then the fault is possibly the diode as the wiring after that is common to the other diodes

[dpathurley]

Yeah my temp sits just under half too all the time and I’ve sat in some serious queues like 30 mins stop start to get into some events never had the fans come on in normal driving conditions though. Would you have a diagram or pic of what I’m looking for please?

[jimi]

I can put the electrical diagrams up on my website if you want to download them ?
As for the diodes, I have no idea what they look like or where exactly they are (never needed to find them) haven't been able to find any pictures of them anywhere. At a guess it will be a small encapsulated block, the only thing I can find is this

[dpathurley]

Would too small an ampage battery affect it?

[jimi]

No can't see that, might have other effects but the idle up vsv only needs a tiny current to energise and from your descriptions it works with the inputs only the rad fan one that doesn't.