E300D Air Con Auxillary Fan not engaging

[Solid]

Dear All,

I am having problems with my auxilary fan not engaging. I had the air con regassed after buying the car and lovely cold air was coming from the vents!!!...............however the auxilary fan would not engage to cool the system down, so the garage had to remove the gas to prevent the system exploding. They recommended I take it to an electrical specialist to trace the fault, but I thought someone here might be able to advise the correct procedure to diagnose the fault.

I have taken a direct feed from the battery to the fan which then worked fine, so I figure it can be one of three things (open to other suggestions!), the relay(s) controlling the fan, the pressure sensor for the air con or finally the temperature sensor which causes the fan to cut in at 100deg. I have a multimeter but my electrical experience is fairly limited.

I need an electrical guru!

Many thanks

Phil

 

[Bolide]

The compressor is cycled on & off via the low and high-pressure switches and is unrelated to the fans

The fans use a thermostat as you say - if you trace the wiring back you should be able to find the fault easily with a meter. I'd check the wiring joins, fuses and relays for corrosion

The "exploding" a/c and removing the gas sounds like a load of nonsense to me

Nick Froome
www.w124.co.uk

 

[Solid]

Nick

Thanks for your valued reply; so in your opinion was there no need to remove the gas? Would therefore the end result have been, in the absence of the fan, that the temperature could rise to a point that the engine overheated?

To help a novice do you happen to know which position the relay occupies in the fusebox, I have 3 relays which working from back to front are numbered 001 542 78 19, 001 542 96 19 & 001 542 01 19 with ratings of 40A, 30A & 15 A respectively.

Excuse my ignorance but how do you test a circuit which I suppose only becomes operational when the temp rises above 100 deg? Do I have to let it rise above this because I think I will have to leave this until next summer with the poor weather we are having!! The viscous fan appears to be working fine, I guess it is kicking in at 85 deg as it is supposed to do.

Many thanks

Phil

 

[Bolide]

so in your opinion was there no need to remove the gas? Would therefore the end result have been, in the absence of the fan, that the temperature could rise to a point that the engine overheated?

Why remove the gas? The off button is still there

The fans come on at quite a high temperature and probably 99% of the time they're not running.

To help a novice do you happen to know which position the relay occupies in the fusebox, I have 3 relays which working from back to front are numbered 001 542 78 19, 001 542 96 19 & 001 542 01 19 with ratings of 40A, 30A & 15 A respectively

I am not sure that there is a relay. If you find the temp sensor, and it's a simple make/break switch, short it out and the fan should come on

Excuse my ignorance but how do you test a circuit which I suppose only becomes operational when the temp rises above 100 deg? Do I have to let it rise above this because I think I will have to leave this until next summer with the poor weather we are having!! The viscous fan appears to be working fine, I guess it is kicking in at 85 deg as it is supposed to do

With the temp sensor shorted, work back till you find a live feed. The fault is then just ahead of your last test point - unless there's more than one fault

I suspect a fuse / connector / relay or decayed wiring across the top of the engine

if there is no relay I'd fit one as the startup & running current of those fans is pretty huge

Nick Froome
www.w124.co.uk

 

[Solid]

Nick

Thanks again, I will do as you say and report back.

Cheers

Phil