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Hi Horatio, many thanks for your reply.
It's about as clear as mud, isn't it?
First of all, what are you using as the earth for your testing? It could be that it's being 'pulled up' when the voltage is applied to the system, and it's giving you a false reading. Ideally, you need the negative meter probe to be touching the car's chassis. Failing that, the outer shroud of the cigar lighter socket is usually a good substitute.
I'd been using the blower motor connector negarive with the motor disconnected but I'll try using the car body tomorrow (it's dark now!)
If it's still doing what you say, even with a known good ground, then it sounds like you have a high resistance on the wire between the F37 fuse and pin A3. This will give 12v when the circuit is not under load, but will drop much lower (possibly to zero) as soon as a load is applied. Though I'm still confused, as energising the top relay should effectively connect the top A1 pin to A3 anyway, so you should be seeing the same voltage on both unless the relay is faulty and the unit is back-feeding from pin B2 on module 107. If you're really getting different voltages on these pins with the top relay energised, then I suspect you have a faulty relay.
I'd be inclined to remove both relays and join pin 30 to pin 87 on each socket. This should bypass the resistor pack (but not the thermal fuse) and prove that the motor and the rest of the circuitry is either good or bad.
Thanks, I'll try that tomorrow and come back to you. I really appreciate your help, I have torn most of may hair out over this
Currently in wilbyforce's garage:
lot, walks and cycles sometimes but rarely runs.