Fan Relay

[dougal]

Afternoon, it's me again, yet more sparks probs.

This time it's the cooling fan relay, on the way down to Le Mans, which was a pain.

Relay starts to buzz, very quickly latching and un-latching, sounded a bit like a Bee, the fans are turning, but slowly due to current being turned on/off very quickly. First thing I try bridging the Otter, same symptom, so not the otter, swap the relay with a different one, same thing, this is all with the engine running, engine off, no relay buzz. So I jammed the relay closed with a piece of plastic then carried on, all was fine, I just have to take the relay out when I turn the engine off now otherwise the fans carry on going.

Does anybody have any idea what might do this to a relay?

Any help much appreciated.

[scoule]

Either you aren't getting a nice solid 12v across the coil side of the relay, or the load is so great that the contacts are sparking...

I'd tend to believe you don't have a good 12v across the coil side. Measure across terminals 85 & 86 on the relay ... It could be bad wiring / connection to the otter switch and shorting the switch still means the bad joint / wire is in circuit.

[dougal]

Thanks for the reply, this is what I would tend to think, I just don't know enough to really start looking in the right place. What should I be looking for on a meter? I don't understand why having the engine running would make it happen and not when it's off, if anything I would have thought it would be the other way round?

[GreenV8S]

Perhaps a brown-out on the switched ignition feed? Current would be higher with the engine running hence more voltage drop. Should be easy enough to [dis]prove with a voltmeter.

[dougal]

OK, got to admit to being a bit thick here, but what do you mean by that?

[GreenV8S]

When the relay closes and connects the fans, the fans will suddenly draw a lot of current. If there's a resistance in the supply, this will make the voltage drop. If the circuit that is controlling the relay has the same source then the voltage drop may cause the relay to open again. If this happened then it could happen repeatedly, but you would probably be able to stop it by disconnecting the fan. If so, that would tend to confirm that it is a current/voltage drop related problem. If not, there's some other effect at work.

[red griff 500]

I had exactly the same symptoms on my 500. The problem was due to not having 12v across the fan(s) relay coil. The Otter switch is designed to pull one side of the relay down to 0v (the other side is permanently at 12v), thus you have a voltage drop (aka potential difference) of 12v which activates the coil which in turn pulls the switch contacts together.

To check this, when the otter switch is open i.e. the engine is cool measure the voltage (ignition on)between each of the two wires and earth. One of them will be 12v and one should be 0v (although you may find that it measures up to 7v). The 12v wire is the one which is connected to the relay coil and the lower voltage one should be connected to earth (0v). If you measure anything other than 0v, then this is your problem. This was certainly mine, and the solution is easy, simply make up a wire with a spade connector and connect this to the 0v side of the otter switch, and connect the other end to a decent earth. If yours is the same as mine, then the "earth" thAT tvr selected is not a decent earth, but it is a "pseudo" earth from the alternator (you may find that the voltage you measure is the same as a wired connector pin on the back of the alternator). On mine, this voltage varied between about 1v and 7v.

Just for the sake of completeness, when the otter switch is closed i.e. engine hot and fans on, both wires on the otter switch should be at 0v because the switch is pulling the relay down to 0v.

I hope this is clear.

Steve

[spend]

Steve, the 'W' terminal on the alt is for the rev counter on some Landies. I think the earth lead you are talking about is supposed to be connected to the alt casing (like the suppressors) - but some cars definitely have it connected to the 'W' terminal??????

Not sure if the factory originally connected it to the casing or to the W term and left the other end disconnected? But if it is connected to ground it will probably cause some strange affects.

My 'W' terminal has no connection, and the black lead is connected to the casing - 6k miles no problems.

Dave

[dougal]

Thanks for the replies chaps, gives me something to work with.

[red griff 500]

Spend, I have not connected the W terminal (I didn't know it was called that) to ground, but merely disconnected the ground side wire from the otter switch and made a new ground connection to the otter switch. That way the W terminal is not connected to ground. By the looks of the wiring looms, I'd say that it was wired to the W terminal from new and if that's the case the previous owners must have put up with a load of unecessary hassle

Cheers

Steve