Killer question Graham; No, I don't have an accurate wiring diagram for a 1988 +8 efi - does anyone?

The only one I've found on the web (again, the GoMog site) is generic and suggests that the Otter switch is breaking the earth wires, but without testing yet, my own fitting looks ominously like one black and one red, and red usually implies Hot!

Red.
Yes, but the switch could still be on the earth side of the relay. You'll have to get the test meter (or lamp) out I'm afraid...

Cheers,

So are you suggesting Graham that both leads could be "dead" (as in Not Live)and that jumping across the terminals on the Otter (and/or introducing a switch) will complete the earth circuit, energise the relay and start the fan ...?

No, not at all.
If the switch is open (i.e. drawing no current) then the 'hot' side will be at a nominal +12V, regardless of whether it is on the live or earth side of the relay coil.

'Fraid I'm not much good at trying to draw things on a PC (pencil & paper works best for me every day ), but in the simplest form of the fan control circuit, imagine a wire from the +12V side going to the relay coil. From the other end of the relay coil another wire goes to the 'hot' side of your otter switch. The remaining wire from your otter switch goes to earth.

The alternative is for the +12V to go to your otter switch, then from the switch to the relay coil, thence from other end of relay coil to earth.

Normally, if the switch is used to switch to earth it would have only one terminal, the earth connection provided by the switch body where it is fitted into the radiator - but this isn't always the case.

Before you can fit a manual switch you need to know in which order the relay and switch are wired. Probably easiest to do by pulling the fuse and measuring with an ohm meter what connects to where. My guess is the relay is probably hidden behind the dash, which makes things a trifle more difficult...

Cheers,
Graham

Thanks Graham, I can't wait to start poking around with a test-light and a fire extinguisher
Hopefully I will advise more in the morning ...

Good luck!

Further note, if you are using a test light:
If the 'earth' side of the otter swich goes up to 12V when the switch is on, the switch is on the +12V side of the the relay coil.
If the 'hot' side goes down to 0V (light goes out) when the switch activates, the relay is on the +12V side of the relay.

HTH

Sorry Graham but when I said test light, that's what i meant, so the result is, test light stuck onto red wire and earth, light comes on (hot); same light onto black (with orange tracer) - No Glow
So now I have power and earth to and from Otter (makes sense?) and absolutely no sign of a relay with similar leads (all wires just head into or out of general loom)dare I add a switch and warning light into that circuit, should it be fused in any way, and other advice??
Thanks for being there - John

OK John,

Do the earth side test with the engine hot, so the fan is running.

If you have 12V on the earth side when the fan is on then the otter switch is on the +12V side of the relay (or fan if there is no relay). This would agree with the wiring colour code (see below).

If that is the case you could connect a manual switch across the otter switch connections. Also makes fitting an illuminated switch easier.

I'd suggest a reasonable quality switch - if there is no relay then the cheap switches won't cope with the current for very long.

Under BS-AU7a wiring codes (vintage 1983 ) black with an orange tracer is "radiator fan motor to thermal switch". However, this may still go via a relay. If you are going to tap across the otter switch itself, then the circuit fuse will be sufficient, however, if you plan to take +12V from another source to the switch, down to the black/orange wire then fit an extra fuse. Also, keep the same colour codes as you presently have - it makes fault tracing years later somewhat easier!

I use Vehicle Wiring Products (a Google search will find them) for most of my electrical supplies.

Cheers,

Brilliant Graham - next stage is going to have to wait for a few days as there are domestic jobs required this weekend, but I now have a plan ...
again, kind regards
John