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MG MGB Technical - Fan relay

Advise please on wiring a relay for the fans on a V8. I have just replaced the relay but seem to have lost the note I took of what went where, i.e., terminals 30, 85, 86 and 87, help!
B Clayton

This pinout data should help you

http://www.teamrocs.com/technical/pages/relay_basics.htm

volts to pin 30 and switched output to 87, coil between 85 and 86. I note in this article the author has 86 grounded and this is almost certain to be -ve ground, which a V8 will be. This matters because otherwise you will blow up the diode which I call the suppressor. Relay willwork afterwards but my "splash" onto other kit.
Stan Best

This diagram should help if you want to add a manual override switch for the fan

DK McNeill

Note the original V8 fan relay only had three terminals, the green 12v supply on C2 supplying the current for both the relay winding and the fans, orange/black from the thermostatic switch on W1 and C1 to the fans on black/green.

To use a more conventional 4-terminal relay but keep the wiring the same you will need to link one of the winding terminals with one of the contact terminals, say 85 and 30 with the green on that, with 86 having the orange/black from the thermostatic switch and 87 the black/green to the fans. For a Lucas 4-terminal relay you would link W2 and C2 with the others being as before

Note that some relays have two spades on one terminal, so you have to look at the individual riveting on the base, not simply count the number of spades.

But you will get better performance from the fans if you connect a brown 12v supply to the contact, via an in-line fuse, and use the green supply just for the relay. And even better if you add individual earths for each fan motor to where their brackets bolt to the slam panel.

That diagram doesn't make sense for a V8 as the sensor only has one terminal, picking up an earth to operate the relay from the inlet manifold. If it was added to a 4-cylinder then pin 86 should be wired to an ignition switched source such as the green terminal on the fusebox, not an 'always live' terminal, otherwise the fans could be running all the time the car was parked.
PaulH Solihull

Sometimes know I'm a bit slow on the up-take, it took me ages to realise what "C" and "W" meant on a 6RA, i.e.Contact and Windings. In other words "C" are the large current carriers and W are the windings for the electro-magnet and therefore take the, smaller, "triggering" current.
Allan Reeling

I am confused here.
I had my fan wired through a relay for several years and all worked fine. I changed some electrical last year and improperly wired the relay in which resulted in a constant power drain.
While trying to figure out how to wire it properly via different internet site and my notes, I ended up confusing myself with too much info.
Ok, I am getting older!!

I simply wish to have the relay supply power to the fan thermostat so the fan will operate (if needed) even if the ignition is off

Will this work
30 = 12V power
86 = Ground
87 = goes to terminal on thermostat
Wire goes on the other thermostat terminal to fan
85 = wire to fan

MGBV81

30 12v supply
85 12v supply
87 to fan
86 wire to thermostatic switch and thus to earth
Allan Reeling

Thank you very much Allan for simplifying it for my poor muddled brain

Bruce
MGBV81

Correction, the *fans* on a 4-cylinder won't be running all the time as they are ignition powered anyway and the suggested relay only bypasses the thermostatic switch, but the comment still holds true as far as avoiding the relay being operated all the time while parked. A V8 only needs an earth from a manual switch to be connected to the black/orange wire at the relay to operate as a manual override, and if it is an illuminated switch as mine is you can see that the thermostatic switch is operating when it should.

As Allan says for the factory V8, as that only has one identifiable terminal on the thermostatic switch, the switch picks up an earth for the relay by dint of being screwed into the inlet manifold. Bruce's statement:

87 = goes to terminal on thermostat
Wire goes on the other thermostat terminal to fan

implies to me two terminals on the thermostat which is more like the 4-cylinder thermostatic switch and that would need wiring differently to operate with the ignition off (which in my view is a Bad Thing in any case).
PaulH Solihull

I was just heading out to wire this up and took at quick look at the BBS

I want the fan to work even if the ignition is off.
The temperature in the engine bay actually rises when I shut it off after a drive on a hot summers day. I want it to be at least at the preset temperature when I re-start the car.

The thermostat temperature control has 2 terminals as does the fan itself.

I had it wired to do this before but really do not want to get into what I did to mess it all up.

Will Allan’s post re relay pin work for what I want to do?
MGBV81

Bruce-

30 = B+ 12V (always hot)
87 = Load (fan motor)
85 = control - (ground through thermoswitch AND/OR manual switch)
86 = control + (Power from IGN or always hot)

You can fit a manual override switch in parallel with the thermoswitch.
IF you want the fans to operate with IGN OFF, then 86 must be always hot.

FRM
FR Millmore

Thank you FRM
I will wire that up.

Bruce
MGBV81

This thread was discussed between 11/03/2011 and 21/03/2011

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