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Fan Relay feed wire

Postby Bob Owen » 30 Aug 2017 18:34

Following a telephone enquiry about the fan relay feed wire, I investigated and found the situation to be more varied than I thought. Others may have further input or find the following useful (O&OE!):

The original wiring is different for different V8s. I know of three variants – there may be more…..

For the following descriptions note that the original fuse box has 35A 1 ¼” glass cartridge fuses 1 to 4 and terminals 1,3,5 and 7 on the left and terminals 2,4,6 and 8 on the right, all numbered top to bottom and looking on to the fuse box. The load side of the fuse is normally the right hand side/even numbered terminals.

Scheme 1: The Workshop Manual Supplement for the MGB GT V8, AKD 8458 Issue 1, had the fans fed from fuse 3 input (fuse box terminal 6) a branch from the ignition switched circuit (unfused). Following BL/Lucas (BS-AU7) convention this ignition switched wire was coloured green. This is the likely wiring on early V8s.

Scheme 2: The Driver’s Handbook, AKD8423 4th Edition, has the fans fed from fuse 4 input (fuse box terminal 7) which is direct from the battery (unfused) and so independent of the ignition switch. Following BL convention this always live wire was coloured brown. This may be the scheme on late V8s.

Scheme 3: My 1974 N registered chrome bumper V8 had a different (unrecorded?) scheme. This is also the case with others of the same vintage as I found when helping Tudur Jones with a fan problem on his N registered chrome bumper V8 whilst on the 2017 Portuguese trip. Perhaps it was the result of a service note? Here the fans are fed from fuse 3 output, terminal 5. This fuse is fed from the ignition switch and in this scheme also feeds the tacho, fuel gauge and indicators. which is convenient as a fuse failure, so loss of the fans, will show by the loss of these secondary functions. The fans have the advantage of being fed from a fused circuit but they are still ignition switched. My fan circuit and battery feeds are changed from the original but Tudur reports that his fan relay fused feed wire is green/black. BL wiring colours state this is indirectly from the ignition switch and specifically a fuel gauge wire so this makes sense.

The disadvantage with Schemes 1 and 3 is that the fans turn off with the ignition and so can turn off while running or will not turn on if heat soak after stopping the engine operates the fan thermostat. This can result in local boiling in the block with the resultant pressure exceeding the 15psi cap limit and blowing water out of the expansion tank. Most modern cars have the fan circuits independent of the ignition switch so that fans can run on after switch off, and can even start up after switch off (there’s usually a “Health and Safety” warning to this effect on the bonnet slam panel.) Although the water pump is not running, thermo-syphon effect will still cause some water to circulate if the fans are blowing air through the radiator so preventing post switch off water loss (some now use electric water pumps).

The disadvantage of schemes 1 and 2 is that the fans are unfused. A seized fan motor or other fault poses a fire hazard.

A fourth scheme is to use scheme 2 but add a fifth fuse so that the fans will run irrespective of ignition but are also fused. This is the scheme I use. In my case the battery feed is also split so that the starter solenoid and ignition circuits are via a battery isolator switch but the cooling fans and horn and (added) clock remain always connected. This partial isolator is then useful as a security enhancement as well as for safety when working on the isolated circuits.
Bob Owen
Posts: 49
Joined: 14 Dec 2012 09:18
Location: Reading

Re: Fan Relay feed wire

Postby John Young » 31 Aug 2017 15:37

Well, mine has the same arrangement as yours which meant when the fan motor seized, I lost the rev counter and indicators. It is an early '76 model and the problem was cured by a new motor and I don't have overheating problems although it does turn off with the ignition.
John Young
Posts: 18
Joined: 18 Oct 2015 10:27
Location: Cheltenham

Re: Fan Relay feed wire

Postby Bob Owen » 31 Aug 2017 17:46

John, thanks for that input.
So it looks like V8s built from '74 to '76 use a different scheme to those shown in the quoted versions of the Manual Supplement and the Driver's Handbook, which in turn differ from one another. I wonder which V8s (if any!) conform to which documentation?
Bob Owen
Posts: 49
Joined: 14 Dec 2012 09:18
Location: Reading

Re: Fan Relay feed wire

Postby Robert Rose » 01 Sep 2017 07:34

The British Leyland handbook as supplied with your new MG ( or any other BL vehicle at the time ) did not always reflect the up to date specification for the vehicle supplied. At first glance this appeared somewhat bizarre especially when one viewed the publication date of the Edition supplied, at the rear of the handbook.
I have previously contacted the Nuffield Press to query the disparity between the publication dates of the Hanbooks and the information provided therein. The response I received was that the edition dates shown at the back of the hanbooks were "projected" publication dates and not the actual date of publication. This does not fully explain all the anomalies but add in the Industrial chaos of the 1970's and it surprising that there are not even more disparities.
# 1646
Robert Rose
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Joined: 21 Aug 2015 15:00

Re: Fan Relay feed wire

Postby Bob Owen » 01 Sep 2017 12:07


You make a good point. In fact I remember from my own early engineering experience just how laborious producing documentation was back in those pre-computer days. You couldn't just edit a piece of text and all drawings had to be done by hand on a drawing board and changes were difficult and messy. Armies of people were employed in technical publications and drawing offices. And often they had only modest technical knowledge of what they were writing about or drawing and the originating engineers would often be impatient having to spend time with the "tech pubs" people. Of course, as ever, we didn't know any different at the time - that's just the way things were done. Only as technology advances does doing it how it used to be done give one feelings of despair at the prospect - and perhaps admiration for those that did it and managed as well as they did.

Interestingly I note that my Driver's Manual, in the Electrical - Fuses section P38, does in fact describe pretty closely the situation I listed as Scheme 3 (as per at least 74-76 cars seem to be) although the corresponding wiring diagram P46 shows my scheme 2.
However, even this text itself has disparities in that under "Fuse Connecting 5-6" (my fuse 3) they omit the tacho but include the temperature gauge, which on mine at least is a traditional capillary non-electrical type.

Perhaps the old engineer's reply "Read The B****y Manual" is not as reliable as it might be!
Bob Owen
Posts: 49
Joined: 14 Dec 2012 09:18
Location: Reading

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