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MG MGB Technical - Blown a fuze

Literally (3rd fuze from top) ! Twice now - while the fans were running - one of the green circuits both fans turning normally - standard BL fan motors /V8 loom with towing electrics - everything working seemingly normally.
Anyone know how much current these motors are supposed to pull ? Could the fan relay short out when hot ? The thermostatic switch is from the classic range rover - (large/brass with two contacts) - should this be added to the list of suspects ?
Thoughts gratefully acknowledged. Thanks

R
Roger Walker

I gather from your opening statement, the fuse blows out when the fans are in use. Check to see how much current these fans are taking vs. what the fans "should" be taking.

My 79 MGB had "low performance" with electrics. One by one I replaced the female connectors and cleaned the terminal tabs on the fuse box as well any male connector I could see.

Cheers

Gary
79 MGB
gary hansen

IIRC -10A together, but more if one of the fan motors is "struggling". Each fan is probably slightly more than half that. (Most meters won't read more than 10A)

Presumably you can test the thermo switch by joining the wires together. If you disconnect both fans you should hear the click of the relay (ignition on, engine not). Or a meter in place of the fan would show 12v when the relay was energised. If it is just too much for the fuse you could disconnect one fan temporarily, then the other.
I think the early wiring has both primary and secondary thru the green circuit (and ignition switch). Later a feed was taken from a brown wire and only the small primary relay current thru the green circuit. The latter secondary circuit off the brown was not fused unless a mod was added.
Michael Beswick

Gary & Michael - thanks - on both occasions the engine was hot (in traffic) and needed the fans. I wasn't paying much attention but noticed the tacho drop off and the indicator lights fail. I pulled over and changed the fuse and both fans started together - the fuse is 35A so the normal 10 amp needed by the fans is well covered - I am beginning to wonder if there is a temporary short while something is switching on . Good advice about testing the relay - I have the standard dash control mod to switch the fans on manually - which operates normally.
Roger Walker

Those glass fuses are only 35 amp for a momentary surge, like when first switched on. The continuous rating is 17 amp.
Mike Howlett

The original V8 fan wiring powered both the relay and the twin motors from the main green circuit fuse, using an unusual 3-terminal relay. This is along with the HRW, ignition, brake lights, reversing lights and everything else including tach and indicators. That has to be really pushing it.

I think later things were changed to a 4-terminal relay powering the fans off the brown at the fusebox, but unfused, as mine and another I've seen were modified in the same way. Later North American spec (same twin fans) had a separate fuse to begin with, and then a thermal cut-out.

I didn't like them being unfused so fitted one (I also fitted a relay and fused the HRW off the brown rather than the green). Just recently when one of the motors stalled it didn't blow the 17/35 fuse, but it did melt the holder.

Both fans running take 10A, but that is without the engine running, it will be higher when the alternator is charging. With one motor stalled that goes over 20A.

A partial short anywhere in the green circuit could be tipping the fuse over the edge when the fans are running, it may be nothing to do with the fan circuit itself. If when they are switched off both fans come to a halt at more or less the same time then the motors themselves are probably OK. If one motor stops very quickly then it is binding and taking more current than it should, which should be felt when moving them with a finger-tip ... unless both are binding equally of course!

The original thermostatic switch only has one terminal, as it sends an earth picked up from being attached to the inlet manifold to operate the relay. If your switch with two terminals does the same (with the addition of a wired earth) then it won't be anything to do with the switch.
paulh4

Mike - thanks - I should have known that - it puts a different complexion on the problem.
Paul - thanks again - I have always thought the 3 pin fan relay arrangement a bit odd. Modifications coming up ! It will be a 30 amp relay. The simple fan test tells me nothing except both motors being 8 years old might share an age related problem.
The cooling system has a Range Rover pipe from the thermostat, hence the Classic RR two pin switch. It's useful to be able rule that out !

Thanks again to everyone

R

Roger Walker

Another useful mod is to move the fan relay 12v supply from the ignition to the accessories circuit. This takes the load of the fans off the battery when hot-starting i.e. fans running. It does mean that they continue to run for a while with the engine stopped if you are sitting in the car with the key in the accessories position i.e. listening to the radio, though.
paulh4

Paul - A useful thought - thanks
R
Roger Walker

Paul has it covered there, but I would add ...

While making changes you could -
- add a manual override switch, in parallel with the thermal switch.
- add an indicator light, in parallel with the relay output, so you know when the thermal switch or you have turned the fans on.

A very worthwhile change is to make better ground connections.

I don’t recall the exact details, but both fans eventually share a ground point with the headlights, way back near the fuse box, and so there are a number of bullet connectors along the way - including a multiple connector that is vulnerable to age and weather. The best thing is to arrange ground points nearer the fans ( and lights ) and run more direct wiring to them ( for the lights also ). The bolts holding the bonnet latch assembly are probably a good place.


J N Gibson

"A very worthwhile change is to make better ground connections. "

Amen to that! The twin fans in the V8 harness share the headlight/indicator earth that goes back to by the starter relay. I added local earths for each fan i.e. in parallel with the original earth, and even when adding the second one while the fans were running resulted in an additional increase in speed.

Another mod was to use the spare output spade on the alternator for a brown back to the brown terminal on the fan relay, which effectively results in a ring-main which halves the volt-drop under high-current conditions, and for the fans as they now have a 12" path back to alternator rather than one of several feet, the volt-drops are a fraction of what they were.

Another source was in the relay itself, which was losing 0.8v and getting quite warm, so replaced.
paulh4

JN - thanks - the manual over-ride was the first mod I made to the car - the points about earthing are really important especially for rubber bumper cars where the airflow is restricted. The fans are the most important ancilliary in the V8 and need to work properly.
I am running a more powerful alternator Paul without a spare terminal but will look at the ring concept.
R
Roger Walker

OK, then you could tap another heavy gauge wire into the existing output wire by the alternator. Personally I would not do this with connectors due to the inevitable slight increase in parasitic resistance and consequent lost voltage with high currents that would cause. I'd solder, and only by cutting a section of insulation off the existing wire, not cutting and re-joining.
paulh4

the fan motors and the green circuit all check out. Everything is working - all together ! The sole untoward event hasbeen a failure of the green circuit revealed when I used the manual switch to turn on the fans. Nothing happened ! My initial reaction was to suspect the fuse had again blown but inspection revealed it was loose thereby ineffective.
Should one suppose that a poor contact in the fusebox led to the fuse overheating and eventually failing. On the second occasion that it failed, I noticed that some solder had dropped onto the bottom fuse ?
Roger Walker

Certainly a bad connection will heat up when it's passing a current, and that could get hot enough to melt solder. Given the solder on the fuse below, I think it almost certain that is what happened. Normally one would first become aware of a bad connection in the green circuit from the indicators being slow, and slowing even further if the brake or reversing lights are on. It's not long ago that I replaced both of my fuse boxes, as being preferable to trying to clean them up.
paulh4

Paul,
Thanks - that's reassuring.
Gary,
Looks like your advice was spot on but, i'm too old and lazy to clean the terminals. I shall follow Paul's advice and get a new box.

Perhaps there is a lesson here for us all.
Roger Walker

This thread was discussed between 12/11/2017 and 24/11/2017

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