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MG MGB Technical - Slow signals and gauges with fan on, ideas?

Hi all,
I'm trying to find the reason why my signal lights slow down and the fuel and temp gauge drop significantly when I have the heater fan on.
I've tested the voltage at the cig lighter and it never drops below 13.4 so I don't think it's a weak alt.
I've cleaned the grounds at the battery, in the trunk and all of the sockets. Anyone have any ideas on what it could be? I did replace the voltage stabilizer last year.

thanks

Ron
Ron

Check your voltage at the green wires on your fuse box with the same circuits on. You may find your voltage lower than at the lighter. If it's more than .1 to .2 volts lower you need find where voltage is dropping. Consider the ignition switch, fuse box connectors, bullet connectors and switches for possible voltage drops.

Clifton
Clifton Gordon

Ron, Clifton has the right idea, you've got some resistance in the circuit, most likely at the fuse block itself. I've found that Lucas fuse blocks are prone to corrosion on the back side where the spade connectors are rivited to the fuse clip. They corrode and loosen over time and can cause voltage drops or intermittant circuits. Try removing the fuse block and soldering the connectors to the rivit using a soldering gun or large iron and lots of flux. I think you'll solve your problem.
Bill Young

There are a group of ground wires bolted to the body under the dash to the right of the wiper motor, you may want to check them also.
John H

It won't be the ground wires behind the dash as the signal lamps use a different ground. A bad fan ground will slow the fan, but paradoxically this will cause *less* slowing of the turn signals. Bad grounds at the lamps will cause the flashing to be slow even when everything is off, which is quite likely to get even worse with the fan or anything else on. Unless you have the identical problem both sides one side will be slower than the other if it is lamp connections or grounds.

If they slow worst with the fan then the problem is in the green circuit as described, the fuse to holders connections can be bad, also there is a rivetted connection on the back of the fuse block, and it can be internal to the fuse as well. It could also be anywhere in the brown and white circuits before it gets to the green. The hazard switch is a very common cause.

The best way to track this (or quite likely 'these' as you probably have more than one) is to bypass the turn signal flasher so when you oeprate the turn signals the lamps glow constantly, this will make it much easier to track the voltage from the battery cable lug on the solenoid, through the ignition switch, fusebox, hazard switch, turn switch and out to the lamps. Also check for a small voltage on the lamp holder and casting which indicates bad ground there.
Paul Hunt 2

Ron, I know this sounds crazy but my fan was sounding like a strangled donkey when I put it on , and also affected the hazard warning lights,slow to virtual stop. I assumed that being an old car these were the type of things to live with. Anyhow to come to the point I had to disconnect the fan at the three pronged connector to get under the wire for another matter. On reconnection, nothing else, the fan works perfectly and so do the hazards.I've since revisted and cleaned just in case. Went out today,forty mile run everthing working perfectly. It makes you wonder doesn't it? Pure luck or what?
Bob
R Etches

The *hazards* should not be affected as you describe. The hazard flasher unit is designed to work with anything from 1 to 4 bulbs working i.e. very variable current, and with low voltage i.e. being left on the side of the road for some time after a breakdown or accident.

FWIW while poor connections in the fan plug will affect the performance (and sound) of the fan they will result in other circuits working better than before, if anything.
Paul Hunt 2

Thanks for the suggestions, all. I will remove the fuse box and have a look at it.
I recently installed an electric fan thermo switch from Brit-tek and have it wired to one of the power spades at the front end of the fuse box. Could this be an issue?
Ron

Your heater fan may have sticky old grease in the bearings, making it draw a lot more current than it should. I'm not sure if that would cause your symptoms, but if it were my car, I'd be cleaning and regreasing the fan motor. Cleaning connectors is always a good idea, and solves lots of problems. Look at a gun shop for a little wire brush to get inside there.
Tom

You really need to investigate where the volt-drop is occuring first, you could work on or even replace several components and not land on it, which will be very frustrating. If it were anything to do with the cooling fan then the problem would happen when that were on, not the heater fan. Cooling fans take quite a bit more current than the heater fan so some slowing with that is inevitable with the original alternators, even without bad connections.
Paul Hunt 2

This thread was discussed between 24/03/2008 and 27/03/2008

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