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MG MGB Technical - Lucas Gremlins Captured

Many thanks to Les Bengtson, Kimberly in San Fran and Paul Hunt for their assistance in helping me solve my rheostadt and lighting problems. It turn out that not only was the rheostadt faulty, but the lighting switch on the steering columb was shot. Removed the rheo and replaced the switch and all is functioning well at the moment.

Again, thanks "fellow B-Keepers , Davy Crocker
Davy Crocker

Ah, the classic case of "Don't assume that because you have found one problem you have found the ONLY problem.". Hunt's Third Law of classic car ownership, originated by Nory many years ago.
Paul Hunt 2

I have recently lost the dash lights on my 76B the reheostat is an open circuit. After connecting the leads together they became hot and were removed before any damage was done to the wires. The head lights and all the marker lights are operating. After reading your thread I thought someone may have some insight to help solve this problem. Does the rheostat have an amount of built in resistance and can it simply be replaced or is there a dead short in the instrument lighting circuit?
R.J. Spinozzi

R.J.-

If the reostat's internal contacts failed and are touching the case, it could be causing the short. Remove the reostat and install an inline fuse holder in it's place and see if the wire heats up.
Kimberly

If you are bypassing the reostat and the wire is getting hot you likely have a short in one of the sockets for the instrument lights. It may have been what killed the reostat as exessive current may have burned it out.
John H

The instrument lights were always dim at full power. I am thinking, that I should remove all the bulbs and see if the wire still heats. If it does then it must be grounding somewhere. If not then the rheostat has an amount of built in resistance in it.
R.J. Spinozzi

The rheostat is a variable resistance from about zero ohms to 8 ohms. All the ones I have seen have caused the dash lights to be dimmer than they should be (and they were never that bright to begin with, making the rheostat rather superfluous), because they were of the printed circuit variety and partially burnt out due to excess current.

The normal diagnosis for dim dash lights is to bypass the rheostat, which is basically what you did, but in your case if the wires are getting hot it sounds like one (or more) of the dash lights (or wires to them) is shorting out (they are unfused). So don't replace the rheostat until you have investigated that and got all the lights working properly or you will simply burn out the replacement, and they are expensive.

Ideally you would isolate each light and test it individually, but on a US (I'm assuming) 76 there appear to be no less that 11 or 12 lamps involved. Not only that but there appear to be two groups of sealed multi-way connections in the circuit - one with 7 wires and one with 6. Only two circuits (cigar lighter and hazard switch) seem to be easily disconnectable using bullet connectors, the rest seem to be 'hard wired'. The heater and main lighting switches may have spades on the back to isolate the switches themselves, and in the gauges the lamp holders can be unplugged from the case to eliminate the possibilty of a short in a bulb or holder. But that still leaves the heater controls and the possiblity that it is one of the red/white wires that is shorting out. It's a big job.
Paul Hunt 2

Thank you Paul,
That is pretty much what I suspected. Your expertice is valuable before embarking on a new mission.
Thanks again,
RJ
R.J. Spinozzi

Hello
Preved Acuna Matata!
Yxjsl

This thread was discussed between 25/06/2007 and 09/07/2007

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