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MG MGB Technical - Rheostadt bypass

I removed my non-funtioning rheostadt to make way for a manual choke cable on my 1979 B ( Fed. Spec.). I soldered all of the wires ( 3 r/w & 1 r/g) together and the dash lights appeared brighter than ever. Tonight, the dash lights are no longer working and the tail lights do not light, but the turn signals, brake lights and emergengy flashers all work. By soldering the terminals together, have I added too much resistance to the the 3 r/w wires ? Any help in catching the "Lucas Gremlins" that have invaded my B would be appreciated.

Also, taillight bulbs are halogen which have been in for about one year.

Cheers, Davy

PS: Hope I get some sleep tonight !
Davy Crocker

Check the headlight switch, it may have failed. Many people have bypassed the reostat in similar manner you have with no problems to have brighter dash lights.
Kimberly

Davy. I agree with Kimberly. I have two vehicles modified in the manner you describe. Both have worked well for several years. The basic modification is not the problem. Do not know what effect the halogen lights will have on the system. If the current draw is equal to, or less than, the factory bulbs, that should not be a factor either.

An expanded copy of the wiring diagram, laminated in clear plastic, would allow you to trace out each non-functioning circuit. Begin your trouble shooting at the common point between the circuits.

Les
Les Bengtson

When my Rheostat gave up I tacked a tempory wire from the map light to the instrument lights (I would have to read the circuit again to see where this is.) I do recall I did this because I was in a hurry both points were easily accesible and electrically common with what I wanted to do. It worked so well it has becopme permanent, I think I did this in the 70s.
Stan Best

If the tail and instrument lights are not working it is the first position of the main lighting switch (or the connections to it i.e. the red/green) that is the problem, not your mod to bypass the rheostat. If that had failed it would only affect the instrument lights, not the tail lights. You should also find that with the main lighting switch in the first position the parking lights in the headlights are not working either. Both front and rear parking lights run through the top two fuses in the fusebox, but again this would only affect the parking lights and not the instrument lights, so the main lighting switch does seem to be the culprit.

It's quite common for bypassing the rheostat to make the instrument lights brighter, all the ones I have seen have been printed circuit types, and the failure mode of these is for the low-resistance section that gives maximum brightness to burn out, leaving a higher resistance section which results in dimmer lights. I used to think that the rheostat was marginal in its rating, but having replaced it in three different cars when I first bought them, I haven't had a problem since. Also the number of lights went up from 4 on the ealier cars to about 9 on the later, and these seems to affect all ages of cxar not just the later ones, indicating that rheostat rating isn't the problem. I now think that what happens is that one of the instrument lights shorts out at some point, which will burn-out the rheostat in short-order.
Paul Hunt 2

So, does this mean that my rheostat is burned out when turning the knob has no affect on light brightness?

Is it worthwhile to open up the rheostat and try to clean contacts? I won't consider buying a new rheostat if it will only produce the same dim light and be subject to burning out when a dash bulb burns out.

The lights come on, and you can actually see this if it is a moonless, starless night and one throws a dark tarp over the windshield to create that wonderful experience of being in a cave!

Are those aftermarket higher wattage bulbs another solution for brighter dash lights?

Thanks for the assistance.

BobA
R.W Anderson

Likely it is. Your best bet is the bypass mod described above. If this isn't bright enough, you can fit halogen bulbs, and remove the instruments to paint the insides white.

I think we are just used to modern instruments, where the numbers are illuminated, that or it doesn't get dark anymore.

My old midget had the bulbs under the dash, and illuminated the instruments because they had holes around the rim, with coloured celluloid inside, I never had any problem reading them in the dark back in 1983.
(Or perhaps eyes really do go dim as you get older!)
Martin Layton

The rheostat doesn't burn out when a bulb burns out, but when the wiring shorts out. They are expensive to replace.

If turning the rheostat makes no difference to the brightness, but the lights are on, then it may have been bypassed already, the lights are hardly too bright at best.

With the printed circuit types if the low-resistance circuit is burnt out it will still vary the brightness but only between off and something significantly less than full brightness. If completely burnt out I'd expect the lights to not glow at all. Opening it up is pointless, other than out of interest.

I've heard tell of wire-wound rheostats but never seen one. If the contacts on these are bad you may get flickering as you turn the knob and erratic brightness, and opening one of these up to retension the slider contact may be of help. When these fail I'd expect them to go out altogether.

There was a flurry of fitting Radio Shack 7.2v bulbs some time ago but they are bit like a firework - bright but rather short lived! Halogen bulbs is a popular recommendation now, but they run much hotter than tungsten even for the same wattage, so much so that only 2w versions are recommended for some instruments as the excessive heat can damage the instrument. For other 5w and 10w are recommended, but then you are getting the extra brightness from the higher wattage as much as the modern technology. Higher wattage versions also pull more current, and may burn out the rheostat on later cars with lots of bulbs.
Paul Hunt 2

This thread was discussed between 09/05/2007 and 15/05/2007

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