Moss Motors, Ltd

My 1979 rubber bumper has had one failure in 15 years, but always had a very low illumination in the speedometer, tach and the other two dials. (almost unreadable in night driving) On the contrary, the hazard and fan switches are very bright, Does anybody know how may I fix this problem.

I would suggest three possible scenarios.

#1 The grounds to the cases of the tach, speedo, oil/temp and fuel gauges are a little on the shoddy side and therefore a lot of the voltage to the lamp is being dropped there and not across the bulb filament as it should be.  So, check grounds.  However, having said this I would also say that it would be EXTREMELY remote that ALL the grounds are too high in resistance.  Mother natrue doesn't generally work that way.

#2 Take the gauges out of the dash, disassemble and repaint the interior of the case a glossy and bright white.  It is very likely that the paint niside the cases has deteriorated, darken and even flaked off. I did this with my 72 Midget and it helped some.

#3 It could be the that the rheostat controlling the lamp illumination is also too resistive even at its minimum resitance (brightest light) setting.  I don't think it would hurt to by pass the rheostat entirely for a little bit to see if there is any noticable difference.  I don't believe (but I could be wrong as I don't have a schematic in front of me now) that the lamps on the switches are in the circuit with the rheostat that controls the gages.

Bear in mind however, that even in the best case the illumination of the gages was fairly poor even when new.  At least that is my experience, and they will never compare to a modern vehicle.

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Bob - Safety Fast

But drive it like you stole it!

Thank you very much Bob. I will start at point #3 to leave out the rheostat. Also check voltage at the lamps. I suspect my car losses a lot of voltage all along the wires.

I did a little checking. And, according to the wiring diagram I have for a "late model" MGB, the lamps for the gauges and the switches are both fed from the rheostat.  So, if you don't see any dimming effect on the switch illumination, then you have something funny going on with the wiring.

Cheers.

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Bob - Safety Fast

But drive it like you stole it!

For comparison the instrument panel rehostat control on my 73 Volvo was doing the same thing......that is not illuminating the bulbs properly to the gauges.  The light was dull and feint and you could barely read the gauges.  The switch squeaked when rotated and small white particles of residue dropped off the knob spindle as dust below the knob entering the switch.  I sprayed CRC which is a type of WD40 into the shaft that entered the switch, kind of like a contact cleaner.  Problem solved.

Hi

I had the same problem and according to Ohmmeter it was broken. I removed the rheostat yesterday and took it apart, it was full of dirt and i cleaned it up with WD40 and a bit sanding on the brass surfaces, and it works well after that, between 1-12 Ohm resistance. (It was a really thick layer of dirt on the contacts) Time to do this 1/2 h,  the bulbs was also changed the old ones was black of age.

 

Had the same problem.  Measured the output voltage from the rheostat and it was only

5.5 volts.  By-passed the rheostat and the illumination is bright.

Thanks to everybody for your help.

This is what I did yesterday:

1. with engine not running, I switched on parking lights (no low beams on)

2. removed one lamp socket from one instrument

3. removed lamp from socket

4. checked voltage at the end contact inside socket, while slowly turning rheostat

5. voltage started to increase from zero to about 7,7 volts, around mid turnig of knob

6. still turning voltage dropped to zero

7. turned a little more and voltage showed up again

8, reaching end of turning, voltage reached only 5,4 volts

9 removed rheostat for change or repair

10. checked voltage at incoming wire to rheostat : 11,6 volts

11.my conclusion: faulty rheostat, that also eats a lot of voltage

12. will try to open rheostat by breaking two visible rivets. I don't know if I will be able to close it again.

13. momentarily, I by-passed rheostat by connecting the one incomming cable to two outgoing cables. Ilumination is little better now. Not extraordinary. Will try to ckeck earthing elsewhere in the dash.

 

Thanks to everybody for your help.

This is what I did yesterday:

1. with engine not running, I switched on parking lights (no low beams on)

2. removed one lamp socket from one instrument

3. removed lamp from socket

4. checked voltage at the end contact inside socket, while slowly turning rheostat

5. voltage started to increase from zero to about 7,7 volts, around mid turnig of knob

6. still turning voltage dropped to zero

7. turned a little more and voltage showed up again

8, reaching end of turning, voltage reached only 5,4 volts

9 removed rheostat for change or repair

10. checked voltage at incoming wire to rheostat : 11,6 volts

11.my conclusion: faulty rheostat, that also eats a lot of voltage

12. will try to open rheostat by breaking two visible rivets. I don't know if I will be able to close it again.

13. momentarily, I by-passed rheostat by connecting the one incomming cable to two outgoing cables. Ilumination is little better now. Not extraordinary. Will try to ckeck earthing elsewhere in the dash.

 

If you get "not extraordinary" lighting bypassing the rheostat it begs the question, why would you want to add install something that only makes it dimmer?

 

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Bayless
Never express yourself more clearly than you can think
'48 Prefect
'67 Sprite (project)

mine when this thing happened in my volvo, i did some DIY.. here's how i did it.

first is to locate the knob or dial that controls the brightness of the dashboard lights. 

make sure that the brightness control is turned on.

check the fuse that controls the dash board lights. 

if still not working try to consult a volvo repair mechanic.

11.6v in front of the rheostat is about right, and remember that a rheostat can be thought of as nothing more than a "volume knob" which can have shorts, dead spots, etc etc etc...

You can easily short around it, and if you STILL don't have what you believe should be a solid AND CONTINUOUS 11-12v, you might also think about swapping out the voltage regulator/stabilizer under the dash.

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tob

1979 MGB

I don't believe the lights will be on the stabilizer so that shouldn't make any difference.

 

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Bayless
Never express yourself more clearly than you can think
'48 Prefect
'67 Sprite (project)

The lights are not controlled by the voltage stabilizer.  The stabilizer only provides 10 volts to the temperature and oil pressure gauges and nothing else.

 

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Cheers,
Dave
http://homepages.donobi.net/sufuelpumps/

Yes, you are (both) correct... I mis-read my wiring diagram; a fat-finger crossed over to a GAUGE wire

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tob

1979 MGB