MGB & GT Forum

When I rewired my 1975 MGB roadster, I replaced the small dash bulbs with LEDs and left the rheostat in place. Now, everything is really dim. Unfortunately, I'm having to remove the dash again to put in a different speedometer, speedometer cable and heater cable. so while it's off, I need to do something with the dash lights. I can put the old bulbs back in, or leave the LEDs and get rid of the rheostat - which I prefer. I already have enough switches that no longer do anything, so I would prefer to put a filler in, rather than abandon the old one in place. I don't see much reason to have a switch there to turn them off and on, so I wonder if anyone has found anything that will fill the hole and look ok.

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My mechanic told me, "I couldn't repair your brakes, so I made your horn louder." (Steven Wright)

The doctor must have put my pacemaker in wrong. Every time my wife kisses me, the garage door goes up.

“There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves.” Will Rogers

Well with Lloyd's help I sourced the parts necessary to have a low oil pressure (15psi) idiot light there but then that's onmy Mk1 where the "dimmer" is between the speedo & tach.

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Mk1: CKD 11/66 first registered 8/5/67; owned since 3/77. 18GB +40 balanced. Peter Burgess BVFR head. Piper 285. 123. FidanzaFW. 4synch c/r box. Lots more as I did a nut and bolt rebuild; finished 2015. Tartan Red.

GT: December '78. VW Golf guards, flush fit front and rear valances. Torana XU1 vents, frenched indicators & Mk1 rear lights. 'Worked' Rover V8 with Monsoon ECU for EFI. GM4L60E, Lokar tiptronic & Quick4 controller. Vintage Air A/C. FC IFS. CCE 4 link rear. Salisbury with Quaife. Jaguar Storm.



Edited 1 time(s). Last edit at 2017-10-15 10:17 PM by MGB567.

I still retain my rheostat, but in the same vicinity, I have added a tell tale light which illuminates when the electric cooling fans spring into life, either on their own, or when I turn them on. The light is very small. and when off, a small dimple is the only thing visible, but I have the old metal dash.

LED's are dim? Did you try bypass the rheostat when you installed them? When I bypassed the rheostat on my car I simply left it in place, I like the dash to look as original as possible. You can look for plastic plugs available at local hardware stores, Lowes/Home depot or eBay that will fill the hole.

BTW- I do like Barrie's idea of an oil pressure idiot light. That's worth looking into!

I deleted the rheostat and added a green light for the OD when it engages.
You could also add a fuse in place of the rheostat since this circuit is originally unfused (I witnessed a dash harness fire before because of that!)

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My 1958 MGA is not a car, it's a CAREER..!! ...

Thank you all. Those are some great ideas. I will add a post to this thread when I get it done. I really like the idea of an OD light. I already have a fan light on the console below the radio.

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My mechanic told me, "I couldn't repair your brakes, so I made your horn louder." (Steven Wright)

The doctor must have put my pacemaker in wrong. Every time my wife kisses me, the garage door goes up.

“There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves.” Will Rogers

Ken,

I have a couple questions and suggestions...

1) You noted that your LED dash lights are now dim and that you have left the rheostat in place. Have you measured the resistance of your rheostat? I can't say what the rheostat specs are, but mine measures between 6.2 ohms and 2.4 ohms when the rheostat (potentiometer) wiper is on the wire winding. At full CCW, it is open circuit, and at full CW it is a closed circuit at 1.4 ohms.

2) These rheostats can often go "bad", but, they rarely go "BAD-BAD". They can usually be brought back to working like new by spraying some contact cleaner into the housing (WD40 also works just fine) and then rotating the knob back and forth a bunch of times. What happens is that crud and/or oxidation builds up between the wiper and the wire winding and, by exercising it and flushing it with cleaner, this brings the contact resistance between the wiper and winding back down to where it should be.

3) When you are using LEDs, because they draw such small currents, they are more sensitive to the effects of the contact resistance between the wiper and winding in the rheostat than the incandescent bulbs.

4) A few months back, Herb Adler (ozieagle) posted a potentiometer circuit that he successfully used to dim his LEDs. As I recall, he used a 390 ohm resistor between the rheostat and ground and connected the 12 v LEDs to the "high side" of the 390 ohm resistor. I do not recall if he was using the original rheostat or one with a different resistor range, so, if you're interested you can search for LED dimmer and Herb Adler to find the post.

You might be able to bypass the rheostat without pulling the dash? I guess it depends on the size of your hands....been a while since I looked behind mine.

One common way to bypass is to move the output wire over to the side of the input.....both sides have two blades, so moving the output over the power just comes into the tang and right out to the lights.

You could do that quickly when you pull the dash loose to atleast verify that fixes your problem. Other problem is a bad ground. I don't recall if Rick Astley in his book or on here, recommended adding another ground path from the instruments to the body...which I did and landed it over where the ground is on the wiper motor mount.

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1974-1/2 Roadster, "Bumble Bee", Corvette Yellow - in shambles, wire wheels
1976 Roadster, "Virus", Sandglow - "driver" condition (stock + 32/36 Weber DGEV, cast iron header, 25D distributor), bolt on wheels, ON the road!

When I installed a supercharger and needed a place to route the vacuum/pressure line for the boost gauge, I ran it where the rheostat originally was. Worked out great. RAY

On my 77/80's I put the choke cable on that spot, stable because the metal of the dash is behind there, but hey those don't have the 75 dash.

I tried the easiest suggestion first. Removed the switch and bathed it in CRC cleaner. It now rotates easier and the lights are much brighter, so I'm going to go with that. I checked the ground with a direct wire to the battery and it didn't make any difference. Thanks to all, again.

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My mechanic told me, "I couldn't repair your brakes, so I made your horn louder." (Steven Wright)

The doctor must have put my pacemaker in wrong. Every time my wife kisses me, the garage door goes up.

“There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves.” Will Rogers

Ken
These instrument lights are all on a separate 'mini' harness. (below is similar to yours)

Make your life easy and check the harness, the dimmer, earth etc. for integrity while you have the dash off the car. Easy peasy and no more behind the dash shenanigans.

B

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Life's most persistent and urgent question is, "What are you doing for others?"

I bypassed the dimmer switch on my 1979 B and the dash came alive! Going to convert to HS 4's this winter and plan to put the choke cable there.