Why did I blow out 2 rheostats in 1 week?

[Guest]

Does anyone know the correct voltage for the hot wire from the light switch to the rheostat? I cooked my second dimmer yesterday. I tested the volts and it is 12.4 volts constant, engine running or not when the master light switch is on. The second dimmer worked well for about 2 total hours and then very dim only.

I have since bypassed the dimmer and dash lights are full bright and wires are not warm to the touch. I am stumped as to why both switches cooked at only 12.4 volts.

 

[LarryK]

Have you checked a wiring diagram? Sounds like a short on the hot side? Check your grommets going thru firewall and fenders and anywhere along to lights. Check around dimmer for chafed wiring around steering column.

 

[TR3driver]

The source voltage from the headlight switch is supposed to be full battery voltage ... 12.4 is close enough with the engine not running, but it can be over 14 with it running.

Any chance you've upgraded your dash lights with brighter bulbs, or additional bulbs? Maybe for a radio or something ? As I've written before, the rheostat is very sensitive to extra load. Even the wrong type of bulb as a load may cause the rheostat to overheat when dimming the lights and burn out; while not overheating the wiring or blowing a fuse.

I'm currently leaning towards replacing mine with a simple 3-position switch, in the classic Lucas positions : Dim, Flicker and Off.

 

[71tr]

One of the british parts suppliers is selling a push button start switch that fits into the rheostat dash location. Perhaps this would be a better use for the real estate.

 

[Guest]

My camera automatically adjusts to low light and it was actually dusk when I took these tonight. I retested the bulb sockets and they are still at 12.4V constant. I'm going to just leave the dash lights with the dimmer bypassed. They look brighter in the pictures than they are, especially the hazard and wiper markers, but they are nice and bright as is. I just wanted to be sure I didn't have too much voltage going to the bulb sockets.

 

[Gordo]

Boy, I wish my dash lights were half as bright as yours! I drive home from work at midnight and keep a flashlight handy!!

Gordo

 

[Tinster]

12.4V seems fairly close to normal battery output.

DPO Pedro had the rheo replaced with a cigarette lighter.
A new rheo will yield dim, dimmer and dark dash lights.

I installed a nice looking on/off, lighted rocker switch
for my dash lights. I also took apart all the gauges, cleaned out the crud and polished the insides. Much brighter now.

I say forget the rheo.

d

 

[Dennis]

I'd definitely check the load like they're saying, too much current draw through the rheostat is causing it to heat up and burn through. You have me now thinking of cleaning mine up so you can actually see them when you turn the lights on!
Dennis

 

[Guest]

The rheostat is now no more than a dash ornament. I'm done. I don't like screwing around behind the dash any more than necessary, it's a problem just waiting to happen. They're on, they're bright, they're working, they're done.

 

[piman]

Hello Stirkle,

just to answer your question, "I am stumped as to why both switches cooked at only 12.4 volts."

It has nothing to do with the applied voltage. It is, as someone correctly pointed out load, which is amps. Connect too many extra lamps or replace the standard lamps with higher wattage ones will overload the rheostat.

Alec

 

[Guest]

Thanks Alec,
So the only way to get the true value from the wattage of your dash bulbs is to wire direct to the 12.4 volt hot wire from the master switch. That is where I'm at right now. I took the 4 new bulb holder assemblies from RB that had 5" lead wires and made a whole new harness for the 4 small gauges. I also made a 4 spade terminal, one plug is hot wire, one is 2 large gauges, one is 4 small gauges, one for future use if needed. Grounded the new harness to the back of the large gauges like the large gauges are already grounded and I was able to bypass the rheostat and original problematic wiring to the small gauges, which started my problem in the first place. The original bulb holder for the fuel gauge popped the spring inside and was touching the + to - inside the bulb holder. Thankfully the newer bulb holders from RB are better quality albeit plastic than the originals. These (not so good) pictures give you an idea of the newer bulb holders from RB.

 

[poolboy]

I see you made use of the ash tray opening,too.

 

[2wrench]

All said, I did bypas my rheo and glad I did.

Also, my "hazard" light is ver bright. Wondered
if I put the wrong bulb in or something, it's so
bright. It's like driving and boom! "HAZARD" is
jumping out of the dash at ya. Doesn't seem to be
a problem.

My turn signal indicators are not so bright, though,
and wonder if I stuck the wrong buld into the wrong
locale.

 

[Guest]

My hazard marker light is annoyingly bright also. I think you can pull it out from the dash and pull the bulb holder back a little so the bulb is not so close to the front. In this picture, you can see that the bulb looks too close. I'm going to try it next time I get in the car.