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Triumph TR6 - Smoke from Under Dash

1972 Triumph TR6. Long time MG owner, first time Triumph owner. Bought this car three days ago and am going through that long, often scary process of figuring what I got, what is good, and what is bad. Day one - temp gauge was working and I noticed it because it seemed a bit high to me (running just under the red H section). Day two – temp gauge not working at all, check fuses etc, nothing, all seemed good. After reading the TR6 archives, I tried testing voltage at temperature sensor lead which I read should be around 10 volts, I had 11.5 and thought that was probably close enough (now, maybe not). When I grounded the lead, the gauge went to full hot so I figured “OK, it is the sensor.” Day three – replaced sensor, during test ride, temp gauge immediately (like in 30 seconds) goes to hot so I think “Crap, I got a faulty sensor.” As I am heading home, a whiff, then actual smoke from between my knees. I stop the car and check but can’t see anything burned or melted. Roll it home – current condition: temperature gauge AND fuel gauge not working. 0.05 volts at the temperature sensor lead. Grounding the lead shows no movement of the gauge at all. What the heck is going on? Any help or advice would be greatly appreciated. This is not a good start; good thing I didn’t sell the Midget yet. Thanks, Matthew
M Moment

Welcome...long time MG Midget owner here thats recently converted to the TR6 myself. The temp and fuel gauges run through the "voltage stabilizer" MG should of had one too. On my 74 TR6 it's located on the back of the speedo unless I miss my guess. On my 71 MG it was behind the passenger section of the dash.
JT White

Oh, it's a small rectangular device...if the 72 and 74 are similar.
JT White

So, are you suggesting that the voltage stabilizer is now "toasted"? Do you suppose it was a faulty new temperature sensor that I installed that caused the stabilizer to burn out? Obviously, I want to repair the original problem, not just replace parts. Thanks, Matthew
M Moment

Matthew, I'm just letting you know that is what both intruments have in common.
JT White

Matthew

A high voltage in your circuit will creat a high reading on the guage. These gauges are current meters and subject to ohms law V=IR (voltage equals current times resistance). R is dictated by the sensor. At a given sensor temperature, R will be constant. Thus higher V will read higher I (the meter reading) .

It sounds like your voltage stabilizer has fried. As JT says - it supplies current to both of those gauges and nothing else and is a likely cause.

I am assuming you have checked the fuse again after this and it is OK.

I would be wary though as to what may have caused it to fry. The fact that your indicator climbed immediately to H indicates the sensor may have been faulty or grounded. This would potentially have caused it to overload the voltage stabilizer.

But it would be very unlucky to get a faulty sensor I would think. Haven't heard new sensors to be DOA before. Others may have a different experience though.

I would go with faulty sensor or grounded some where it shouldn't be and that it fried the stabilizer.

This is also assuming that the fuel gauge was working properly and is not a possible red herring in the problem.

Let us know what you find.

Regards
Mike Petryschuk
Michael Petryschuk

Mike, he says he "grounded the lead" to test the temp gauge...while a plusable test it could have "done the deed" on the voltage stabilizer.
JT White

Another thing worth mentioning is for anyone buying a british car...take a look at ALL fuses and make sure someone hasn't put U.S fuses in ther...especially 35amp U.S...most things British will "burn up" before the U.S. rated fuse blows.
JT White

Guys: The reason I tried grounding the sensor lead was that was one of the recommended (more than once) tests to check that the gauge was not faulty, from an earlier TR6 archive. Nobody objected in those posts. I just crawled under dash and took some voltage readings on the voltage stabilizer : the "power in" (solid green) side is reading 11.8 volts (battery a little low) and the instrument side "out" (light green / green) is reading 0.07 volts with engine off, key on. Sounds fried to me; that side should be 9-10 volts right? Now, what fried it? I'd be very interested to know if grounding the sensor lead as a test of the gauge is a "no-no" (so I won't do that again)! While I also question the DOA sensor theory, it's the only thing that makes sense at this point. The only thing I did prior to "the fry" was the gauge (grounding) test and installing that new sensor (purchased from Car Quest btw). Also remember, I drove for about 5 minutes or so before getting the puff of smoke from under the dash; it didn't happen while sitting in the garage when I first started it. Thoughts appreciated.
Matthew
M Moment

Matthew

Just to clarify- My comment on grounding was intended to pin point a ground in the circuit while you were driving. Not the grounding of the gauge you did to see if it would climb. I am sure excessive current (through a short circuit) in the stabilizer will over load it. I will say that while possible I can't think a ground while driving is the likely happening.

I have no experience with grounding the temp gauge to test operation but I will comment that overloading the temp guage with too high a current can potentially damage it. I don't know what too much current is though and another factor would be how long it is in an over current position. Another way to test the gauge is use hot water and dip the sensor in it. Have a thermometer available to confirm the reading. This avoids grounding it.

Maybe the whole problem was the voltage stabilizer (about to fail) and the fact that it followed shortly after the new sensor is just coincidence.

Maybe your alternator voltage control regulators are failing or failed and you are getting too high a voltage in the circuit. It would manifest itself when you are running at higher engine RPM as the alternator will be generating higher voltages and current. Is your volt meter reading on the car gauge consistent with the reading you get at the stabilizer?

Did you buy a new stabilizer and install it? Is it working OK?
Michael Petryschuk

Mike:
Thanks for your feedback, I appreciate it. I am expecting a package today from Moss with a new sensor and voltage stabilzer among some other items (Bently workshop manual!) sometime later today. I installed a new battery and two new cables yesterday. All contact points were well cleaned and coated with dielectric grease. I'll let you know what happens.

Glad you enjoyed McCartney show, saw him myself a few years back in NJ; great performance!

Matthew
M Moment

Closure for this thread (for now). Everything is now working properly; fuel gauge works and the temperature gauge not only now works, but is now sitting dead center while at normal operating temperature. Here is a summation of what I did that corrected this issue; remember, I just bought this car:
Installed a new battery (old one was 9 years old).
Installed new battery cables.
Cleaned all battery cable connections very well, and used dielectric grease.
Installed new temperature sensor (from Moss).
Installed new voltage stabilizer (old one DEFINITELY toasted, black burn found on back of circuit board). Made sure it was grounded properly.
I don’t know what originally caused what happened (owned car for three days at that point), and if my “grounding the temperature sensor/gauge test“ contributed to problem or not, but maybe this information can help someone else down the line; I hope so.
Thanks to Mike and JT for your input.

Matthew
M Moment

This thread was discussed between 08/08/2010 and 12/08/2010

Triumph TR6 index

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