Faulty TR6 Fuel Gauge? - Fix Lucas Electrics with a Voltmeter!

The Triumph TR, the MGB, even the XKE and Astons of the time... all used Lucas electrics. Lucas has a pretty poor reputation, but it's undeserved. In actuality, they were pretty robust. If you're having trouble with that overdrive switch in your Austin Healey, remember that the switch is probably 60 years old--well past the intended design life.
A common issue with the Triumph TR6 (the Spitfire and GT6 too) is with the temperature and fuel gauges. These indicators are more or less both voltmeters that read the voltage of a circuit that includes a variable resistor of some kind (kind of like a dimmer switch). The fuel tank sending unit is basically just that, and the temp sending unit is similar but varies with heat rather than using a float like in the fuel tank. These gauges are supposed to operate on a fairly clean 10 volts, which is where a lot of problems with inaccurate readings happen.
Rather than throw parts at the problem (why replace the fuel sender when that's not actually broken?), it makes sense to learn to check a couple things with a voltmeter. That way you can diagnose your trouble before spending any money. Replacing parts is not only more expensive, but it muddies the water in trying to find the original problem--because now you've made numerous changes trying to fix something... and may have thrown more things off.
Hopefully this helps! Let us know what trouble you may have discovered!

[@davidwarr8600]

Good basic instruction video. Whenever I see someone working on the electrics on a British car I generally recommend Rick Astley’s book, Classic British Car Electrical Systems. It is a great resource. It has many colour coded diagrams and lots of troubleshooting tips. Astley also has a companion book just for MGB. Both books are very thorough.

[@midwestmotoring]

I haven't read it, but I'm interested. Thanks for the tip! I'm not going to get Rick Rolled am I?....

[@davidwarr8600]

@@midwestmotoring completely different Rick.

[@midwestmotoring]

I know, but couldn't resist making the joke. :)

[@MGB-learning]

Great video! It is paramount that the voltage stabilizer has a very good ground. In the case of MGBs the screw that secures the stabilizer to the back firewall behind the dash is the ground. The Triumphs are mounted to the gauge. In this case a good ground is equally important.

[@midwestmotoring]

That's a great point! Yes, a poor ground is often a failure point, causing inaccurate readings.

[@davidwarr8600]

I do agree that Lucas gets an undeserved poor reputation. Many people that criticize have no idea at all about British cars.

[@midwestmotoring]

From what I've read, it wasn't until the late 70s when British electrics started getting a bad reputation... and that's likely due to labor problems, poor management, taxes, exchange rates, poor government intervention, increasing competition... all the stuff that collapsed the industry. Hard to make a good product under those circumstances--even when it's been well engineered.

[@phildavis9504]

Great video as always. Question what octane of gas should I buy for my 75 TR6? Thanks

[@midwestmotoring]

If it's not a modified engine with higher compression, then regular pump gas is just fine. The compression in the stock engines is relatively low, so you don't need high octane.

[@jamcdona]

Great video. First off, as an electronics technician, I fall into the 'shotgun' approach to parts replacement very easily myself- and this is due to the fairly cheap cost of replacing parts. I've found that method can introduce another issue, as new parts can be fail on install depending on the part, and now you have more issues! BTW, my trip miles indicator works but I can't do trip reset on my speedo. The little knob is frozen. What do you suggest?

[@midwestmotoring]

Thanks! Most of the systems in these cars are very logical, and just need some patience and an easy diagram to figure out. Actually, many original parts like wiring and switches can be repaired rather than replaced too. For the trip reset, it's a twist knob and not a push button. I'm confident you know that, but someone reading the comment might not be. Anyway, this involves pulling the speedo, but you should be able to disconnect the cable from the gauge. I'd want to see if it's the cable that's binding up or the gauge. If the cable, you could try a little WD-40, or replace the cable with a new one or a used one from another gauge. I wouldn't go shooting WD into the gauge though, so if that part's frozen you might be looking for someone to do a rebuild. Might not want to mess with it until the winter.

[@jamcdona]

@@midwestmotoring So True! I recently installed a headlight relay kit, and the fuse holder was junk. Took a couple nights in the garage to figure it out. Then, my light dimmer stalk was intermittent on both high and low beams; I cleaned what contacts I could reach with out dissasembly, then sprayed contact cleaner. Worked like a charm! I was proud.

[@midwestmotoring]

@@jamcdona you should be proud. That's the sort of logical thinking that's necessary to keep these things running well. Sometimes solving those puzzles is my favorite part of the hobby.

[@frankmgallo]

I can relate to that oh yea no fun but hey it’s rewarding.

[@midwestmotoring]

After I finished I thought to myself it would have been a whole lot easier if I had just put the car on the lift, but I was too lazy to move the Healey.

[@terryatpi]

Mine runs out of gas at a 1/8 tank and the temp gauge barley gets over “ a quarter “. Is this similar to yours ? Did this correct the problem ( I assume it did ) Finally , you were able to leave the speedo in the car and got it from under the dash ? Thank you. Subscribed , Terry

[@midwestmotoring]

It's actually much more difficult with the speedo in the car, but yes it's possible to reach if you have small fingers. You're probably best off removing it for access. As for improper readings, if you can get a voltmeter up there, check for the voltage coming out of the stabilizer. If it's more than 10, then it's not working correctly. Caveat to that: the old stabilizers work on an average voltage--they turn on and off to make the average 10V--so you might briefly see higher than that if you're using a digital voltmeter. One problem with your car is that the fuel gauge seems to be reading high while the temp gauge seems to be reading low. The stabilizer might be at fault, but you probably also have something else going on at the same time. Your fuel sender might need a little bend, or your temp sender might be giving an inaccurate signal.

[@terryatpi]

@@midwestmotoring wow! Thank you. I’ll let you know how I make out. I appreciate it. I’m north of Syracuse (ny)

[@roselh1]

Neither my 59 Bugeye nor my 60 BT7 have voltage stabilizers as original. My 64 Jag that I no longer have did have a voltage stabilizer. Were earlier Smiths gauges 12 volts? If so, why did they change to 10 volts? Thanks. Lin

[@midwestmotoring]

Something you'll notice is that on the bt7, the fuel gauge needle flicks back and forth as the fuel moves around in the tank. The later cars were redesigned so the needles were more stable, and i believe about that time they were switched to 10v and damped needles. The fuel gauge on a TR6 behaves much more like a modern car. I'm not sure if there's a reason for the 10v, but it might have to do with the damping.

[@roselh1]

@@midwestmotoring yes. I have a Porsche 356 dampener on my BT7 to “calm down” the erratic needle. I am wondering if the gauge itself changed or just the current that flowed to if.

[@midwestmotoring]

I'm not sure but I bet it has to do with making the max voltage more predictable.

[@chriscook509]

I replaced my voltage stabilizer a year ago with a solid state one. Lasted a few months before it fried... chinese crap....

[@midwestmotoring]

I've had that happen before too. I wonder how difficult it would be to open one up and repair it? I'm probably not qualified to do so, but... how hard could it be?

[@ozwud9085]

@@midwestmotoring Probably not worth the effort. Just go to your favorite electronics supply vendor and get an LM2940T-10.0 voltage stabilizer, a 0.47uF tantalum capacitor for the input side, and a 22uF tantalum capacitor for the output side. Wire it all up and you'll have rock-steady 10v as long as the input is at least 10v. Pay attention to polarity on the capacitors. The LM2940T is made for automotive use and should last a long time. I did this for my GT6, the cost of the components was only a few bucks, much less than what the vendors charge.

[@midwestmotoring]

I'd love to do this. Is there a writeup somewhere, or some photos?