MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - What the ....Lucas?

My 67 has a fuel gauge that started drifting towards empty then back up to the correct level...then drifts back down near empty and so on. I replaced the gauge with no change, then the sending unit... no change. Then I am driving this eveing and my left headlamp goes off, then on every few minutes, and it is not a loose connection that I can find. 12 volt single battery converted to negative ground. Unmolested harness is several years old and car NEVER gets wet. All other gauges work fine and lamps are fine? Any thoughts? Ground issue on battery cable?
Ralph

Dodgy voltage stabiliser? There is a thing up under the dash somewhere that provides the power for the fuel gauge. I believe what it does is slowly vary the voltage up and down so that if averages out to about 10 volts to drive the fuel gauge (to average out changes in voltage you get when the car is running and the alternator is charging the battery I guess). If that has gone wrong maybe it would cause the gauge to do what you saw. Not sure how it would affect the headlights though?

Simon
Simon Jansen

Ralph,
"it is not a loose connection that I can find"

Keep looking, you have one. Things going on and off while driving is a strong indicator. It may represent a loss of voltage or ground. Your first obstacle is getting these things to remain dysfunctional until you get back into your garage.

Check your fuse block. Push on your fuses and spin them with you finger. I once spent hours checking wire voltage and connectors only to discover an oxidized fuse clip was my problem.
Also suggest that you check all secondary ground connections, they're easy. Look for black wire clusters attached to the body. You'll find them in the engine bay, behind the dash and in the trunk (behind the tag). Your primary grounds are those braided steel straps attached to your left motor mount and on the left side of your transmission.
If the above doesn't offer a solution, prepare yourself. Make an enlarged copy of you car's wiring diagram and with a voltmeter start checking wire voltage, moving back from the dysfunctional gauge or light. Check the wires and their connectors for continuity. If your leads are long enough, check voltage and continuity from your gauge to your tank sending unit (green/black wire).
I believe your fuel gauge sends 12 volts to the tank sending unit (green/black wire) and your "Bi-metal instrument voltage stabilizer (BMIVS)" supplies 12 volts to the fuel gauge (light green/green wire). The BMIVS gets its voltage from the inboard side of the lower fuse (green wire). This fuse gets its voltage from the ignition switch, via the outboard side of the lower fuse (white wire).

You need your ignition switch on to check for voltage in your fuel gauge system. Suggest that you remove white wire from your coil before doing any long term evaluations.

Have patience, you'll find it.
Steve Buchina

Ralph - Simon is correct in guessing a dodgy voltage stabilizer, but somewhat off in the operation of the thing. It works like the flasher of the turn indicators, in that it is a bimetalic strip with a coil of resistance wire around it. Like the flasher is makes and breaks the current path and the on and off periods average the voltage at around 10 volts. It sounds like the stabilizer in your car is stuck in the closed position, therefor supplying full voltage all the time. As the engine is reved up, the generator produces a greater voltage and the gauge reads higher. When the revs drop, the voltage drops and the gauge reading will fall. You can check this out by just sitting still in the car and reving the engine up and down while watching the gauge - if it follows the revs, then the stabilizer needs to be replaced. You can even make your own, solid state stabilizer, following the instructions on Rick Astley's web site at http://www.mgcars.org.uk/electrical/ Click on Instruments, then Stabilizer. Good luck - Dave
David DuBois

Ah, OK. I didn't know if varied smoothly or just switched. I have one sitting somewhere out of the car. I'll have to go put a meter or scope on it and watch what it does!
Simon Jansen

Hi Ralph.

The headlamp symptom is due to a bad connection or possibly bad bulb.
The headlamps share a lot of common wiring, the fact that only one is misbehaving tells us that the problem is local to this lamp.
If it happens on main and dipped beam it is in the ground side, if it only happens on dipped it is in the feed side, possibly in the bullet connector at the front of the car where the LH / RH 'split' occurs.
Blue / red is for the dipped beam, blue white for main beam, black for ground.

Don

Don

Think this is two seperate issues. The fuel guage and the headlights are totally seperate systems on the 67.

Stabilizer is the probable issue for the fuel line, especialy if you have the original. only things that are tied into the stabilizer are the blinkers and they come off the input side so would not be affected if the unit failed.

Since the headlight wires, including the dimmer switch run by the fuel guage, I wonder if you pulled one of the headlight wires when you changed the guage? I would start there.
Bruce Cunha

Thanks guys. The headlight did indeed have a broken wire right at the three prong plug. Wire coating was intact but the strands were separated. The fuel gauge today runs up with the key on...then upon starting the gauge begins to slowly recede unless I rev slightly...then it climbs again only to settle and start over. Last night while driving, the gauge settled at empty and stayed there no matter the speed/rpm level until just before I arrived home and then it began to wander again. I will replace the voltage stabilizer next but I thought it would go bad altogether as opposed to this fluctuating?
Ralph

Ralph the stabilizer is completely bad. It appears to vary from on all the time, allowing the gauge to fluctuate with the rise and fall of the generator voltage, or it opens (no voltage, which cause the gauge to go all the way to empty) and remains that way for a period of time. Good luck - Dave
David DuBois

Ralph, I seem to remember that the voltgage stabilizer supplies both the fuel gauge and the temp. gauge. Is your temp. gauge fluctuating also? If not, it may not be the volatage stabililzer.

JIm
74B Roadster
James Conner

The temp gauge on the '67 is mechanical. Ray
RAY

If the tach continues to read when the fuel gauge drops then it could be the stabiliser, the green 12v feed to it (but after the tapping for the tach),the light-green/green between stabilser and gauge, the gauge, the green/black between gauge and tank, the sender, or the sender ground wire. If a voltmeter shows 12v at any of these points the problem is further towards the tank ground. If it shows 0v then it is back towards the stabiliser and 12v supply.

If the tach fails as well then it is in the 12v supply to both, the fusebox is the usual culprit.
Paul Hunt 2

This thread was discussed between 19/09/2005 and 21/09/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.