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MG MGB Technical - erratic gauges

I have a 1974 mgb converted to electonic ignition,saturn alternator, modern fuse block with relays. The other day I noticed my tach bouncing at low speed but stablizing at higher speed.I then noticed gauges working and then not working.The car has plenty of starting power. Now it seems to be running like it is trying to loose a cylinder.
Could this be a coil going bad. I have noticed a little moisture from the heater valve dripping on the distributor . Which could be having an affect but would that tie into the gauge issue.also has new electronic voltage stabilizer for gauges.
B Stone

The poor running is probably antifreeze shorting out your distributor cap. Better rectify that problem soon or the engine won't run at all. It could also be the cause of the tachometer needle bouncing around. Make certain that the voltage stabilizer has a clean ground, if not it will cause the gauges to give false readings. RAY
rjm RAY

Electronic ignition can cause problems with tachs, it depends what type you have and how it has been connected, although usually 73 and later models with the voltage pulse tach are OK.

Whilst water in the distributor will certainly affect the HT and hence running, and could affect points to cause the tach jumping and a misfire, with electronic systems it's very unlikely to affect the tach.

A coil? There is only one coil, if it was going bad it would affect any cylinder and you would be getting a jerky misfire in time with the tach jumping about if it was the LT winding, or no effect on the tach if it was the HT.

The instrument voltage stabiliser only feeds the fuel gauge and the electric temp gauge. If the oil gauge is dropping as well, that isn't fed from the stabiliser, so it's more likely to be the ignition supply, which could equally well be the cause of the tach and the misfire i.e. a single cause.
Paul Hunt

Additional info about the gauges. The tach stops working at idle but works fine other wise. and the fuel and water temp also stop working but at times will start to work again. The turn signals will not work and seems to kill the gauges when i use it.Could it be the voltage stabilizer affecting gauges and turn signal? it is a new turn signal switch and electronic stabilizer. Engine runs fine otherwise.
B Stone

some tachs sometimes don't work straight away from start up but this might be irrelevant in your case

have a look at Paul's post again the VS doesn't affect the turn signals, the fuel and temp might be something to do with the VS, not fitted securely so not level or poorly earthed, poor connections, or it might be part of another problem

the turn signals were on the same fuse and line as the VS and gauges so you could look there

more information is on Paul's excellent web site and a link to downloadable colour (color) wiring diagrams - http://www.mgb-stuff.org.uk/hammertext.htm

or you check all your connections on your modern fusebox and relays including the actual fuses and fuse connections and fuse box connections

plus the electronic igniter's delicate leads inside and outside dissy for chaffing

your possibly leaking heater valve will not probably help matters you'd best sort that, they often seize up and leak through lack of use or regular testing, you might be able to remove yours clean the crud out of the the engine and heater valve then clean and lube the valve and the control cable and operating mechanism to get it working if it isn't already - new gasket will be needed of course
Nigel Atkins

This is indicative of a poor connection in your electrical system. Check all of the connections than you can get to and make certain that all of your grounding points are clean. RAY
rjm RAY

I had a tachometer that would not move until the car moved. It was the needle was sticking onto the little rest post on the face.It would also work if you tapped the face. I put a miniscule piece of plastic heat shrink on it and it worked well after that.
Sandy
ss sanders

"The turn signals will not work and seems to kill the gauges when i use it"

That's the classic symptom of a bad connection - good enough to power the gauges OK, but as soon as you apply any extra load such as the indicators, which take a lot more than the instruments, the bad connection breaks down altogether cutting everything it feeds off.

The brake lights take the same current as the indicators, so if they don't cause the problem that indicates (ho ho) the problem isn't with the fuse or its holder - assuming the brake lights work of course! It could still be that one of the green wiring connectors isn't making a good connection with its spade at the fusebox, this would be the one with two green wires in that go into the main harness. The third green wire is usually separate and feeds the brake light switch.

Another diagnostic is the reversing lights. Because they come off one of the two wires that share a connector, if they fail as well then it is that fusebox spade.

But if they continue to work when the other things have failed, then it's the 4-way bullet connector (I think there is only one) behind the dash. One wire from that feeds the tach and the voltage stabiliser, and another wire feeds the heater fan and the indicators. As both those circuits are failing, it must be the wire from the fusebox that is feeding that bullet connector that is making the poor connection.
Paul Hunt

This thread was discussed between 23/04/2014 and 05/05/2014

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