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MG MGB Technical - Gauges Dead

The Tachometer, Fuel Gauge and Temperature gauge, as well as the signal lights on my '80 MGB are intermittently cutting out. Not sure where to start troubleshooting.
Joe M.

Start with the fuse box.


Joe. The place to start is the green wire circuit which comes off of the rear of the third fuse down from the top on the fuse box. Use a wiring diagram to trace forwards from there is that is not the root cause. You will also loose your turn signals and brake lights if the fuse is bad or you have a bad connection in this area.

Les Bengtson

Same problems with my '80 turned out to be a bad relay. Also check the connections. Ben Cordsen
Ben Cordsen

Hi Joe

I had the exact same problem last summer on my 73 B - it would blow fuses after driving for a few minutes. In my case, the green wire that leads to one of the switches to the gearbox (I think it was to the seat belt warning switch) had come off and was dangling beside the gearbox. When I rounded a tight corner at speed, the wire would swing to the side and touch the gearbox and short out the system. The connector on the end of the wire has a rubber cover, but it was worn off on the corner probably from being like this for years (I disconnected the actual buzzer as soon as I bought the car so never noticed when the wire came off).
Incidentally, this problem began at one of the slalom events, which obviously involves lots of cornering!

What helped me to trace the system was to photocopy my electrical diagram, (enlarged it abit too) then have it laminated, so now I can draw all over it with markers and wipe the marks off when I'm done - got that tip from another post in this BBS. Also, my mechanic pretty much new where the problem was without even looking at my car!

Erick Vesterback

Fuses. I have had this problem recently, no tacho, no fuel gauge and no indicators. Fuse looked unbroken but when I tested it on a meter it was dead. New fuse, no problems. Fuse is 2nd from the bottom, 17A constant blow at 35A.

On my V8 this fuse also covers the electric cooling fans, so things got a bit warm !
Andy Torode

Are your brake lights working? Wipers? Washer pump?

It is common for the contacts in the fuse box to be corroded, especially on the back.

You can get a nice, colored wiring diagram from <>

Print two copies, one to make notes on as you check the circuit and one for reference.

A problem with the ignition relay on a North American *should* affect the ignition as well, but not on a UK spec. If the ignition is unaffected then almost certainly it will be associated with the fusebox - either a dodgy fuse, corroded fuse holder, corrosion on the rivetted connection on the back, or where the connectors on the wires go on the spades. I'd advise using a voltmeter on the circuit with everything connected rather than an ohmeter, it is the nature of bad connections that they will be more likely to fail the greater the current that is forced through it, and an ohmeter only generates micro-amps. OTOH such a small current can sometimes be too small to burn through the normal oxide that develops on switch contacts for example, whereas 'normal' currents are fine, leading you to believe a component is faulty when it isn't.
Paul Hunt 2

Everyone was pretty much on the mark, but Andy hit the nail on the head. Based on the initial feedback, I pulled the 3rd fuse and it looked ok. Put it back in and all was well - I presumed that the fuse and/or holder was dirty. Headed out from Virginia to Ohio, and all was fine until 50 miles from our destination in Columbus. This time I pulled the fuse and cleaned the fuse ends and holder - no luck. So, I replaced the fuse and all was well again. Upon a closer inspection of the original fuse, I found that the solder connection holding the fuse wire to the end cap was broken. Hence it was a hit-or-miss as to whether it worked.

Thanks to all.

Joe Mercer
Joe M.

This thread was discussed between 14/06/2006 and 20/06/2006

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