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MG MGB Technical - Cooling fan, temperature gauge not working

Hello all,
I recently brought my car back from an autobody shop where it was painted and now there a few electrical items are now no longer working.

At first, the turn signals/indicators were not working and when I brought it back to the shop they managed to find the problem- there was a green wire running to a connection next to the carburator that was loose.

Now, however, the cooling fans and temperature gauge do not seem to be working - a bad combination to fail. (I drove the car 40 miles home in occasionally bumper to bumper traffic and the fans never came on once and the temperature gauge always reads Cold. This does not seem normal. The fans usually came on after a 3-4 minutes of idling or slow traffic.) The cooling fans were working when I took it back to have the turn signals fixed. The temperature gauge may or may not have been working all along.

One mechanic who tried to fix the turn signals apparently replaced a relay on the dashboard on the passenger side or so he said. He also found one burnt fuse and replaced it. All fuses seems to be fine. I checked under the dash to see if I could find anything obviously wrong I found two rectangular boxes with connections. One had a green and green/brown wire connected to it. And the other rectangular box had no wires connected. Not sure if this normal or related.

Any suggestions would be appreciated. I have looked through the archives and found a lot of helpful tips but I thought I would present the specifics of this problem to see if anyone could give me a better idea of where to start.


thanks,
Jim



Jim

I would doubt that a green connection near the carbs would have any effect on the turn signals unless it were partially shorting to ground and bad connections in the fusebox were dropping all the voltage instead of blowing the fuse.

I presume it is a late model with factory electric fans. The fans have their own in-line fuse, probably a brown/white to a green near the fusebox. The temp gauge shares the same stabilised voltage as the fuel gauge, so if the is working the stabiliser itself is OK, although the light-green/green may have become disconnected between the two. The temp gauge is operated by a variable resistance ground on the green/blue from the sender in the head. Try grounding this wire both at the gauge and the sender - just long enough to see if the gauge moves - and that should tell you whether the gauge, the sender, or the connection between them is at fault.

The two boxes are the turn flasher and the hazard flasher. The turn flasher uses green and light-green/brown wires, the hazard flasher (which seems to be disconnected) brown and light-green/brown, but note that the two light-green/brown wires are *not* electrically the same and getting them crossed will cause problems with the hazard and turn flashers. Make sure your turn signals light up as soon as you move the switch, then only go off after a delay. Also check that with one bulb disconnected they either glow but don't flash, or flash at double speed. If they flash at the same rate it is a serious safety hazard. Hazard flashers usually come on shortly after the switch is operated and are designed to flash anywhere from one to four or six bulbs at more or less the same rate, but must not be used as turn flashers.

Paul Hunt


I forgot to mention it's a 79 mgb roadster with two factory cooling fans.

I have a question about how to ground the wire at the sender. First, to be sure, is the sending unit wire the green wire that runs to the head from the harness below the air pump and near the alternator? If so, there is a female connector at the end that it uses to connect to what I assume is the sending unit. Do I remove the female connector and simply touch the wire the chassis? Also, if this is the correct wire I would probably have to temporarily disconnect the another wire that runs to carburator - the same one the autobody mechanics said was loose - in order to be able to touch the wire to the chassis. Is this correct way to ground this wire?

Regarding the hazard flasher, someone has connected the brown and lightgreen brown wires to a silver cylinder (a relay?) with marking 552 12V Wagner. The hazard lights are working. Should I just leave this or reconnect them to the hazard flasher?

I checked the cooling fan inline fuse and it seemed to be fine. I remember one mechanic had replaced the original inline fuse with a more "modern" fuse saying it was more robust. The fuse is a transparent green plastic covering inserted into black rubber. It does not appear to be burnt or blackend so I will assume for now it is ok.

Finally, the only other electrical item that I have noticed not working in the car in addition to the cooling fan and temperature gauge are the "key-in- ignition-and-door-open" buzzer.

Thanks for your help.
Jim
Jim

Like I say, the temp sender wire should be a green/blue. Just use a seperate wire (like a meter wire with a croc-clip on one end and a probe on the other) between a good ground and the temp sender, the wire doesn't have to be off the sender, and it is better not to have to pull other wires off in order to make a harness wire reach a ground point.

The cylinder sounds like an alternative flasher unit, just make sure you do the safety checks on both hazards and turn signals as listed above.

You will need a voltmeter to track the 12v supply through the inline fuse and the rad switch via the blue/green to the fans to see what is wrong.

For the key buzzer you will again need a voltmeter to track 12v from the ignition switch on the purple/pink through the door switch onto the purple/green to the buzzer unit, which also has connections to the drivers seat belt (yellow/brown), warning light (yellow/purple), purple (fused always hot) and black (ground), and maybe a white/red from the ignition switch if you have a service interval counter.
Paul Hunt

This thread was discussed between 12/11/2002 and 13/11/2002

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