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MGB Late MGB Wiring Questions

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glemon

glemon

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I just got a '79 MGB LE, all is pretty good with the car except for some of the electronics.

I am working on getting a manual, but haven't yet, I am pretty familiar with british cars, but everything I have had has had 60s or 50s era Lucas wiring, the late '70s cars have a LOT more stuff going on.

Anyway,I have some issues I am trying to sort out, any ideas on likely problems (or better even yet solutions!) appreciated.

Gas guage does't work, new sending unit didn't help, everything seems to be hooked up OK.

Temp Guage doesn't work, everything seems to be hooked up OK, any way to test the sensor?

Back up light no work, new switch on tranny (I didn't install it the previous owner did) I get power back there, but the power doesn't seem to be enough to light the lights, lights light when hooked up to good power elsewhere, 12 volts at rear lights until I hook up the 2nd back-up light, then it drops to about 3 volts, weird.

Any suggestions of likely culprits or good tests appreciated, I have a multi-meter and know enough of how to use it to be dangerous.

P.S. Hazard lights don't work, plug is not hooked up under Dash, and back half of switch appears to be missing, pretty low on my list of concerns, mostly mentioning it in case that might affect the rest.

Thanks, Greg
 
2nd fuse from the bottom of the fuse bloc - grounds for the fuse bloc, connections at the fuse block.

Bet the wipers & turn signals also don't work...
 
Thanks Tony, The turn signals and lights work, not sure about the wipers haven't tried them on this car I am thinking my chances on the wipers are 50/50.

I will check the wiring and ground around the fuse box.
 
Greg,

a good wiring diagram always helps and test light or meter always helps.

Go to: https://www.advanceautowire.com/

Look on the left side of the page for schematics and follow the link.

The gas gauge and temp gauge are powered from the voltage stabilizer. It is a small silver can located high up on the firewall near the inertia fuel switch under the dash on the left side. As Tony pointed out it, is fed from 3rd fuse (the green circuit). It takes 12 volts in and outputs from 12 to 0 volts (giving an average of 10 volts).

Check your grounds at the back and front of the car. In the back, the grounds are usually tied to one of the bolts holding one of the license lamps on. On the front, the grounds are on the right fender near to the fuse box.

Let us know what you find. Good luck.

Pat
 
Thanks so much for the schematic, I was surfing with no luck for one, but knew there had to be something somewhere, Greg
 
The wiring diagram helped, I had two problems, the voltage stabilizer was wired wrong, with the wire going in and the wire which was supposed to be going out hooked up to the same terminal.

ThenI used the diagram and advice of Tony and Pat and poked around the fusebox area.

I was getting power when reverse was engaged (and ignition on) but it was weak.

Kept looking had one Green Black (GB) wire coming out of a four way union that should have been all green, and a GB and Green going into a single union.

GB is the fuel guage sending unit, so I had the weak voltage from the sending unit going to the reverse lights and full voltage going to the gas guage.

Swapped them to correct and Bingo!

Cannot test the temp sending unit/guage without waking the wife, which would not be a good thing, but I am guessing that problem is solved as well.

Thanks again for the help.
 
Update, the tempguage is alive, on a long drive today iy crept off the peg just barely past the green "cold" section. Is this normal for a late 70s MGB, has been desmogged, maybe it runs cooler?

Anyway happy it is working as long as I know what my normal reading is it can surve the purpose of showing what is nromal and what is outside normal.
 
Glad to hear you have had some success today.

My bet is the sending unit and not the gauge is the problem. You can check by disconnecting the wire from the sender and connecting it straight to ground. Turn on the key for a few seconds and see if the gauge swings all the way over to the HOT side. If it does, that says gauge is good and it is time to replace the sender.
 
Sounds like it's running too cold! I'd put in a new thermostat.
 
I just bought the car, I assume it has a thermostat, but haven't checked, thanks will do the sensor guage test as suggested.

I hope if anything is needed it is the sender, as I recall they are cheap, and I can see easy to access.

Greg
 
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