MGB 72 MGB Fuel and temp guages

[sonuvagunsmith]

In trying to troubleshoot an inop temp guage, replaced the voltage stabilizer with a solid state one. Now my fuel guage is inop as well and the bezel light doesn't work. Any quick hints on what I've done wrong?

 

[Kimberly]

Knocked something loose?

 

[PAUL161]

Well "Smith", Make sure all connections are tight. Also make sure the ground wire that goes to each instrument is in place and tight. A bad ground behind the dash, and elseware, will drive you crazy. /ubbthreads/images/graemlins/crazyeyes.gif Just another possibility /ubbthreads/images/graemlins/rolleyes.gif PJ

 

[sonuvagunsmith]

/ubbthreads/images/graemlins/smirk.gif...actually it's supposed to be son-uva-gunsmith...but I'll take anything! Checked all my connections including the taboo thing of grounding the sender side. Seems the guage is OK. Noticed on the sender, though, that there is a second "wire tab" in the center of the sender disk with nothing attached to it. Is this a ground and should it have a wire on it? I'll try to run a wire from the sender to the guage tomorrow. Also, ran a wire from the temp transmitter to the guage. Everything normal up to it...I think the guage is busted. Anywhere to find out how to rebuild it, or should I just try to find one online?

 

[sonuvagunsmith]

/ubbthreads/images/graemlins/frown.gifAlways possible, but I can't find anything.

 

[jlaird]

I think I would try grounding that strange tab and see if it works, but then I am not an expert on Bs, what does the wireing diagram say?

 

[aroostok]

I replaced mine with a solid state one, too, and now I think my fuel gauge is reading too high. I started the b (76 with dual HS4's) with a quarter tank showing on the gauge and ran out of gas! I don't think this is normal, but temp gauge works okay.

 

[Guest]

There is no extra tab on my '72, just the one hooked to the wire going to the guage. The sensor is grounded where it is screwed into the head and shouldn't need and extra ground.
The wiring diagram I have shows one side of the guage connected to the sensor on the head and the other side connected to the regulated output of the regulator. I believe the output of the regulator should be 10 volts, the input is connected to 12 volts from the fuse holder pin 6. Maybe one of the fuses is corroded slightly and by cleaning the contacts it will work. What reading did you get on the guage when you grounded to sensor wire. I believe you should have shown maximum temperature. If so, this would say your guage is working OK and maybe you have a bad sensor.
Bob

 

[dklawson]

As stated by others, the electrical gauges on cars with stabilizers run at 10V. The stock Smiths stabilizer is a switching device turning 12V on and off very quickly so it's hard to get an accurate measurement with a volt meter. With the solid state stabilizer/regulator you can use a volt-meter between the stabilizer side of the gauge and chassis ground to confirm you have 10V supplied to the gauge. If you measure anything higher, make sure the stabilizer/regulator is hooked up properly. If your gauges are getting higher voltage... they will read higher.

To test the general operation of the gauges is straight forward. For both fuel and temperature gauges all that is required is to switch on the ignition. At the sender, disconnect the wire going to the gauge, then ground that wire. The gauge should move toward the high side. If the gauge fails to respond to this, there is a problem with the power to or from the gauge.

Regarding extra spade lugs and dedicated grounds on fuel sending units, I have no direct experience with '72 Bs. However, look closely at the empty spade lug. If it's mounted in an insulator it is more likely a low fuel light connection (use on some BL cars). If the spade lug is spot welded directly to the metal disk, you can certainly attach a dedicated ground wire to it. BMC/BL/Smiths/Lucas (whoever is to blame) changed things over the years as they learned. Some early BMC cars used the metal braid on the Petroflex fuel lines to make the ground connection. When/if that was replaced the fuel gauge stopped working. If in doubt it wouldn't hurt to try a dedicated ground wire (with an alligator clip) attached to the bayonet ring (commonly used to hold later senders in place) to give the sender a better ground.