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MG MGB Technical - MGB Fuel tank sender & gauge

Hi All, My 1966 MGB fuel tank has a Jaeger fitting for the sender but the gauge is Smiths and it has never worked since I bought the car! I'm trying to fix it, and I was wondering if you have to have a Jaeger gauge to work with a Jaeger sender? Any suggestions would be welcomed if polite of course!
Thanks Jim
J Reckitt

No, any make gauge will work providing it covers the same range of resistances that the sender is putting out. You really need a multimeter to check these things out, but often failures are due to dirty and corroded terminals at the tank where they are exposed to road muck.

First thing then is to pull off the wires and clean the terminals on the tank and make sure that the earth wire really is earthed. Since the gauge measures resistance, any corrosion on the terminals is going to cause problems.

Then you need to check you have around 10 volts on the green/black wire at the tank with the ignition on. Inside the tank is a float which as it moves up and down rubs a contact across a resistance wire. Ideally you need to know what the resistance is when the tank is empty and also when it is full. For example my sender has a resistance of about 25 ohms with full tank and 270 ohms when empty. You then need a gauge set up to go full scale between those readings.

As you use the car you can easily check if the sender is working by measuring the resistance between the green/black wire and earth, either at the tank or at the gauge. Read the resistance after filling and then after driving 100 miles or so. If its different then the sender is probably OK.

If you have changing resistance readings and a 10 volt (approx) supply then the gauge could be faulty. Ideally you need a mate with a similar car whose working gauge you could substitute. But if you buy the correct gauge for your year of car from any of the main parts suppliers it ought to be close enough to work with your sender. If it isn't send it back!

But first clean those connections!
Mike Howlett

Have a look at Paul Hunt's excellent mgb-stuff web site for comprehensive information on fuel gauges and lots more.

The fuel gauge section link below includes details on Early vs late systems, Fault diagnosis, Tank sender, Voltage stabiliser, Calibration, Gauge identification and more. -
http://www.mgb-stuff.org.uk/electricstext1.htm#fuelgauge
Nigel Atkins

Thanks for all yuour comments. Really helpful. Regards Jim
J Reckitt

I second Nigel's suggestion about Paul Hunt's web site. Really useful.
Mike Howlett

I have had problems with the electronic replacement stabilizers and high gauge readings. In both the 1970 B and the 1971 midget, I have added a voltage regulator to the circuit between the stabilizer and the gauge.

Filling the fuel tank completely and trimming the regulator to drop the voltage until the needle is directly on the F mark will give proper readings when the tank is full, half full, 1/4 full and E is really empty. On the MGB, this lowered the engine temperature readings to logical levels so that now the car doesn't say it's overheating all the time. Calibrating the fuel gauge effectively calibrated the temperature gauge in the same exercise.

I have ordered these regulators from Amazon. They are inexpensive, with a tiny potentiometer to trim the voltage.
Glenn Mallory

There was an intermediate system used for a few months that used a different screw-in sender with the Smiths gauge. That sender is NLA, so on the face of it you have to swap the tank for the later type (which has very different mountings) or use the Jaeger gauge. But there is a gadget from Spiyder https://www.spiyda.com/fuel-gauge-wizard-mk3.html that reverses the logic between the two (amongst other things) and enables the early Jaeger sender to work with the later Smiths gauge.
paulh4

After fitting the later tank for the extra capacity on long distance rallies, I ran for years with the original Jaeger gauge and the later sender, reading back to front. Just regarded E as meaning enough, and F meaning ****-all
Paul Walbran

This thread was discussed between 07/06/2020 and 08/06/2020

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