Fuel Gauge doesn’t work – Replacing Voltage Stabilizer, swapping gauge in a Triumph Spitfire

So my fuel gauge was pegged full, and smoke on the inner glass. Then it stopped registering altogether. The fuse was blown, so I replaced that. Took me awhile to figure it out because the circuit diagram didn’t show it going through the fuse.  Still didn’t work.

The fuel and temp gauges get their power from the Voltage Stabilizer. The voltage stabilizer gives an average of 10 volts out to the gauges. It works by switching on and off, 12volts, then 0, then 12 volts, etc. The gauges react so slowly that you can’t tell.

To get at the voltage stabilizer to test it, I wiggled onto the drivers floor back down.  I tested the voltage stabilizer by first putting a test light on the B side  (battery) of the voltage stabilizer.  The B side has the male terminals sticking out on the original.  Replacements may have different  It is marked on the brown fibre board, but hard to see.  I had power to the B side.  I then checked the I side (Instrument).  It should cycle on and off with the test light.  It didn’t.  So I removed it, and took it apart.  The nichrome wire that heats up the bimetallic strip inside was broken.

I ordered a new one from British Parts Northwest for about $12.95.

It had all male connectors instead of 2 male, 2 female, so I had to make a small female to female patch cord.  The replacements are solid state, and put out a steady 10 volts to the I side.  I tested it upon arrival.  12v to B, and grounded it.  Then put a voltmeter on it.  No 10 volts out on the I side.  Dead on arrival.

There are various posts on the forums showing how to make your own inexpensively.

http://www.britishv8.org/articles/mgb-voltage-stabilizer.htm

http://www.bullfire.net/TR6/TR6-44/TR6-44.html

http://www.74tr6.com/voltagestab.htm

So I went to Santa Cruz Electronics and got the parts for about $5.95 and made my own.  At first I tried to get it to fit in the original voltage stabilizer case, then realized it wasn’t worth the effort and just assembled the bits.  It worked fine.  Output 9.46volts.  After reading the datasheets I added a .22 MFD 20 volt tantalum capacitor between the 12 volt and ground.   The .1MFD wasn’t necessary. Here is my original circuit.

Because Santa Cruz Electronics didn’t have exactly what I needed, I swapped in a NTE 1932 voltage regulator, 10V out, 1 amp rating, and a .22 MFD 100volt capacitor.

After installing it, my fuel gauge was reading past full.  So I checked the Ohms of the sender, by disconnecting both leads from the back of the tank, and checking across the two terminals, and it was at 24 Ohms.   I also checked at the black/green wire into the fuel gauge.

I found the following at  https://www.mgexp.com/forum/mgb-and-gt-forum.1/ohm-rating-for-fuel-sending-unit.2904591/

20 Ohms is full
35 Ohms is 3/4
65 Ohms is 1/2
105 Ohms is 1/4
222 Ohms is empty

Full is 20 Ohms, the tank is almost full, so it isn’t the sender.  I had replaced my temp gauge with a Smiths Dual Oil/Temp gauge, so I grabbed it.  The fuel and temp gauges have identical guts.  The Temp gauge read like approx 7/8 of a tank, or just below H.  So it must be the fuel gauge innards were damaged when it overheated.  So I swapped the innards of the temp gauge into the fuel gauge, and everything worked great!

I let BPNW know, and will probably have them send me a replacement.  Mine is kind of funky.  We will test it, see how it goes.