TR2/3/3A Help with fuel gauge blowing fuse

[TRclassic3]

A little background....while engine out for rebuild I put in a new wire harness. Everything had been fine, but while out for a ride yesterday I blew a fuse. I was able to eliminate the wiper, brake lights, and turn signals as the cause, leaving fuel gauge/sender. As seems the norm, my heater fan picked up power from the gauge. With that wire off the gauge the fuse does not blow, but the gauge isn't working. I,m not well versed on how to proceed I appear to be getting 12 volts to the gauge, but only about 5 at the sending unit. How can I determine if the problem is gauge, sender, or neither?

thanks

 

[TR3driver]

Make sure the body of the gauge is grounded. The original setup relies on the clamp to ground it, which is far from secure especially if the panel has been painted or powdercoated. I actually added a ground wire to mine, which made the gauge behave much better. Also make sure the green wire is on the correct 'B' terminal. Green/black goes on 'T' (for tank). If it has power and ground, and you are seeing 5 volts on the sender but nothing on the gauge, then the gauge is dead.

 

[TRclassic3]

Thanks Randall. I think that my initial conclusion that the heater circuit was the problem was in error. I had popped the gauge out of the dash, removed the heater wire and the fuse didn't blow. However, at that point I had no ground on the case so I'm assuming not a valid test. That said, even grounding the case doesn't blow the fuse but gauge does not work. Voltage is still as stated. Does this all still point to the problem being internal to the gauge? Wonder why fuses blew before but not now?

 

[dklawson]

Your fuse issue and the gauge not working are not likely to be related except that they draw from the same power source.

The wiring inside the fuel gauge is tiny/thin and would blow long before a fuse. The gauge's "B" terminal would have to short directly to the gauge case to cause the fuse to blow and that is unlikely. However, if there is a short or break in the green wire going to the gauge, that would disable both the gauge and heater.

If you have a fresh fuse in the circuit and turn the ignition switch to the run position, you should be able to measure 12V from the gauge's B terminal and ground. If you don't find 12V, then there is problem along the green wire.

If you find 12V on the B terminal and the gauge does not work, look at the gauge wiring carefully. As Randall said, the gauge case has to have a good ground connection. With a good ground connection, disconnecting the green/black wire from the gauge's "I" terminal should make the gauge go to full. If you make a jumper and short the I terminal to ground the gauge should go to empty.

 

[TR3driver]

I agree. The short may have simply been intermittent (as they so frequently are) and so the fuse didn't blow right away when you reconnected the circuit that originally had the short. The gauge failure is unlikely to be related to the short (although might be a result of R&R the gauge).

Intermittent shorts are so much of a pain to find that, many years ago now, I substituted a self-resetting circuit breaker for the fuse. Unfortunately the self-resetting ones don't seem to be available any more, but this type should do well enough. You just have to remove any load (including the short) before they will reset
https://shop.advanceautoparts.com/p...chTerm=circuit+breaker+glass+fuse+replacement

(I do wish I'd stocked up on the self-resetting ones, though.)

 

[TRclassic3]

Thanks Doug. The plot thickens a bit. This morning I hooked up a ground (gauge still out of dash) turned the key and the gauge went to full (which it is) and no problem with the fuse blowing I put things back together and all ok. I then routed the heater lead to aux fuse block I had put in when rewiring. Works fine and did not blow a 10 amp fuse. So everything is working, but I still don't know what caused the problem. I'll carry my extra bottle of Lucas smoke just in case.

 

[dklawson]

My son's Spitfire has/had an intermittent short with part of the turn signal circuit. I think it was a bullet connector that was only partially inserted into one of the inline black couplers and MAY have been bouncing against the sheet metal behind the dash. I say "think" because while it hasn't blown fuses since I reseated all the connectors, I still wait for him to tell me it is happening again. To trace the problem I went so far as to temporarily fit extra inline fuses in various places to help me track down where it was.