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MGA Negative grd- fuel gauge question

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Marvin Gruber

Marvin Gruber

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I traded for a 57 MGA last fall with a bad motor. PO had painted it, new upholstery and chrome. It has been converted to neg grd and MGB 1800 engine & trans. I just finished putting a fresh rebuilt 1800 back in it and working out any bugs. PO also put a new gas tank in, new sending unit as well. I got to drive the car some today and noticed the fuel gauge reading past full. Since its converted ( with alternator), am I going to have to change sending unit?


Marv
 
More likely the voltage regulator. The gauge wants a 10V supply. Sender should not care whether it gets positive or negative ground. Look for (do a search here on the forum) Doug Lawson's article on a solid-state upgrade for the regulator. And get the component from Mouser, on line.
 
Did the MGA have a voltage regulator for the gauges, Doc? The Bugeye (similar-ish vintage) doesn't have one.
 
Kinda think so. Can't recall fer certain, Drew.
 
Marv -- the MGB did have a separate regulator for the gauges as Doc mentioned. Just can't remember when they started using them.
 
The sending unit can also be installed upside down quite easily. I'd have to look but I can't remember if the terminal is on the top or the bottom. Since it was just changed , I'd check that first. Polarity doesn't have any effect on reversing the reading. If the sending unit is installed properly , then reversing the leads on the guage should fix it.
 
Looked at a couple of wiring diagrams last night. Since its got the Alternator ( and no mech tach drive) I'm going to wire it like an early MGB and use a voltage stabilizer. Will see what that does for the fuel gauge and new tach.

Marv
 
Marvin, just a thought, the voltage stabilizer may be one answer but even then you will properly have to reverse the wires on the back of the gauge and the temp gauge also, unless you are still using the old temp/oil gauge.

Wayne
 
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Plans changed today and didn't get to work on the A today. Oil and temp gauge are working fine. My mother has me scheduled to help her plant a garden tomorrow. Car show on Saturday. Always something to do.

Marv
 
Marvin, I think you are on the wrong track. Look at the MGA Guru website (Barney Gaylord). He has thousands of pages of technical stuff. I don't remember seeing anything about having to change the fuel sending unit or its wiring when changing over from positive to negative ground.

It is most likely that you do not have a good ground at the sending unit - assuming it is an MGA type unit. Since it is a one wire unit, on a positive ground, the ground is from the tank. The sending unit goes from 0 to 70 ohms, I believe, and the gauge also reads from 0 to 70 ohms, but I'm not sure which is full or which empty. My sending unit is disconnected and it reads full, so there is no electrical going through. Barney also has tons of stuff on the sending unit and on the fuel gauge.

The sending unit / fuel gauge are constant tinkering issues on the MGA.
 
The gauge is a simple reader of resistance. The wires should not need to be reversed on it. It works on V(in) and resistance to ground through the sender.
 
I remember when I checked the tail lights, they weren't working either, I had to do a new grd and they worked perfect. So I will check grd first. Another question. Since I have a later B motor, there's no mech tach drive. Can I use a pos grd electrical tach guage. Will I need to change direction of wire loop?
 
The elec tach won't work without being internally re-wired. It's not as easy as just changing the direction of the loop. If it is the loop type, then one possibility would be to use a DC-DC converter to supply it with the -12 volts (relative to ground) that it wants. Haven't tried it myself, but should work.

MGA definitely used a "balanced" fuel gauge, no VS required. An electronic VS won't hurt (the gauge will still work on a steady 10v) but an original type VS will keep it from working properly. Notice that the gauge housing (assuming you have an original MGA gauge) needs to be grounded for proper operation. Without the ground, they will typically read either past full, or past empty.

It's been too long, I don't recall for certain, but I think I remember a buddy's MGA being wired the same as a TR3, which is to say they relied on the gauge panel to ground the gauge. If so, I suggest adding a ground wire for it.

With the balanced gauges, an open circuit to the tank sender (or a broken wire inside the sender) will cause the gauge to read past full.
 
The fuel gauge is reading past full so this gives me more food for thought. Guess I could use a later B tach ( its the big 4" dia) from a RB car, Face won't match but better than not having a tach I guess.

Marv
 
Marvin, you just have a few of the Lucas electrical gremlins. Definitely go to MGAGuru.com. Everything you need is there. Makes for late night reading.

1) check the ground at the tank. I think that in the past I have run a test wire to the ground connection on the frame from one of the sending unit mounting screws. Also, the gauge does need to be grounded. I believe that is done through the brace that is used to hold the gauge to the dash.

2) Barney also explains how to convert the tachometer. Doesn't look that hard, but if push comes to shove, I suspect that Nysonger's (sp) can do it for reasonable money. Barney also explains how to re-wire the electrical tach. I ran an electrical tach on my positive ground car for a few years. One wire was connected to the coil, one to the post on the distributor. That's not Barney's method, but for normal driving it worked. Unfortunately, I mostly drive and shift at WOT so it was rather jumpy for that.
 
Checked MGAGuru, thought I needed to change direction on the white wire loop. Swapping the internal wiring shouldn't be an issue either. Thanks for reminding me about the MGGuru site. I went there a lot when I had the Twin Cam. Lots of good info.

Marv
 
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