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MG MGB Technical - Fuel gauge electrical

Hi everyone,
I'm trying to set up a fuel warning light for my 78B and discovered a problem.
The light pulses at a semi-steady rate around twice per second. The speed seem to fluctuate slightly but not greatly. I leave it until the needle on the gauge has stopped climbing and the light is still pulsing.
I understand there is a 12V circuit that goes through the gauge and the sender in the tank, with the sender acting as a variable resistor according to the level of the float. I thought the supply was a constant 12V?
Any help appreciated.
Dave,
Sydney, Australia
D O'Brien

Dave, you are very likely looking at the effect the voltage regulator has on the fuel gauge supply. The old style units operated on a make/break circuit to regulate the voltage, which pulses the voltage to produce an "average" voltage supply of about 10V. You can buy solid state regulators that supply a constant 10V.

You have two choices, buy a solid state regulator or put up with the pulses :-)

Cheers
Tony
Tony Oliver

One little-known benefit of the MGB factory system is that at switch-on the gauge rises faster than it does with a 10v solid-state regulator, because the gauge gets a full 12v for several seconds before the regulator 'warms up' and starts pulsing.
PaulH Solihull

Oh, This is something new to me. Do MGB have a low fuel warning light?, Mine hasn't, it is a 72 GT. For many months I was dip sticking the tank, before I got fed up and bought a new sender. All fine now even with (probably) the old regulator. But I wasn't aware of a warning lamp how does that work? Mike
J.M. Doust

Thanks for the advice. I think I will change the regulator as I don't mind if the gauge takes a bit longer to "warm up" at switch-on. Good info though. I am continually impressed at how some of those bearded guys in that shed out the back of Abingdon came up with such nifty innovations and how they knew when the old way was still better than the new way. Solid state tech is great, but not always the best.

J.M.Doust as far as I know none of the MGs (pre 1980) had fuel warning lights. I bought a small kit from Jaycar called a "voltage switch" which simply lights an LED green when the supply is above a set voltage and red when it drops below. I planned to mount the LED in one of the blank switch panels near the fan and hazard switches.
I know it's not original. So? :)
D O'Brien

D. O'Brien, what a great job, I'll see if I can get one. Ta, Mike
J.M. Doust

Well I've given up for now.
I spent some time looking for a solid state voltage regulator (Jaycar say they don't have them and Dick Smith isn't really an electronics store these days). No luck.
Meanwhile I dug up an old site I bookmarked with a very comprehensive description of the electrical system and the gauges. I was intrigued by the workings of the gauge being thermo-mechanical rather than electro-magnetic as I had thought.
Anyway I learned a lot about the gauges, in particular how to adjust the "F" and "E" readings for the fuel gauge. So I'm going back to original again. No warning light for now unless I stumble across a solid state voltage regulator.
P.S. If you still want to do it JM, I suggest you look at a couple of the other kits Jaycar have. The one I got was the cheapest with a simple red/green LED that changes colour at the threshold you set via a small trimpot. They have others that activate on/off relays and might be more useful. On the other hand, my one is also the smallest circuit and you would have no trouble tucking it behind the dash.
D O'Brien

http://www.britishv8.org/Articles/MGB-Voltage-Stabilizer.htm if you are that keen.
PaulH Solihull

Hi there,
MY 64ish gauge has not worked for 30 yrs.
I just reset the trip meter when I fill it up.
However it would be nice if it worked. Where may I buy a new sender?

Trackless
J Millsom

Hi, I am back to this thread. J. Millsom, I bought the new sender unit from Concourse spares in Melbourne. I will look up their address. They appear not to have a web site. But they are very good at supplying parts, very trusting too. They send the parts straight away, then you send a cheque, or Cash-on-delivery. The new senders are electronic too work well. Mike
J.M. Doust

Make sure you get the right sender. 62 to 64 gauges were undamped i.e. swung on cornering, had no instrument voltage stabiliser, and I think the sender screwed to the tank with several screws.

Some time in 64 (chassis number 48766 or GT) it changed to a damped gauge, with stabiliser, and the sender was held in with a locking ring.
PaulH Solihull

This thread was discussed between 01/05/2012 and 29/05/2012

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