Reliant 6V fuel guage/sender problem

[Luxobarge]

OK, here's a tricky one that I've been battling with all day.

Situation: my Reliant Regal "special" resto is nearly finished, and I was recently refurbing the fuel tank and noticed that it had a sender unit. Bear in mind that apart from ignition and charging, this car has NO electrics or guages, not even a speedo. However, I decided it'd be nice to have a fuel guage, so I got a 6-volt fuel guage (new) that's for a 6-volt Willy Jeep, I thought that would do.

So I connected one side of it to the ignition-switched 6V supply and the other end to the sender, switched on and it rose gracefully to show a full tank of fuel. Trouble is, the tank is nearly empty.

I've now removed the sender and when I move it from "full" to "empty" there's virtually no movement on the guage.

The resistance on the sender unit varies smoothly from zero to about 30 ohms, and looks in very good condition, I honestly don't think there's anything wrong with it at all, it just maxes out at 30 ohms.

Now 30 ohms isn't anything like enough to drop the current through the guage so that it reads empty - so what am I doing wrong?

Any ideas?

I know I could fit a voltage regulator but this would only provide a stablised 6V to the guage, I doubt that it would help drop it to a zero reading with only 30 ohms if resistance from the sender.

Should I fit a resistor in series with the sender? If so, what value? How would that work?

Or were Reliant fuel guages wired up differently, or did they have some sort of gizmo that made them work with this sort of sender? If so, what?

I'm not often flummoxed by electrics, but I am this time!

All help gratefully recieved - especially JPB, as I know you're a bit of an electronics guru....

Cheers!

[harvey]

Guages and sender units need to be matched to work properly, but there's a gizmo that you can buy that will match any guage to any tank unit so that they work. I can't remember the name, but some creative googling should enable you to find it. The hotrod and custom boys have the problem all the time so they use them.

[Luxobarge]

Thanks Harvey. Hmm, so that prompted some intensive internet searching, there is indeed such a thing but I could only find one device in the UK and it's for 12V guages.

John - don't fancy making me one do you?

I think I might be into getting another sender unit for it, but first I need to know the resistance range I need, so I will need to get out there with a bunch of resistors (or ideally a variable resistor) and find which value makes the guage show empty.

At least it looks as if quite a lot of senders have standard tank fitments (a round plate with 6 holes) so my chances of finding a suitable sender are fair.

I'm learning a lot here.....

Any other bright ideas anyone?

[TerryG]

I can think of a couple of possible solutions
The ultimate cheap "bodge" of a sight tube.
Replacing the potentiometer in the existing sender unit to give you the range you need.
Fitting a new "universal" level sender or getting one from a different car, making an adapter plate for the mounting and bending the arm to fit then using the appropriate gauge
Building a 6-10v / 6-12v (regulated) step up transformer to give you the correct voltage you need for the gauge you have found without needing to touch the sender

If it was mine i'd opt for making the circuit (or to be strictly accurate, getting one of the guys at work to do it so it actually worked).

[Luxobarge]

Yeah, I'm looking at all of those options. The last one isn't valid though, I have a 6V guage on a 6V system, and if I feed the guage with 6V it goes nicely to full deflection, so no problem with the voltage as such.

I've just had another play with some fixed resistors, and found that it needs 580 - 600 ohms to read "empty" - I've looked around and the highest value sender I can find is 220 ohms - this is going to show over half a tank when it's empty, so still no good really.

Getting stuck here!

[Luxobarge]

So, done some more research and fiddling, and I think I've come up with the reason for the problem, if not necessarily the solution.

The problem lies in the fact that the guage is a moving coil type, and the case of the guage is one of the connections. It should be connected like this:



So I fiddled with the wiring and it kind-of works. With it wired up as shown it actually works backwards, i.e. if the sender sends 6V then it reads empty, then the lower the voltage the "fuller" it reads. So I wired it up like this, and with the sender wire sending 6V, it reads empty, if I introduce a 100 ohm resistance in the sender then it reads full - so far so good.

Trouble is, a) my sender is only 30 ohms and b) the guage is negative earth as shown on the diagram. My car however is positive earth, and so even if I had the right sender I can't get it to work as one side of the sender is connected via "earth" i.e. the chassis. I've had a look at the sender internals and there's no way I could isolate both ends of the rheostat and connect a second wire to it so that's out.

I've also had the guage apart and there's also no way I can reverse the polarity of the guage.

Regarding the ohm range of the sender, it needs 80 - 100 ohms to correctly read "empty" and I've found a direct replacement sender that's 84 ohms, so that's a possibility.

That leaves the polarity issue. Now, as I've already said there are hardly any electrics on the car, so changing the car from positive to negative earth is a real possibility. The dynamo regulator (I bought a solid-state one, great bit of kit, works perfectly) can be configured to work on negative earth, so that's no problem. However, can someone tell me how I change the dynamo to generate negative earth instead of positive earth? I seem to recall that it can be done quite simply?

That leaves the starter motor. Anybody know how I can change that, or do I need a different starter motor? This might be the show-stopper actually.....

Assuming that the starter motor is effectively a show-stopper (getting a negative earth 6V starter motor that fits might be a challenge or cost £££) then one further option is to get a little 6V battery pack to power just the fuel guage, and put a switch in to switch it on when I want to know how much fuel there is - i.e. not use the car's power supply at all.

Or any other bright ideas?

Cheers!

[harvey]

Polarising the dynamo: take a lead from the battery live and flash it on the dynamo field terminal. (That's the small one).

Starter motor shouldn't be affected by the polarity.

As for the tank unit, when I fitted a 40 gal tank to my truck and the sender didn't work with the guage, I found the tank was 40cm high, so I bent a steel rule to use as a dipstick and every cm of fuel level was a gallon. Simples!

[Luxobarge]

Hi Harvey - many thanks!

Hmm, I'm pretty sure this starter motor is polarity sensitive but I do tend to trust what you say, so I'll go and check.

Thanks for the tip about dynamo - so if I do that it'll just reverse the polarity? Magic!

Yeah, this is an awful lot of faff just to know how much fuel I've got, but I like a challenge and this is mostly for the fun of getting it all to work. Anyway, I can't use a dip-stick because the fuel filler neck is sort of S-shaped, and the tank (a long barrel-like thing) has three baffles in it, with the fuel filler coming in from the side, so even if I did get a dip-stick in there it'd be going in at no more than 30-40 degrees and would hit the baffle sideways before it hit the bottom of the tank, so a non-starter really.

I have to say I'm starting to wish I'd gone with Terry's idea of a simple sight-tube - doesn't get simpler than that, and very accurate!

I've actually now ordered an 83-ohm sender unit (off an MGA, which a source tells me uses this ohm range) which should work. Also ordered a 4 x AA battery holder, from which I'll power the guage independantly. A bodge, but it'll work fine for what I want. If I find I can easily reverse the polarity of the vehicle I can always wire it up as it should be i.e. without the battery pack.

This is where I find that the new sender doesn't fit, isn't it?

Cheers!

[JPB]

The starter is absolutely, definitely, beyond a doubt not polarity sensitive, so will work just fine and besides, the bendix gear can only throw in one direction so even if polarity were an issue here, then the worst thing that could happen would be a non-engaging starter but it'll be fine, as will the wiper motor if you have such a thing. I've swapped from +ive to -ive earth on several old BMC things and only devices with transistors, diodes and electrolytic & Tantalum bead caps (some pulse-wound clocks, radios without a polarity change switch, etc.) need cause concern.
Some folk I know have reported that, even with out flashing the field coil as Harvey wisely suggested that you do, it's sometimes possible to get away with simply swapping the battery leads (as well as the switch and contact terminals at the coil) though you would still have a spark if the coil were wired "back to front".

[Luxobarge]

That's helpful John - many thanks. No wiper motor on this - no point really, as there's no windscreen. No windows at all in fact. Actually, not even any bodywork to speak of, just a chassis, engine & running gear - hence the ability to have minimal electrics! It does have a single headlamp - all of 21W!!

In that case I think I'll have a go at reversing the polarity of the car, I have a (hopefully) correct sender in the way so I should be sorted after a bit of fiddling.

I'll post back on how it goes.

Thanks all!