Fual Gauge Resistance

You definitely should be using a 10V stabiliser, because otherwise your readings will vary according to the charge going into the battery and the drain coming out of it.

Once you have set up your stabiliser, find out which way your dial works

- it might read Full with an open circuit or it might read Empty. Vice versa with a short to earth. Probably as the gauge came from a Triumph it will read empty, but you don't say which Triumph, so either is possible.

Likewise, your tank unit showing 80 Ohms when empty doesn't tell me whether when it is full it goes to 0 Ohms or to a number higher than 80. Again, either is possible, and you need to find out which.

Once you have got this information, you are again faced with two possibilities - one of which is that dial and tank units work in opposite directions.

Find some answers to the questions above and I might be able to advise you further.

Jim

Yes - if it's a 'hot wire' gauge. This is the type which reacts slowly to a change in voltage - unlike moving coil etc which do so near instantly.

It might be worth making an electronic one yourself using a variable voltage regulator. Then you could fine trim the readings. At least so it's reasonably accurate at the empty end.

Thanks for the response, in answer to your questions, I don't know what Triumph it came from just a Fleecebay bargain, it reads full on a short circuit, it does has 382 on the side if that's any help & not just the QA testers number. My sender currently reads 80 Ohms on empty & 10 ohms on full these were just some resistors I bought at 10 Ohms each just to prove it's viability. Once I know what resistance makes the gauge show empty & what resistance shows full they will be changed, it's getting those numbers I'm struggling with.

Alan...

Provided you use a 10V voltage stabiliser, I reckon 50 ohms will just get the gauge to Full, and 1000 ohms will be just about empty. If you bought a handful of 10 ohm ones, you should be able to test this, but assuming your 80 ohms reading 3/4 full is when using a 12V battery, it is a working assumption that the full mark is reached when 200 ma are running through the meter. So that is what I based my calculations on.

You might want to make the 1000 ohm one a bit bigger so that when the gauge says empty there will be a little drop of petrol left in reserve. You could try 1600 or 1800 ohms.

Don't forget that the 200ma current running through the 50 ohm resistor will make it warm. To be on the safe side, get a 2W resistor for that one.

Jim

Not sure where you can get the figures - VDO made a variable resister tester which clipped onto the wires at the tank end, if you set the scale on the tester gave a value on the gauge.

What was supplied with it is a chart with those resistance settings for most types of gauges.

I do have on somewhere but unsure where it is now.

try these

may be of help.

And I always thought that the E on the fuel gauge meant Enough. :)

Once again thanks Jim, got a few busy days so might not get back onto it till after the weekend, but will call round at either RS or Maplins to pick some resistors up. If you recommend 2W for the 50 ohm, what do you recommend for the others.

Alan...

have a look at the reference I mentioned it has various ranges and you should be able to work something out from that.

You didn't say how many steps you were planning to include from full to empty, so I have done some calculations based on steps of 1/8 of a tankful.

The other thing I would advise is that you get your 10V stabiliser and do a couple of tests with the resistors you have got to see whether the assumptions I started with (eg 200ma gives Full) are actually right before you buy higher wattage resistors. You can get away with lower wattage ones for testing but don't forget that a fuel gauge is powered all the time the ignition is on so there is a gradual heat build up: don't under-scale the ones you are going to use permanently. You can over-size the wattage with no ill effects but undersized ones are going to fail eventually.

So wire up the voltage stabiliser and the gauge and check that 50 ohms is FULL and 80 ohms is about 5/8 full and 100 ohms is about half full (the calculations are on a logarithmic scale not a linear one). If you get somewhere near those results (they wont be exact - most resistors are nominal value plus or minus 20%), then you can trust the rest of my calculations. If you don't, then tell me what you do get and I will rework my figures.

If you get somewhere near my rough results, then based on the resistor values that are actually sold

are aiming for these values (the wattage shown is for the last one in the chain): FULL 50 ohms 2W

7/8 50+6.8 ohms 2W 3/4 50+6.8+10 ohms 2W 5/8 50+6.8+10+12 ohms 2W 1/2 50+6.8+10+12+22 ohms 1W 3/8 50+6.8+10+12+22+33 ohms 1W 1/4 50+6.8+10+12+22+33+68 ohms 1/2W 1/8 50+6.8+10+12+22+33+68+200 ohms 1/2W Empty 50+6.8+10+12+22+33+68+200+1000 ohms 1/2W

Jim

Have a quick search on Ebay - there used to be a firm which supplied packs of assorted high wattage resistors. For a fraction of the Maplin price. I'd be surprised if any Maplin store stocked them.

First thing to note is that most Triumph gauges were "hot wire" types, in which the needle is moved by the expansion of a bit of wire as current goes through it. This is a simple way of smoothing out the rectangular pulses produced by a bimetallic voltage stabiliser.

That means that it's an ammeter, and a non-linear one at that, since the resistance changes as the wire gets hot.

Since I am Nice People I have just nipped out to the garage and stuck a digital meter on a spare Herald sender I have knocking around. It's not been used for a while, so the track is a bit mucky, but with a bit of wiggling I got some consistent results: empty is 2000 ohms (ie 5 mA at 10V) and full is 200 ohms (50 mA at 10V)

Good luck,

Ian

I thought it stood for Eeeeeeeeeeeeeeeeeeeeeeeeek

Ian

That's rather a chicken and egg situation. Hot wire gauges are cheap to make - and have the benefit of slow reaction which is ideal for petrol tanks where the fuel slops around. But need a regulator. A moving coil meter etc can have voltage compensating coils built in.

>

Thanks again Jim, just waiting for the stabiliser to arrive, might struggle to get so many resistors in the sender, it's only a 15mm tube, already has the reeds in there, was hoping for just 2 per set. As you appear to be knowledgeable in this area if I wanted a light to come on at say 1/8 full is that possible?

Alan...

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You only need the reeds in the tube. You can run wires out from the reeds to a little box conveniently placed which holds the resistors, and you will probably have to do that because that many 2W resistors might not fit in a 15mm tube.[1]

As for the light coming on, it is possible, but you will need more components. I will have a think about it, but which components will depend on your reeds. Are yours going to be normally open and closed by the magnet, or normally closed and opened by the magnet? Or have you gone for the posh double pole type?

Jim

[1] After what Dave Plowman said about Maplins further down this thread, I remembered I had a Maplins catalogue at home and I looked up their resistor range. The bad news is that their 2W resistor range doesn't go below 100 ohms. So from Maplins you are going to be getting 3W resistors which are physically a bit bigger (but go all the way down to 0.1 ohms). The 3W range are 33p each (inc VAT), so not dreadfully expensive in the grand scheme of things. I haven't got a Radio Spares catalogue so I can't do any comparisons.

>>>

Thanks again Jim, of course you're right about not having the resistors in the tube, it was just me not doing the lateral thinking, I've put them in for testing. Currently & this is open to change if a better idea comes along, the reeds are normally open & are closed by the magnet, they again are currently are in pairs which are offset to each other as I found that a single reed goes closed/open/closed as a magnet passes over them. However this does change if you orientate the magnet the other way, you then got just a single close but for a very short duration.

Alan...

There are a couple of blokes on eBay who make and sell electronic stabilisers. I bought one for the Herald: it works OK on the bench but I haven't fitted it because it will need a wiring mod: the Herald has a double connector on the unregulated side.

Ian

That's where I've ordered it from, I've used a couple before from a seller in Mansfield on previous kits, they worked very well, just wish I could make one for myself.

Alan...

Just found an interesting article on the stabiliser & Maplin's do the chip for 95p.

Alan...