Fuel Gauge Driver Board

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addo
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Fuel Gauge Driver Board

Post by addo »

One for the boffins...

S1 405 with working fuel sender, continuity to dash connector plug.

From the four pin Veglia fuel gauge it has a little "piggyback" board with one side basic caps and other common bits, and the other all SMDs. Foolishly, I forgot to photograph these pieces when the dash was apart.

Gauge when stripped of this board, deflects to about halfway with an independent feed of 1.5V. I'm guessing that's fairly normal. Grounding out the low fuel switched feed with ignition on, doesn't light the low fuel light.

My presumption is the gauge receives a stabilised feed with less than 12V to full deflection, and there is some sort of latch for the low fuel lamp, like on Xantias. Seem realistic for 1990 technology?

I'm not the greatest when it comes to circuit diagnostics of this sort. To get no result - not even a tickle - suggests complete failure of some key aspect.

Has anyone here tinkered with this type of stuff? I'm interested in pointers to follow with a troubleshooting regime.

Cheers, Adam.

(This is the trouble with blue-collar careers - your mind runs riot on the hobbies...)
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spider
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Post by spider »

I'll try to figure it out from some circuit diagrams (read: rats nest) as I have these for S1 in a tatty BOL.

I though the low fuel light was not actually fed from the gauge signal, I seem to recall it has a separate switch (built into the sender) , but I could be totally wrong here and possibly thinking of another make.
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Post by addo »

I've got the BOL and it just has the dash as a monolithic block with wires going into it. Barely useful, especially as many of the numbers on the wires don't tally well.

Complicating the matter is my car's a S1.5 so the sender is a two wire jobby; below a certain fuel level there is a dead short.
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Post by spider »

I'll check mine later today (once I can find it, I know roughly where it is) as its an earlier one so might not be a BOL :?:

I'll go look later this afternoon for you anyway :)
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Post by spider »

Looking at the diagram (well, a kind of diagram) , I see what you mean.

There's nothing to show the internals of the instrument panel.

However, the fuel sender has three connections, both with a common feed (or earth, I guess feed), the two outputs:

One is a switch (so simple on / off) this will obviously be the low fuel level sensor / float

The other is a potentiometer, obviously the sender.

Not sure if that's any help though. Should be three wires from the sender by the looks of it...

EDIT... Just re-read your post again, although the PSA senders are usually reliable. I do recall on one of my MK3 escorts (a long time ago) the sender failed on that, anything less than 1/3 tank always read zero, it was strange as you would suddenly see it just fall off the scale. It was accurate above that. Perhaps you have a similar issue.

Replacement sender (if possible) would be ideal, although you say you have a S1+ so I guess you have a K plate (?) as that could be mix...
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Post by addo »

Hi Andy, sounds like exactly the same circuit diagrams as I ran into! By UK registration standards, the car would be a very early H-plate.

I've been through the sender; replaced the wiper arm as the old unit was worn to a stump after 195K miles. :shock: With the mouldymeter I now get imperfect yet usable "steps" in resistance as the float arm moves.

The piggyback board looks like a way to deal with the later two-wire sender; it stabilises the jiggling output and once damped voltage increases (and remains) beyond a certain point it powers on the low fuel light.

If I used a low mileage Xantia resistive track (about 280 ohms instead of the 405's 200 ohm one) I might be able to use the matching dash gauge. The lower "pot" of many fuel pickup/sender units is interchangeable with only the upper portion defining the model - smart design.

I've got to reef out the dash in a few weeks, so will probably have a red-hot go again, at that time. Rehooking the speedo cable (dash end) on these is a challenge...



As an aside (possibly for the tech archives) the tank/sender seals in these cars have a bifurcated lower lip. I see this as meaning you install the seal into the tank, then slide the sender unit through. Also I clean thoroughly the lock ring threads and lock ring, then lightly grease with waterproof grease - makes tightening the lock ring to its match marks, very easy.
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spider
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Post by spider »

Ah, yes I have heard about the speedo cable (but never done one)

To be fair I think with yours having the earlier dash it will be easier. The later ones, the trick was to use an elastic band (apparently) to hold it in place so when it pushed home it locked (the band broke and sat harmlessly in the dash anyway.

The tracks have probably worn away on the sender then by the sounds of it, similar effect to the heater fan speed knob (where they wear into 'steps' rather than being linear) , I know as my 205 did this and its 'fun' to replace.

It should be reasonably smooth in operation (resistance wise) throughout its range.

Your theory on the extra board could well be correct. I guess these only come with the panel (and just a few at a guess)

Although again off topic, I do sympathise when you get these 'in-between' vehicles, I had one of the very very last MK2 Astra's once and nothing (brake wise) from the dealer would fit. I'll spare you the details but it was basically built up in the factory using Corsa braking parts.

Back on topic, I'd say see if the scrappy has any early panels and have a peek.

If possible I'd love to see a pic or two of said 'extra' board.
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Post by citroenxm »

Not sure if Ive read this topic correctly...

Faulty Guage? I had a Xantia Estate in with ths problem...It was a 1997 P plater, petrol

Replaced sender in tank with a known good one... still the same. Continuty check, all checked out fine...

Swapped the guage still the same... scratched the head... So followed the tracks on the instrumen, and found a piggy back board that the wireing from the sender goes through...

Swapped the instrument panel to another ALL worked well!!!

So the small piggyback board can go wrong, but not looked to see exactly what it does...

Paul
Last edited by citroenxm on 05 Oct 2010, 18:24, edited 1 time in total.
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Post by addo »

It's that same problem by the sounds.

I'd reckon the boffin quotient of FCF is enough to see it fixed, though. :)
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Post by addo »

Interesting observation:

The Handbrake warning light draws ground from a source shared with the fuel sender. With a quarter tank, even when brakes released the warning light remained dimly lit - clearly there was a ground being "pulled" backwards through the sender.

Filling the tank extinguished the handbrake lamp - obviously due to the higher inline resistance of the sender.

That suggests on the driver board, a solid state component has failed, allowing reverse current flow. Diode, probably.