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Old 18th January 2021, 23:13   #31
Sonic ZS
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Having been using the car for almost a week now (driving, stopping & starting, parking on different inclines, varying ambient temperatures, etc.) with fixed resistors in place of the tank senders, the gauge has behaved itself with absolutely none of the eratic behaviour seen previously

Last week, I ordered a potentiometer (500 ohm) and today I swapped the fixed resistor over for this, just to see if I could make the gauge move to any desired position without any odd movements. Once again, with the ignition on, the needle moved up & down exactly as predicted, from full to empty or any chosen position in between.

So I think I can now confidently confirm to other members that a correctly working 75/ZT fuel gauge system DOES indeed show any change in fuel level if the ignition is on. You can forget the idea of complicated algorithms, the system works exactly as expected if the resistance is varied - although the needle does move quite slowly, taking maybe 30 seconds to travel it's full range.

Hopefully, I'll get chance to have a look at the sender circuit boards tomorrow....
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Old 19th January 2021, 17:20   #32
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Thanks for taking the time do carry out and report back on your detailed analysis. Looks pretty convincing.
Can you report back when you've replaced the bits. I suspect you'll help many members with infuriating gauges, myself included!
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Old 19th January 2021, 18:21   #33
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Will do Michael - I'd hoped to have a look at the senders & circuit boards today, but unfortunately I didn't get the spare time I'd hoped for

I'll try again tomorrow - hoping to find something obvious like a worn rheostat or loose float, but I'm begining to wonder if it could be corrosion inside the electrical connector on top of the pump....

Will update again when I can
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Old 20th January 2021, 22:10   #34
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Great stuff Paul, you're filling in gaps that should help resolve one of the 75/ZT's great mysteries, the wonky fuel gauge. It's clearly a relatively simple mechanism after all, but prone to the most odd behaviour. The slow needle response to rapid changes in the rheostat confirms the electronic damping imparted by the ECU to drive the gauge. This smooths out the sawtooth effect you'd otherwise get from the fuel sloshing around. What remains to be understood is what causes the float rheostats to give varying values when the fuel level isn't changing. I suspect the answer will be quite interesting.


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Old 21st January 2021, 17:15   #35
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Managed to get another hour on this yesterday and had a look inside the tank at the O/S (pump) sender unit. Didn't get too far as I needed the car for work last night and was worried I might break something ! Will have more time next week though.

Anyway, have uploaded some pics to show the progress so far and hopefully assist others if they want to test their own systems....

a) This was the initial testing of the fuel gauge using a selection of fixed resistors, each of a known value within the normal sender range, to determine if the needle was showing the correct amount. The normal range is about 10 to 360 ohms (I believe, from measuring a couple of old units), so a resistor of around 180 ohms should place the needle about half way up the gauge. I used five different resistors to get the gauge to show 1/8, 1/4, half, 3/4 and full:



b) I then made up a simple link lead to allow me to run the car for a few days with the senders disconnected but the fuel pump attached. I could substitute the fixed resistors once again and see if there was any abnormal behaviour in everyday use. Please remember this was only a temporary thing....it looks a bit bodged !!:



c) I then tried a variable resistor in place of the fixed ones, to move the gauge with everything running. Unfortunately, I forgot to take a pic, but just used a basic 500 ohm potentiometer. With the ignition on, the gauge moved exactly as expected, as it had with the fixed resistors.

d) Then yesterday, I had a close look at the pump side rheostat track. Quite grubby, so a quick wipe with a cotton bud to clean it up temporarily. The tracks do look quite tarnished. Next week I'll have a better go and use some contact cleaner, possibly removing both senders from the tank:



e) Lastly, I had a close look down inside the connector, as corrosion here would give increased resistance, but they actually look very clean (not sure if it'll show up in the image):



It does look more likely to be the rheostats, will dig deeper next week. Does anyone know if the metal tracks can be replaced, or are new boards available without having to buy the whole pump/filter units ??
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Old 21st January 2021, 22:50   #36
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Quote:
Originally Posted by Sonic ZS View Post
....it looks a bit bodged !!

Nay Paul, that's never a bodge. Using the correct colour coded cable for the pump positive and taping the two together is very safe and sensible work indeed.

Simon
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Old 21st January 2021, 23:40   #37
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Quote:
Originally Posted by Sonic ZS View Post
Does anyone know if the metal tracks can be replaced, or are new boards available without having to buy the whole pump/filter units ??
More great info from Paul.

I think the likelyhood of finding new rheostat boards is vanishingly small. I can't say zero because people like DMGRS and Rick-Sta have located components you'd think would be extinct. If a new board would have a market, I dare say they'd be intrested in scouring their sources.

Another facet to your investigation would be to run the Trip Screen Diagnostics while you have the external rheostat set up. In Section 6.2, there's a variable digit (1, 2 or 3) preceding the 6.2 that confirms rheostat output (yes or no, LH float or RH float or both). There's still ambiguity in the way these digits appear and are interpreted. My write-up (Edit 7 October 2019)gives more on this. Any further data you can get will help to refine it.

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Old 22nd January 2021, 13:47   #38
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Totally agree with Simons post.--Not a bodge.--I've spent a lot of time where electronic circuitry is being developed.--Looks like what you are doing but many times magnified.

Good on you.--
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Old 23rd January 2021, 01:25   #39
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My fuel gauge hasn't worked for a few months now, just sits on empty with the fuel light on. I get 400 miles per tank so l fill up, zero the trip & fill up again at 350.
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Old 23rd January 2021, 08:10   #40
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What an interesting thread. Thanks Sonic.

Well done for the efforts and everybody's input along with the explanations and pictures.
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