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Old 7th July 2019, 20:14   #21
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I've been doing a bit of comparison today as I have suspected my gauge of being pessimistic and my gauge was nearly on E so had to fill up.

Prior to fill up I checked on the diagnostics and had

6.0 = --0026
1_6.2 = --0005

I then filled the tank to the brim and then got

6.0 = --0057
1_6.2 = --0057

This corresponds with the 57 litres I put in the tank, but given that the tank on the ZT is 65 litres, does this mean mine is misreading by 8 litres? It would certainly explain why I seem to get less than 300 miles on a full tank. I knew with my other car it was about 320 miles to E.

Last edited by kelvo; 7th July 2019 at 20:28..
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Old 7th July 2019, 20:42   #22
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Default Fuel Gauge Tests

My MG ZT 1.8 (04 Mk1) petrol gauge started playing up. Sticking and generally becoming untrustworthy. I filled the ZT up and added a dose of Redex. Drove the car around locally selecting the routes with speed bumps After a longer journey to London and back the gauge fell to ¼ full at around 250 miles. However, the next day the gauge had risen to nearly ½ full. After some local use it fell to ¼ again, 285 miles from filling the tank. I removed the rear seat out took some resistance measurements.

Note: the Maplins meter I used seemed a bit inaccurate at low resistances, in that touching the probes doesn't get zero, but around 4 ohms. Also my ZT's connector block was 180 degrees different from the one in T-Cut's diagram as far as I can tell. i.e. the open side was in the car centre.

RHS of car, Forward=>




Readings were:
Pins ; Ohms
6-3 ; 9
5-2 ; 4.5
4-1 ; 132

Quite a variation between the different sides of the tank. I note the car is very low mileage so I do not think there will be much wear on the variable-resistor tracks, although it is 15 years old which could lead to deterioration in a corrosive environment like a fuel tank.

Also, I noted that my fuel pump connector was wired differently from the 75/ZT Haynes manual (ed. 2010, Sec. 12:32)

Pin ; Actual Colours ; Haynes Colours
1 ; Brown/ Green ; Brown/ Green
4 ; Green/ Black ; Green/ Black
3 ; Black/ Grey ; Brown/ Green
6 ; Brown/ Green. ; Black/ Grey

i.e. Pins 3 and 6 were inverted. As it was previously working fine and the wiring diagram shows the variable-resistors on the fuel level-sensors wired in series I figured it didn’t matter.

T-Cut suggested I take some IPK Test 6 readings and post to this thread, which I did today around checking the lhs sensor floats and fitting a fuel-filter locking clip (a.k.a. FOC) whilst in there. Figuring the tank in my car was 65 litres (Parkers), after fitting the FOC, I added petrol in 15 litre increments twice (~1/4 tank at a time), after taking an initial reading. Always parking the car on my drive in the same place to take the readings, noting the car was slightly nose down and cantered to the RHS. I then took the final reading on level ground as a control. The fuel gauge indicated realistic readings throughout the test, moving up just less than a ¼ tank per 15l of fuel added.

Readings were:

Test___¼ full___~½ full (+15l)___~3/4 full (+15l)___~3/4 full (level)

--6.0___--0194______--337_________--0484__________--462
Diff__________+143_________+147___________-22

--6.1____-----_______-----__________-----____________-----

1-6.2____---164_____--0273________--0427__________--434
Diff___________+109________+154____________+7

I understand the first number in 1-6.2 indicates both sensors are serviceable.
Looking on both the 6.0 and 6.2 lines, and dividing the number by 10 you would get the approx. litres in the tank. e.g. 434/10 + 43.4 litres. Thus both went up very roughly 15l when that much fuel was added.

If test 6.2 gives total fuel in tank, is test 6.0 doing the same but by another sensing or calculating method?

Test 6.1 gave nothing.

I hope this is useful. Anyway, I’ll run the ZT for a while and hope the gauge is trustworthy from this point on…
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Last edited by gnu; 8th July 2019 at 16:03.. Reason: clarification
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Old 7th July 2019, 20:47   #23
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Quote:
Originally Posted by kelvo View Post
I've been doing a bit of comparison today as I have suspected my gauge of being pessimistic and my gauge was nearly on E so had to fill up.

Prior to fill up I checked on the diagnostics and had

6.0 = --0026
1_6.2 = --0005

I then filled the tank to the brim and then got

6.0 = --0057
1_6.2 = --0057

This corresponds with the 57 litres I put in the tank, but given that the tank on the ZT is 65 litres, does this mean mine is misreading by 8 litres? It would certainly explain why I seem to get less than 300 miles on a full tank. I knew with my other car it was about 320 miles to E.
These results fit with what I found today: test 6.0 and 6.2 seem to give similar readings (within a few litres), although yours are a lot closer than mine.
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Old 8th July 2019, 07:44   #24
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I just tried this as I needed to stick a bit in.
It was showing the following prior to filling:
Range - 22 miles
1_6.2 = _ _ 0045

I added a tenners worth of fuel (8litres) then checked again.
Range - 100 miles
1_6.2 = _ _ 0131

Literally, did this in the station and straight after driving out, parked on a flat car park
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Old 8th July 2019, 10:22   #25
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It's becoming clear that the Diagnostic Routine HowTo for Section 6 needs a complete overhaul. As I comment in the write-up, there are too many differences being thrown up to trust the instructions. So far, it's not clear - to me anyway - what the significance of Test 6.0 is compared with Test 6.1. In no case so far has anyone reported a display as 'abcxyz'. All we are seeing (I think!) is '_ _0abc'. So we haven't had a reading showing the separate tank sides, only totals. However, there are also some ambiguities being seen even within this norrower scope for Section 6. For example, Typhoon190's tank had one faulty sensor and gave Test 6.0 _ _0000 even though it was half full. However, kelvo sees 6.0 _ _0057 with a full tank and both sensors working.
Confused? Just keep taking the readings.

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Old 8th July 2019, 20:39   #26
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TC, thank you for posting what you have so far. Having owned a ZT for less than 2 years I've found your posts instructive and illuminating.

As a discussion piece, Regarding the 6.0 test, my first thoughts would be that its a check of the 6.2 test, measuring the predicted fuel in the tank using another sensing or calculating method. From the Haynes circuit diagram it looks like the green/ brown wire pins 1 and 6 simply joins the variable resistors up in series. With your measurements, showing RHS ranges from Empty 147 to Full 4.7 Ohms and LHS ranges from E 4.7 to F 153 Ohms, when they are joined in series the range would be only be about a 1 ohm per 1/8 tank, 8 ohms total (as one side goes up in resistance, the other goes down). Not a big figure since you proved the variation due to dirt etc will far exceed this. Is the computer is doing something clever with the signal? If it senses anomalies due to u/s sensors does it throw in the strange results? The black/grey wire also links to an air temp sensor, so maybe either 6.0 or 6.2 takes this into account, with the other result being the baseline result?
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Old 9th July 2019, 14:17   #27
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OK, here's a synopsis of results so far. What these show is the probability that Test 6.1 is not part of the analytics and can be ignored.


These are the numbers exactly as reported, so I think there are a couple of errors which I've commented on. I also have a feeling that some of these data suggests there are some worn floats out there.

Can everyone interested in this thread send us some numbers? We need a lot more to get a clear picture of how it works.
Cheers,

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Old 9th July 2019, 20:03   #28
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Yeah, I've checked my notes and all my results have the noughts where expected
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Old 9th July 2019, 21:09   #29
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Quote:
Originally Posted by gnu View Post
Yeah, I've checked my notes and all my results have the noughts where expected

OK, cheers.
The variation between the Test 6.0 and Test 1_6.2 is interesting. In your case Test 6.0 is always larger.


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Old 9th July 2019, 21:14   #30
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Apologies, I have just looked at the pics I took of my diagnostics, and with a full tank both readings were:

6.0 = _ _ 0570
1_6.2 = _ _ 0570

This still doesn't account for where my missing 8 litres is...
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