Following my fuel regulator connection failure I have had two other faults appear. One is co-incidental, No.3 glow plug, so that’s on order. More worryingly, the fuel gauge is reading empty with the low fuel light flashing. The fuel sender in tank is a simple resistive type that is connected to the Body Computer (BC, part of fuse box by the drivers knee). The BC shows 0% fuel on Multiecuscan, but if I unplug the tank unit it gives a fault of open circuit sender. Checking the resistance at the BC (plug D, below EOBD socket, pins 4 and 15) show correct value, 155 Ohms, about ½ full. The range is 300 ohms for 1 litre to 50 ohms for 62l. This all points to a failed fuel level input on the BC. Research on the forum comes up with a similar failure when plug D was pulled. It seems the BC does not like having the fuel sender unplugged with power on. The problem is that the BC is reputedly part of the CODE system, but I can’t find any firm information on what is required when changing it. So I have a number of options:
1/ Pay £x00s for a new one from Fiat – not going to happen.
2/ Change the BC, TEG, Steering lock and ECU for used set – probably not going to happen.
3/ Fix the existing BC fuel sensor input.
4/ Fit used BC and see what happens
5/ if 4/ does not work, identify the “chip” that contains the CODE data and swap to the “new” BC.
6/ Disconnect the sensor from the BC and connect it to a new DIY fuel gauge.
So to start with 3/, 4/or 5/ I’ve ordered a cheap used BC from EBAY. It will be here Wednesday and I’ll open it up and see what’s what inside. It’s pretty poor design to have the fuel gauge fail because the sensor was disconnected/connected with power on. The BC appears to be over-reading the resistance but still working to some extent because it can detect the open circuit state. It looks like it may be driving too much current into the sensor.
Watch this space.


Robert G8RPI

just try another fuel pump

flisko said:

just try another fuel pump

Click to expand...

Why do you suggest that? Apart from wasting around £150 and a lot of time it is going to do nothing for me. I already know the level transmitter is OK because I've measured it's resistance and its in specification. Just to make sure I'd also subsituted a 100 ohm resistor in its place with no change.
Just randomly changing parts is very wasteful. In this case the sensor can be easily checked with a standard multimeter.
I'm trying to quietly teach good diagnostics and fault finding, changing parts is not part of that.

Robert G8RPI.

So an update.
Spare Body Computer arrived so I had a look inside. The bad news is both the analogue to digital converter (ADC) and the CODE memory are in the microprocessor. This is a 164 pin surface mount device and while I have thee correct equipment to replace this device the chance of getting the one from my car off and onto the "new" BC intact are not good so I've ruled that out. At least not for just a faulty fuel gauge. I did try the "new" BC in the car and confirmed that it gave a "Vehicle Protection Sytem" error with a dead car. Just Getting the BC out is a pain as it is held in with shear head security bolts so you have to drill them and use a stud extractor (M5). Investigation shows the circuit was not quite what I expected with a voltage source rather than current with the current resulting from the tank resistance being measured across a 250 ohm (4x1k in parallel to be precise) resistor by the ADC in the microprocessor via a RC low pass filter. I was measuring 12V open circuit on the soures pin before so that ties with the voltage regultor being faulty- high. The resistors all checked out as did the pass transistor. The control transistor cas was cracked when I took it off (microscopic surface mount device) so could not check it. I swapped it with one from the "new" BC. This resulted in a "Full" gauge indication so I've either fixed it or broken it further.
I'd put fuel in the car yesterday but don't know how full it is, the gauge could be correct.. I'll check the sender resistance tomorrow to see how much is in the tank. I've done a lot today and getting tired and don't want to make a mistake and damage something.
Watch this space.

Robert G8RPI.

I checked the tank unit resistance today and it's 41 ohms so full tank. Gauge is reading full so looks like I've fixed it .
The faulty transistor is tiny (SOT-23) 2.9 x 1.6mm
tran-1.jpg
tran-2.jpg
Shown on 5p piece. It's not easy to replace, even once you have identified which one of the dozens of devices is the faulty one. I robbed the replacement transistor from another Body Computer.
The Punto Body computer seems very similar so could have the same issue.
Moral is don't disconnect anything connected to the body computer with power on and disconnect the battery before disconnecting the fuel tank unit.
I have located a company who say thay can clone your existing BC to a used one you supply

j cracknell car sales

www.ebay.co.uk/usr/jcracknellcarsales

https://www.jccars.co.uk/
I've not used them, cost is around £75 plus post and you can't use te car while they have the old ECU.

Robert G8RPI.