:: Fuel economy gauge...

[Qube]

Where does the dash fuel economy gauge take it's reading from?

I notice mine is starting to act up (probably because it knows I'm going to by a TT). Sometimes it doesn't work. Sometimes it stays at zero (instead of off the side). Sometimes it just sit at a fixed position.

I notice that when it's 'broken', the OBC readout in test mode and consumption mode is 0 as well.

Any hints?

[infurno]

I would like to know too. Mine wont go below 24. I remember reading somewhere that the electronics (capacitors?) in the dash went bad and it needs to be replaced. Im not sure if this is correct though.

I hope there is a simpler, and cheaper solution...

[Qube]

If it's that, I don't mind as I have a spare clusterboard... but who knows. That would affect the OBC test group readout as well?

[Morgenster]

I'm pretty sure the gauge takes input directly from the ECU. If it acts weird it can mean two things: either the gauge itself is malfunctioning or the ECU is doing some weird stuff. It woks on the basis of preset values in the ECU and not real world measurements of how much fuel is released by the injectors.

[karl1171]

The OBC and Instrument cluster get information directly from the fuel gauge sender and speed sensor. I assumed these are processed within each unit to produce a fuel consumption figure. The OBC just gives a running average.

I also think its likely the capacitors are at fault.

[myles]

I had a problem with the mechanical movement of the gauge itself. The needle got stuck on something as it passed a certain point on its path. Try doing a cluster self test to see if the gauges move correctly then.

[Ross]

If it says zero MPG then it seems logical that it doesn't recognize the signal from the speed sensor telling the computer the car has moved.
My '89 has a bum speed sensor(Ithink, the speedo is inop). I'll check and report what the OBC reports for fuel consumption.
What is your speedo/odo doing??

[Qube]

My speedo and odo work fine. It's the econo gauge and OBC consumption and instant consumption (test mode) that shows nothing. Keep in mind doesn't always happen. Every once in a while since a few weeks back.

[uscharalph]

I had a problem with the mechanical movement of the gauge itself. The needle got stuck on something as it passed a certain point on its path. Try doing a cluster self test to see if the gauges move correctly then.
How do you perform a cluster sef test?

[myles]

Hold in the check control button (the one on the right) as you turn the key from accessories to on, release the button when the test starts. If that doesn't work try holding the check control button while turning the key from off to accessories. The self test only works on cars with 02/89 and later instrument clusters.

[Ferret]

My TDs started doing this a while back, and then I noticed a pattern forming...

It would get a lot worse when the weather was cold and the cluster was a bit frozen. Took the guage apart and the pivot point/actuator on the guage was shot.

If you think about it, this isnt surprising as the iMPG guage does an order of magnitude more work than any of the other guages - it's constantly on the move all over the place despite the fact your speed and rpm arent changing that much.

I agree with Morgenster above - I think your mpg is calculated by the ECU - if it was based off the fuel tank sensor and speed sensor only you'd get some weird readings every time you hit a bump and the fuel sloshed around in the tank. The fuel sensor is just not accurate enough to pull these readings off. Also there's nothing in the fuel lines that can measure flow; so it's almost a dead cert that it's the ECU doing the math.

If your guage is doing weird things, suspect the cluster and/or physical guage - they're the most likely culprits. If your ECU was at fault I'd be expecting to see the engine not running properly... (though its output driver may have failed - which is very unlikely considering how hardened bosch electronics tend to be.)

[Ross]

HMMM, sounds plausible.
Qube does your "spare clusterboard" include the gauge? I've an earlt cluster I bought for the LCD display which I can't use anyway. It was from a member here, I can't remember who, who was having trouble with his caps. It has new cap I"m told but he was still having some probs, I don't remember what though.
Anyway if you need a cluster or the gauge let me know.