Kilohertz

Hi boys and girls,

I have been working my way thru all the things that need to be fixed for the inspection and I have been frustrated by the MPG econometer, I just can't get it to work. I know it's not required to work for the inspection....I have checked with a scope, both pulses are present at pin 1 from the DME fuel injection signal, black/yellow wire, and the speedometer hall sensor is working producing a good pulse at the HG terminal, so I know the signals are there, but the gauge isn't reading anything, pinned to the right.

Today I put the meter on the workbench and put in a pulse signal from a sig gen and still didn't get anything. I checked the signal by putting it into the tach input, pin 6, got the tach to read 1K to 6K RPM just fine.

So I need some help at the circuit board level of this tach/MPG o'meter thing. Anyone have schematics of the tach/MPG circuit, or been successful in board level repair of the tach/ECO meter?

Any help, would be greatly appreciated. Thanks!!

Cheers

BTW, it's a 1983 944, but I think they pretty much all work the same

benchod

On the 944 I'm going to be taking delivery of neither the tach nor the econometer work. Any suggestions on what should be checked first?

Tom M'Guinn

Are you saying the tachometer is not working when the motor runs, but does work when you supply a pulse from a signal generator on the bench? If so, seems like you just need to get the ignition pulse (and ground) to the tach. I've never played with the fuel gauge, but wonder if it operates like the turbo's boost gauge. If so, try putting a 4k resistor on its input and feeding that resistor a 0-5v analog signal for testing (don't forget the ground). If that works, then like the tach, it would seem the gauge is just disconnected.

Kilohertz

HI Tom,

Sorry, no, I guess I didn't explain it very well. It's the MPG/Eco meter that shares the same circuit board as the tach, that is not working, tach works fine both on the engine and the test bench. It's the MPG meter that stays pinned to the right all the time. They are both mounted on the same round circuit board and it's a bugger to try to figure out what chips are for what.

Cheers

Tom M'Guinn

I see. I thought maybe some of my boost gauge work would translate over but doesn't sound like it, sorry about that.

Dan Shea

Is that all it uses to calculate MPG? Fuel pulses and engine speed? I always thought it was more complex than that on the N/A cars. Mine stopped working long time ago (probably when I did the 16v motor), it would stick far left, far right, and sometimes give me a reading. Come to think of it, don't these cars have a econ shift light too? That hasn't worked on mine in so long I forgot it's there.

Kilohertz

Hi Dan,

It actually uses road speed, from the Hall effect sensor on the back of the speedo, and injection pulse duration from the DME to compute MPG. I have seen video of it's normal operation and at idle it seems to hover around the middle of the range, indicating GPH, then above 20MPH indicates MPG.

Cheers