MPG Gauge wtf?

[Gawernator]

My MPG gauge keeps turning itself "off" and "on". It will work, then randomly not work. Today driving home it kicked itself back on and off three times in 2 minutes!!! After that it worked the whole way home...

??????

This is in addition to the non-functioning volt gauge again, not sure if related

[porschetub]

Tell my about it mine hasn't worked since the 2.7 conversion,no one had an answer I just put my foot in and enjoy the power,love the 2.7.

[AaronM]

If you have other nonfunctional gauges, you're probably looking at a grounding or connection issue within the gauge cluster itself.

The MPG gauge gets an injector duty cycle signal from the DME and also a speed signal (in the early cars the speed signal comes from a hall-effect sensor on the mechanical speedo, in the later cars it uses the same electrical pulse signal as the electronic speedometer), which it then combines to display a fuel usage readout.

Below a certain speed (IIRC it's 12 mph, but I could be wrong) the system will read in gallons per hour (the light grey "gph" scale) while above that speed the gauge will read in mpg (the white scale). In early cars it's fairly common for the hall sensor to fail and leave the gauge reading gph at all times.

[porschetub]

Originally Posted by AaronM

If you have other nonfunctional gauges, you're probably looking at a grounding or connection issue within the gauge cluster itself.

The MPG gauge gets an injector duty cycle signal from the DME and also a speed signal (in the early cars the speed signal comes from a hall-effect sensor on the mechanical speedo, in the later cars it uses the same electrical pulse signal as the electronic speedometer), which it then combines to display a fuel usage readout.

Below a certain speed (IIRC it's 12 mph, but I could be wrong) the system will read in gallons per hour (the light grey "gph" scale) while above that speed the gauge will read in mpg (the white scale). In early cars it's fairly common for the hall sensor to fail and leave the gauge reading gph at all times.

Correct in what you are saying about the signal being differant on the early and late cars,I posted awhile back on this and got that info but no one had a fix for my situation (late motor in early car).

Gawernator as you probably already know there are earth problems for the dash gauges (been there, had that) some of my gauges played up after I removed them for a tidy up.
Your problem kind of points to that more to that as the unit is still working @ times.

[Gawernator]

I'm going to redo the grounds with my new rotary tool instead of hand sanding. Are there any connections besides the football shaped ones? I'm hoping I just didn't tighten the volt gauge connection down enough and it came loose. Are there any grounds I maybe could have missed on the cluster itself? Also maybe source replacement nuts and washers?

[AaronM]

Originally Posted by porschetub

Correct in what you are saying about the signal being different on the early and late cars,I posted awhile back on this and got that info but no one had a fix for my situation (late motor in early car).

If you have an 85.5 car I'd think you could swap out the instrument cluster with one from a 2.7 car, but if you have the early interior there's probably no solution at all. It seems like it would be possible to translate the speed and injector duty signals to suit the older gauge, but the amount of effort required for that means that no-one's done it.

[porschetub]

Originally Posted by Gawernator

I'm going to redo the grounds with my new rotary tool instead of hand sanding. Are there any connections besides the football shaped ones? I'm hoping I just didn't tighten the volt gauge connection down enough and it came loose. Are there any grounds I maybe could have missed on the cluster itself? Also maybe source replacement nuts and washers?

Used rough sandpaper and grimped up the connectors with pliers did the job fine.

[Gawernator]

So it must not be the ground...

[AaronM]

Originally Posted by Gawernator

So it must not be the ground...

Could be an issue inside the electronic tach unit then.

[Gawernator]

I'm definitely have some electrical issues and I'm sure grounding is part of it because I can still make the needles move by flicking on the high beams or turn signal etc...

Are there grounds I can clean on the cluster? Should I replace the nuts and washer on the back or just get em with my new rotary?

Also perhaps the 800 grit sandpaper was too fine? I'm also hoping the connection on the back of the Volt is loose again, I fixed it once before. I will also put in LED bulbs and re-do the reflective material.. aluminum foil with double sided tape?!

[pfarah]

It sounds like the stalk may have ingested some small conductive material causing arcing/grounding. You may want to remove the turn signal assembly and thoroughly clean/inspect all contacts and traces. A few months ago, I had some electrical gremlins come and go unexpectedly and found crap in between the radial traces. I gently rubbed the traces with a #2 pencil eraser (softer than some other types), then used canned air to gently remove all debris. It took me about three times to effectively clean everything out but the gremlins went away (wipers come on with turn signals, washer motor not activating, etc.) Have you ever removed your steering column; I know the early models differ from your 86 but it's not a big issue, just takes some time. You also may want to look at all the cable connections while you're in there. Just my two cents incase you haven't performed this procedure yet.

[Gawernator]

Wait, I have only removed the steering wheel before, and I tried to fix the turn signal cancel by wrapping the black thing with electrical tape but it still doesn't work. What do you mean by "stalk"? Cable connections on the cluster? I was going to clean again with some contact cleaner carefully. +1 on eraser for removing electrical corrosion. Do you have any photos? I'm mechanically retarded, remember.