Author Topic: What controls the "MPG" gauge in the tach cluster? (Read 4685 times)

Cobra Jet · « **on:** July 08, 2008, 10:21:01 AM »

What harness, relay, sensor or sending unit controls the MPG gauge that is located in the tach cluster?

Where does it get it's "signal" from and how does it operate?

Is there a separate sending unit in the fuel tank for this gauge?

haledj · « **Reply #1 on:** July 08, 2008, 04:27:06 PM »

It gets it from the ecu. I beleve it is a function of your RPM, pulsewidth, and speed

EN318isPDX · « **Reply #2 on:** July 08, 2008, 05:16:11 PM »

Seems like its vacuum controlled. I am probably way off but thats just my thought. Ever since i seen the gauges in the M3 I've wanted to ditch the useless MPG and get something useful... Oil pressure or temp! Don't remember which it is but its way more useful than a silly throttle position gauge.

Cobra Jet · « **Reply #3 on:** July 09, 2008, 10:10:24 AM »

Well, the reason I was asking how it operates is because sometimes mine will operate in reverse - meaning that instead of operating normally, the needle operates in the reverse manner when depressing or letting off of the accelerator pedal... it only does this sporadically, and I can never replicate the problem. The first time I noticed it operating funny I just happened to glance down and saw it was operating in the reverse manner of normal... Then I kept watching it and eventually hit a large bump in the road. When I glanced down at it again it seemed to have returned to normal, however, I cannot comfirm my reasoning 100%....

This is why I wanted to know if the gauge is operated by "wire", vaccuum or other?

I've never had the gauge cluster out, so I have no idea what is attached to the back of the cluster, especially in the area of the MPG gauge.

So, how is this gauge actually operated, is it definitely a vacuum line or is there a wire harness I need to look at to see if it's lose or not grounded properly?

Any more help is much appreciated!

batsbats · « **Reply #4 on:** July 11, 2008, 12:52:51 AM »

I laugh when I'm at a dead stop, and revving causes it to spike from 0 mpg to about 15-20 mpg.

AcSchnitzer318is · « **Reply #5 on:** July 11, 2008, 02:02:05 AM »

SI board batts starting to go bad will cause this. Does your tach ever get goofy? If so it's definitely the batts going. That was what happened on mine anyway... can't hurt to check while you have the cluster out.

tony92ic · « **Reply #6 on:** July 11, 2008, 08:48:31 AM »

Mine's been doing that for a couple of years. Unfortunately, I am not really getting 50mpg when I have the accelerator pegged to the floor. :-)

Its such a useless gauge I've never worried about making it work.

I seem to remember it being vacuum operated in the system somewhere, but not all the way to the gauge.

DRtothE · « **Reply #7 on:** August 29, 2008, 12:04:18 AM »

i don't want to hijack or anything, but it seems as though this thread might be dead. My Tach and MPG aren't working but my temp and MPH are fine. any ideas?

AcSchnitzer318is · « **Reply #8 on:** August 29, 2008, 12:30:25 AM »

SI Board or SI Board batteries... do your SI lights work properly?

DRtothE · « **Reply #9 on:** August 29, 2008, 11:13:05 PM »

yea they work.

AcSchnitzer318is · « **Reply #10 on:** August 30, 2008, 10:29:22 AM »

Quote from: DRtothE;55742

yea they work.

Do they just light... or can u reset them and everything? I still think your problem is SI board batts.