Instant and Average MPG Not Working?
Thread Starter
Heel & Toe
Joined: Aug 2001
Posts: 22
Likes: 0
From: Hawthorne CA
Instant and Average MPG Not Working?
My final - always final problem - is the average and instant mpg reads 0 or 99 - sometimes just random number that respond or not respond to the actual driving - it used to work somewhat intermittently.
All other readouts are seemingly correct except miles remining - which I would guess it using the average mpg.
is there any sensor or other item I could check to troubleshhot this?
thanks,
barrypaul
[Modified by barrypaul, 6:21 PM 8/7/2002]
All other readouts are seemingly correct except miles remining - which I would guess it using the average mpg.
is there any sensor or other item I could check to troubleshhot this?
thanks,
barrypaul
[Modified by barrypaul, 6:21 PM 8/7/2002]
Instructor
Joined: Sep 2005
Posts: 135
Likes: 12
My old post ... finally fixed the issue - needed to replace the ECM.
Just wanted to conclude the post in case anyone else is searching. It sure would have helped me out.
Here's my summary:
Just wanted to conclude the post in case anyone else is searching. It sure would have helped me out.
Here's my summary:
I had the mpg readout in error since I got the vette 22 years ago. Since this was the last issue I wanted to get this finally fixed. I checked out the circuit (serial data line) when I had the dash out and it checked out OK.
I've been researching this readout problem for years - I even have some old posts that I'll be updating. I did clean up the ECM connectors a while back, and as I mentioned I just cleaned up the dash connectors. Interesting that I dug out my old paperwork and the previous owner had the ECM replaced when the vette was just 4 months old. Researching on-line recently convinced me that it was the ECM or the PROM - but I've noticed that the range updates when I was idling - meaning that their was communication between the ECM and dash via that serial data line.
So my last research just last week lead me to another corvette forum, and if its OK to quote that one, this is what I found:
Quote:
Decided to contact Gordon Killebrew on his fee consulting line (800-For Your VETTE). As I described the above situation, Gordon explained that there is a serial data issue with the stock ECMs, and was very confident that replacing the ECM would resolve it. When searching the internet, I found two other situations similar to mine (one was a 1986 and the other a 1989) that was fixed using a new ECM. I'll post an update when completed. Right now the weather is 60+ and sunny in Western New York. so I'm enjoying the ride. - See more at: http://www.corvetteactioncenter.com/....oGGz1h9R.dpuf
and his follow up a day later...
Quote:
As promised, here is the update. Gordon was correct and that the issue was the ECM. The ECM needs to send MPG data to the cluster via a serial data line. User access is via connector "E" on the ALDL port. Purchased a rebuild ECM from Advanced Auto for $80. It was Cardone Part # 777165 (AC Part # 1227165), which is used on the 1986-1989 vettes (other cars as well). I opened the case to verify Cardone did indeed clean and recoat the electronics with a special clear sealant when they rebuilt it. They claim improved ECM reliability over the GM sealant. Replacement was pretty easy. 3 screws hold the passanger kick panel, and 2 screws hold the ECM in place. Transferred PROM, ECM cover plate, and two mounting prongs. - See more at: http://www.corvetteactioncenter.com/....oGGz1h9R.dpuf
That was enough to convince me to get the refurbished ECM - so glad I did.
I've been researching this readout problem for years - I even have some old posts that I'll be updating. I did clean up the ECM connectors a while back, and as I mentioned I just cleaned up the dash connectors. Interesting that I dug out my old paperwork and the previous owner had the ECM replaced when the vette was just 4 months old. Researching on-line recently convinced me that it was the ECM or the PROM - but I've noticed that the range updates when I was idling - meaning that their was communication between the ECM and dash via that serial data line.
So my last research just last week lead me to another corvette forum, and if its OK to quote that one, this is what I found:
Quote:
Decided to contact Gordon Killebrew on his fee consulting line (800-For Your VETTE). As I described the above situation, Gordon explained that there is a serial data issue with the stock ECMs, and was very confident that replacing the ECM would resolve it. When searching the internet, I found two other situations similar to mine (one was a 1986 and the other a 1989) that was fixed using a new ECM. I'll post an update when completed. Right now the weather is 60+ and sunny in Western New York. so I'm enjoying the ride. - See more at: http://www.corvetteactioncenter.com/....oGGz1h9R.dpuf
and his follow up a day later...
Quote:
As promised, here is the update. Gordon was correct and that the issue was the ECM. The ECM needs to send MPG data to the cluster via a serial data line. User access is via connector "E" on the ALDL port. Purchased a rebuild ECM from Advanced Auto for $80. It was Cardone Part # 777165 (AC Part # 1227165), which is used on the 1986-1989 vettes (other cars as well). I opened the case to verify Cardone did indeed clean and recoat the electronics with a special clear sealant when they rebuilt it. They claim improved ECM reliability over the GM sealant. Replacement was pretty easy. 3 screws hold the passanger kick panel, and 2 screws hold the ECM in place. Transferred PROM, ECM cover plate, and two mounting prongs. - See more at: http://www.corvetteactioncenter.com/....oGGz1h9R.dpuf
That was enough to convince me to get the refurbished ECM - so glad I did.
Instructor
Joined: Sep 2005
Posts: 135
Likes: 12
Nice Display!
Slightly different than my symptoms, my mpg would be 0 or 99 and go to 28 for a couple seconds when I hit reset.
Either way, sounds like ECM not calculating correctly. Since the range is displayed, like mine was, that bidirectional line between the dash and ECM is talking.
Slightly different than my symptoms, my mpg would be 0 or 99 and go to 28 for a couple seconds when I hit reset.
Either way, sounds like ECM not calculating correctly. Since the range is displayed, like mine was, that bidirectional line between the dash and ECM is talking.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
The ECM sends the injector pulse width to the dash. The dash uses the vehicle speed and that injector pulse width to calculate the MPG and from that it calculates the range using the fuel level in the gas tank.
Safety Car
Joined: Nov 2008
Posts: 4,328
Likes: 15
From: Jacksonville FL
If this is true, where is my car dropping the ball with inaccurate range and accurate MPG? Its really weird because every time I get gas the range increases with the same amount of fuel but it is still nowhere close to where it should be.
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