86 ECM Problem?
Thread Starter
Instructor
Joined: Sep 2006
Posts: 192
Likes: 0
From: Seattle WA
86 ECM Problem?
Hi, I would normally be posting this in the scan and tune section, but I have a track school in 3 days, and I need to get this diagnosed.
I have an 86 that for the time I've had it (since september) has been detonating under 3/4 to full throttle. We got an autoxray scanner, and went to scan it this afternoon. With the scanner configured to scan an 86 corvette, the live sensor values were way out of wack. For example, the coolant temperature stuck at 124*, and vehicle speed at 5mph. Other values just slowly counted up until they got to the maximum value, then reset, and kept counting up. The PROM ID keeps changing as well. We thought that it may be a scanner problem, so we hooked it up to my boyfriend's 92, and everything worked fine. On a whim, we tried scanning it as an 85, and everything read correctly. All of the sensors' values made sense, but with the scanner on, the car idled poorly, and my average mpg displayed on the dash changed to 1.2. When we turned the scanner off, the idle returned to normal, and my mpg rating went back to what it was.
Does anybody have any idea what's going on with my car? I only have a few days to sort it all out!!
I have an 86 that for the time I've had it (since september) has been detonating under 3/4 to full throttle. We got an autoxray scanner, and went to scan it this afternoon. With the scanner configured to scan an 86 corvette, the live sensor values were way out of wack. For example, the coolant temperature stuck at 124*, and vehicle speed at 5mph. Other values just slowly counted up until they got to the maximum value, then reset, and kept counting up. The PROM ID keeps changing as well. We thought that it may be a scanner problem, so we hooked it up to my boyfriend's 92, and everything worked fine. On a whim, we tried scanning it as an 85, and everything read correctly. All of the sensors' values made sense, but with the scanner on, the car idled poorly, and my average mpg displayed on the dash changed to 1.2. When we turned the scanner off, the idle returned to normal, and my mpg rating went back to what it was.
Does anybody have any idea what's going on with my car? I only have a few days to sort it all out!!
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(sorry, I'm not an expert ... just thinking out loud)
The 85 had a different ECM than 86+ ...
Can you change the baud rate of the scanner (older computers were slower)?
To get data to flow on both my 85 and 87 I have to have a 10K resistor between pins A & B of the ALDL (I assume your scanner will do this). But scanning with the 10K resistor sends my cars into "diagnostic mode" (turns on fan, etc) ... maybe this is related to the car "running poorly"
Well, not much help but at least here's a bump for you
Gary
The 85 had a different ECM than 86+ ...
Can you change the baud rate of the scanner (older computers were slower)?
To get data to flow on both my 85 and 87 I have to have a 10K resistor between pins A & B of the ALDL (I assume your scanner will do this). But scanning with the 10K resistor sends my cars into "diagnostic mode" (turns on fan, etc) ... maybe this is related to the car "running poorly"
Well, not much help but at least here's a bump for you
Gary
Thread Starter
Instructor
Joined: Sep 2006
Posts: 192
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From: Seattle WA
Well, we pulled the ECM out, and the PROM is "for offroad use only," so it's definitely not an OE PROM. We read a post saying that the 86 has issues with a cracked circuit board on the ECM with the part number that I have. Though we couldn't see a crack, we figured that we might as well go grab one from the local auto parts store and check it out. We got it home and installed it (the ECM), and it does the exact same bizarre stuff.
Unless BOTH ECMs are bad (possible, but unlikely) I suppose I am looking at a PROM problem.
Any help would be GREATLY appreciated!
Unless BOTH ECMs are bad (possible, but unlikely) I suppose I am looking at a PROM problem.
Any help would be GREATLY appreciated!
Thread Starter
Instructor
Joined: Sep 2006
Posts: 192
Likes: 0
From: Seattle WA
(sorry, I'm not an expert ... just thinking out loud)
The 85 had a different ECM than 86+ ...
Can you change the baud rate of the scanner (older computers were slower)?
To get data to flow on both my 85 and 87 I have to have a 10K resistor between pins A & B of the ALDL (I assume your scanner will do this). But scanning with the 10K resistor sends my cars into "diagnostic mode" (turns on fan, etc) ... maybe this is related to the car "running poorly"
Well, not much help but at least here's a bump for you
Gary
The 85 had a different ECM than 86+ ...
Can you change the baud rate of the scanner (older computers were slower)?
To get data to flow on both my 85 and 87 I have to have a 10K resistor between pins A & B of the ALDL (I assume your scanner will do this). But scanning with the 10K resistor sends my cars into "diagnostic mode" (turns on fan, etc) ... maybe this is related to the car "running poorly"
Well, not much help but at least here's a bump for you
Gary
Melting Slicks
Joined: Apr 2003
Posts: 3,110
Likes: 6
From: Red Sox Nation! RI
I don't know much about scanners, but my first thought was inline with ghlkal's comments. Sounds like too much data out of whack to be anything more than a compatibility issue between the scanner and the ECM.
Can you get your hands on an ALDL diagnostic cable and a laptop? I've got one and they work great once you get them configured.
Can you get your hands on an ALDL diagnostic cable and a laptop? I've got one and they work great once you get them configured.
Thread Starter
Instructor
Joined: Sep 2006
Posts: 192
Likes: 0
From: Seattle WA
It wouldn't scan as an 86, and wouldn't scan properly as an 85, but it scans fine as an 87, and all the sensor values appear to be right on. The best we figure is that it's just a screwy PROM. We were going through vader86's PDFs of the FSM, and it seems as though it's an undersensitive knock sensor. We'll know for sure tomorrow.
Le Mans Master
Joined: Aug 2003
Posts: 9,884
Likes: 3
From: Magnolia Mississippi
St. Jude '03-'04-'05-'06-'07
(sorry, I'm not an expert ... just thinking out loud)
The 85 had a different ECM than 86+ ...
Can you change the baud rate of the scanner (older computers were slower)?
To get data to flow on both my 85 and 87 I have to have a 10K resistor between pins A & B of the ALDL (I assume your scanner will do this). But scanning with the 10K resistor sends my cars into "diagnostic mode" (turns on fan, etc) ... maybe this is related to the car "running poorly"
Well, not much help but at least here's a bump for you
Gary
The 85 had a different ECM than 86+ ...
Can you change the baud rate of the scanner (older computers were slower)?
To get data to flow on both my 85 and 87 I have to have a 10K resistor between pins A & B of the ALDL (I assume your scanner will do this). But scanning with the 10K resistor sends my cars into "diagnostic mode" (turns on fan, etc) ... maybe this is related to the car "running poorly"
Well, not much help but at least here's a bump for you
Gary
Le Mans Master
Joined: Aug 2003
Posts: 9,884
Likes: 3
From: Magnolia Mississippi
St. Jude '03-'04-'05-'06-'07
It wouldn't scan as an 86, and wouldn't scan properly as an 85, but it scans fine as an 87, and all the sensor values appear to be right on. The best we figure is that it's just a screwy PROM. We were going through vader86's PDFs of the FSM, and it seems as though it's an undersensitive knock sensor. We'll know for sure tomorrow.
Depending on which Xray unit you have, you 1st should make sure yours is up to date on software, 2nd make sure the vin # is correctly entered in the unit; 3rd; if you have all the above correct, use the unit to data log and upload to PC to get the values (you can send them to me, and i'll take a look; 4th: If you want i can burn you a stock 86 prom, but you'll need to get the adapter for the ECM from Moates.net to install or contact Chris @ Superior Chevy and order a stock 86 prom from the vin #. If you guys have any questions drop me an email.
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Thread Starter
Instructor
Joined: Sep 2006
Posts: 192
Likes: 0
From: Seattle WA
Sounds as tho you have some aftermarket prom (prob the issue): i'd get a new stock eprom and go from there.
Depending on which Xray unit you have, you 1st should make sure yours is up to date on software, 2nd make sure the vin # is correctly entered in the unit; 3rd; if you have all the above correct, use the unit to data log and upload to PC to get the values (you can send them to me, and i'll take a look; 4th: If you want i can burn you a stock 86 prom, but you'll need to get the adapter for the ECM from Moates.net to install or contact Chris @ Superior Chevy and order a stock 86 prom from the vin #. If you guys have any questions drop me an email.
Depending on which Xray unit you have, you 1st should make sure yours is up to date on software, 2nd make sure the vin # is correctly entered in the unit; 3rd; if you have all the above correct, use the unit to data log and upload to PC to get the values (you can send them to me, and i'll take a look; 4th: If you want i can burn you a stock 86 prom, but you'll need to get the adapter for the ECM from Moates.net to install or contact Chris @ Superior Chevy and order a stock 86 prom from the vin #. If you guys have any questions drop me an email.
Thank you for your offer to help. If our knock sensor diagnosis doesn't yield the results we're looking for, I may very well take you up on your offer. I didn't even think to check for the newest version of the software, but it is a brand new ez6000. I'll check on that tomorrow!
Team Owner
Joined: Jan 2003
Posts: 20,335
Likes: 264
From: Summerland B.C. Canada
It wouldn't scan as an 86, and wouldn't scan properly as an 85, but it scans fine as an 87, and all the sensor values appear to be right on. The best we figure is that it's just a screwy PROM. We were going through vader86's PDFs of the FSM, and it seems as though it's an undersensitive knock sensor. We'll know for sure tomorrow.
Racer
Joined: Oct 2004
Posts: 435
Likes: 0
From: Seattle WA
From what I understand, the 86 and 87 had basically the same computer system, so maybe it will be ok with this alien PROM.The lead that we're chasing now has to do with the fact that under heavy detonation the spark retard is topping out at 11*, when it should retard up to 20*. My theory, which is supported by the diagnostics of the FSM is that the knock sensor is "undersensitive." If we rap on the engine with something the ECM doesn't see any knock counts. My guess is that light detonation is not being felt by the knock sensor, and heavy detonation is only seen as light detonation by the sensor, hence only 11 degrees of retard. Once we hit 11* of retard the heavy detonation becomes light detonation, and the sensor doesn't see it anymore, so the car continues lightly detonating, and the knock counts don't go up anymore.
We've checked all the wiring for shorts and open circuits, so this morning we'll replace the knock sensor and test it again.
Racer
Joined: Oct 2004
Posts: 435
Likes: 0
From: Seattle WA
The final answer is... knock sensor. We swapped in the new knock sensor this morning, and after test driving it I can say the problem is fixed. I guess we got down the wrong path due to that screwy afteramrket PROM, but all seems to be well now.
