Resets - Relearns

Bob Owner/Technician East Longmeadow, Massachusetts Posted   Latest  
Tech Tip
Driveability
1999 Volvo S80 2.9 2.9L (97 B6304S) 4-spd (4T65-EV)—YV1TS97D0X1027010
MIL Flashing
Misfire Codes

I thought this was interesting enough for a tech tip. This was a new customer to me. They had been to several shops trying to get their problem resolved which was a flashing mil and misfire codes set. Someone finally referred them to me. A PCM reset cured this. If you want the details, read on.

This was my pre-scan and as you can see, there are multiple misfire codes stored.

I initially thought this would be a piece of cake. I gave it a quick road test and noted that the MIL started flashing when I started to get into the throttle. I brought it back and did the usual relative compression and ignition system testing. Someone else had already done plugs and some coils and I found no problems there. A timing belt had been done about a year ago so I tested cam-crk sync and did an in cylinder pressure test. I was not able to identify anything obvious so I buttoned it back up and went for another road test. This time I payed much more attention to what was going on. After awhile it became clear that when the MIL would start flashing, the engine really wasn't misfiring. Aha, these are phantom misfires, I need to do a crank profile re-learn.

It turns out that there is no scan tool function to initiate a crank re-learn. I scratched my head for a minute and thought, if the system has misfire detection, it must need to know the crank tone ring profile. It had to have learned it at some point. After thinking about it awhile I suspected that this thing must have learned the profile the first time the pcm was powered up so maybe if I de-power the pcm and cause it to restart, maybe it will learn the profile on it's own again.

I found this information regarding "adaption of the flywheel"

So it seems that the pcm should learn the profile when driving under the specific conditions. The problem was that whatever it was learning was obviously wrong as this thing had the flashing MIL problem for quite some time. It appears to be a software glitch that prevented a proper re-learn.

So I figured I would reset the PCM and hopefully get this thing out of whatever failure loop it was in. I pulled all the PCM fuses for a few minutes. Then I took another road test and the flashing MIL and codes did not come back. I drove the car a couple more times and then one last time after sitting overnight just to be sure. No matter how much or how hard I drove it, the MIL never flashed and no misfire codes were set. Customer picked it up the other day and so far so good.

Post scan

The is one radio code but it's an aftermarket radio so I'm not concerned about that. 

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Helpful
Victor Owner/Technician
Doral, Florida
Victor Default
 

PCM memories can be cleared by placing a 1 ohm 10 watt resistor across battery cable terminals for at least 30 minutes​.​Give it a try.

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Martin Owner/Technician
Brooklyn, New York
Martin Default
 

Recently has an older model 4 cyl Mitsubishi Lancer with waste spark ignition system. Another shop had replaced the water pump and did a timing belt job. Since then the car had an occasional bad idle, loss of power and mil light on when acting up. The same shop did new plugs and coils based on an igniton# 1 dtc. Came to us so we inspected the last area of repair, conversed with the previous shop…

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Bob Owner/Technician
East Longmeadow, Massachusetts
Bob Default
 

I seem to be running into more of these situations where the pcm is locked into some fault strategy or a previous adaption that won't clear up on it's own. If there is no reset function in the scanner then a power cycle of the pcm is the only other solution.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Certainly have read several cases in the last few years. Thank you for sharing, it keeps it fresh in our minds. Another somewhat similar trick, is when you cannot find a Fuel Trim reset function. Disconnecting the MAF, while running, and then starting the vehicle back up (if it stalled) may "shock it" into forgetting the learned trims.

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Bob Owner/Technician
East Longmeadow, Massachusetts
Bob Default
 

Geoff, I had another one from a body shop a couple weeks ago. I forget the model at the moment but they had repaired some light damage to the right front. Afterwards when driving the car over 10 mph the traction control system would just about bring the car to a stop. It was really freaky. I scanned it and there were no codes. Also did a visual check and saw nothing unusual. It seemed that…

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Another good story. Thanks Bob. And Happy Independence Day!

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