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Hello everyone, I am new to the forum and come with an interesting issue that I hope someone of you may have had experience with.
I am diagnosing an issue on a 1992 740 with the 230B engine, REX/REGINA control system. All of the ignition tune components are new and I have verified that they are in good working order. The engine starts and idles well. Then at 3,000rpm it cuts out as if it had a rev limiter. I would suspect overrun fuel cutoff except that the issue is there under load while driving, not just in a bay test. I connected a timing light and can see that spark timing is actually jumping back and forth as much as 30 degrees when this is occurring and I can hear the popping sound of firing in the exhaust. The problem gets worse with more throttle pressure.
I have disconnected the knock sensor to eliminate knock retard as a possible cause.
I've connected a lab scope to the crank sensor and to the timing control signal to the power stage from the REX module. I can see the timing anomaly in the timing control signal. The signal is a 5v square wave and I can see the spaces between pulses fluctuate and the pulses "on time" drop off randomly. The off pulses seem to coincide with the off time spark events. My early model lab scope has poor resolution(my kingdom for a PICO) and I have a hard time making out any matching fault in the crank sensors AC sine wave that might explain the odd firing.
The owner has made several attempts at fixing the issue himself, including swapping REX modules(there are two on the pass seat and another installed) he has also swapped out the power stage and coil. The crank sensor is the "improved version" with the white stripe. My thinking is that since the anomaly is present in the command signal from REX, it is not an issue with the power stage anyway. Sorry for the long post, thanks for any help you can give.
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Take a look at this thread: http://www.brickboard.com/RWD/volvo/1408779/220/240/260/280/cranking_issue_timing_starter.html
It addresses a different symptom on a different ignition, but the interesting thing I learned was a new, out-of-box Bougicord crank position sensor could actually make things worse. I had to use a scope and experimental shimming of the sensor to prove it eventually, but the shortcut would have been to swap in a second new sensor or perhaps one known to be good.
Of course, the difference in my experience was my personal knowledge the symptom did not exist until the sensor was changed. In other words, I was intimate with the history.
The scope traces may not be much help because Rex has a different hole pattern.
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Art Benstein near Baltimore
The ad says: "Automotive Technician Training -- Get Job Ready in Less Than a Year."
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Thanks Art for your response. Was your reference to the thread meant to be a link? If so it isn't working and I can't seem to find it. I would like to check it out.
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Half efforts yeild half results
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Oh, sorry. I've gotten lazy and neglected to add the html to make that a hot clickable link. Seems all the browsers these days recognize a string beginning with "http" and automatically provide a right-click option to select them.
Here it is in link fashion: http://www.brickboard.com/RWD/volvo/1408779/220/240/260/280/cranking_issue_timing_starter.html
Anyway, the shortcut I suggest is replacing the sensor, just as you've planned to do. If this doesn't work, I'm no Rex/Regina owner, but I can tell you many posts on this board show that a Bosch system can run with the tone ring mis-indexed by 45 degrees but those that have actually committed this error have reported different symptoms in the higher rpm and power ranges.
So, if the CPS swap doesn't yield any result, I'd suggest checking the flywheel/flexplate index. Not sure exactly how to do that with Rex, but I could do it through the CPS bracket hole on Bosch, where most pull the starter to see it.
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Art Benstein near Baltimore
A famous Viking explorer returned home from a voyage and found his name missing from the town register. His wife insisted on complaining to the local civic official who apologized profusely saying, "I must have taken Leif off my census."
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Thanks again Art. I am going to rack the car(I'll have to put the darn CPS back in) and take a closer look at the tone ring. On my initial inspection, It looks as if there may be a crack forming. I can see it through the sensor opening but the view is restricted. My bore scope helps but I prefer a bird's eye view.
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Half efforts yeild half results
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Problem solved! Thanks to those who posted replies. It was the bend it the tone ring that was causing the problem. I wish that the resolution was better on my scope. I was unable to make out the effect on CPS signal as the problem occurred but obviously the sensor read the bent area as a gap and produced an errant signal, affecting the timing control only above 3,000rpm. I am going to go ahead and install the new sensor as the one in the car has a deteriorating housing. Special thanks to Art.
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Half efforts yeild half results
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Mark, I would have steered you to this post but I figured something like the bend on this one would show up on your scope.
Photo credit: John Sargent
Post containing details:
http://www.brickboard.com/RWD/volvo/1415680/940/960/980/V90/S90/mysterious_940_failure_bet_youve_never_seen.html
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Art Benstein near Baltimore
An Indian chief was feeling very sick, so he summoned the medicine man. After a brief examination, the medicine man took out a long, thin strip of elk rawhide and gave it to the chief, telling him to bite off, chew,and swallow one inch of the leather every day. After a month, the medicine man returned to see how the chief was feeling. The chief shrugged and said, "The thong is ended, but the malady lingers on."
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Thanks again Art. I am going to rack the car(I'll have to put the darn CPS back in) and take a closer look at the tone ring. On my initial inspection, It looks as if there may be a crack forming. I can see it through the sensor opening but the view is restricted. My bore scope helps but I prefer a bird's eye view.
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Half efforts yeild half results
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Tone ring is okay. What looked like a crack is a scratch left by someone using something like a prybar on the flex plate, probably to hold the crankshaft. I did straighten one spot where it looks like they managed to bend the tone ring. It looks like it should not be a problem. The new sensor will be here on the next delivery and we'll see if it helps. I definitely see where the Bosch system's crank sensor scope trace is different. The REX/REGINA trace has a double flat with a spike in between at each 180 mark. I can see on the tone ring why that is.
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Half efforts yeild half results
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Have you checked the tone ring for damage? It is the only mechanical input for ignition timing on this engine. What OBD1 codes were present prior to disconnecting the Knock sensor? Checked Ohms on Crank sensor?
http://www.brickboard.com/RWD/volvo/1369776/740/760/780/rpm_sensor_ohms_reading.html
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Thank you for your response. There were no codes in memory. I have been thinking about the tone ring possibly being damaged. It's just odd that the engine runs so well at idle and up to 3K. I will check just the same and also ohm test the crank sensor. It seems as if this almost has to be related the CKP sensor since it is the only reference input to the REX module. Just want to be more sure before I go that way.
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Half efforts yeild half results
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CKP sensor reads 200 ohms warm. This is at the lower end of the scale. I have hesitated to condemn the sensor itself since there is no signal drop out. I am seeing an odd signal from it on the lab scope though. It's $39 my cost for a Bougicord brand replacement. Maybe I'll just try it.
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Half efforts yeild half results
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Good call. Cheap, and they do go bad. Be careful when removing the CPS, the bracket that holds it is flimsy pot metal. Break it by yanking too hard...and you'll be removing the tranny to replace the bracket!
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Thank you again for your help. On anything delicate like, I always just use my 1/2" drive 1200ftlb big mama air impact and/or my 4 pound hammer. Then I blame the mess on someone else. LOL. Actually, the "Impulse Transmitter" as it was listed in the catalog, came right out with no fuss using my 1/4" drive ratchet. I'm going to take a close look at the tone ring while I have the sensor out. Then, I'll post back with the results of the swap.
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Half efforts yeild half results
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