Mystery problem

Flash2004 · January 4, 2007

I Have a 2001 Canadian Spec 2500HD SLE Sierra with 8.1L gas engine and Allison. It has done 74,000 kilomters now and the problem I am experiencing began at 21,000 KM.

I am in New Zealand and there is no dealer service available here for North American origin GM trucks whatsoever. It has been professionally converted to Right hand drive from new by a very experienced operator.

I think, but because it was so long ago I can't be totally sure, the problem started after the battery terminal came loose one day.

The problem: The truck drives brilliantly but it will stop dead without warning. The red charge light comes on and the truck will coast to a stop unless it restarts by itself if the engine is still being turned over as it decelerates when it will drive away a bit sluggishly but clear within a mile or so and back to normal. Other wise I sist on the side of the road and pretend to be making a phone call while attempting a start every minute or so until it eventually starts again. Usually it will occur only once on a journey and can be OK until the engine is stopped normally and it might happen again or it might go fine for another month. I have never timed how long it runs before stopping, but have noticed that if its going to do it, quite often it will happen at the same place on the way to or from work.

Another thing it will do, and I think its related, is it will occasionally refuse to start. This can last at worst for 6 weeks, but usually it will decide to start within 10-15 minutes. It turns over normally and seems totally fine - it just will not start. Today I tried disconnecting the battery and leaving it disconnected for thirty minutes - which usually works - but it would not start again until just as the tow truck I called arrived.

Back in 03 when it started doing this there was no check engine symbol. Lateley though the CEL is on all the time. My mechanic, who is very good with auto computer systems, bought a laptop and ODBII diagnostic program. He keyed in the VIN and connected to whoever you connect to. The fault code that came up initially was camshaft postion sensor. All connections were checked and voltage testing said the sensor was OK and the fault codes reset. Now the CEL is on continually and reads as Crank position sensor faulty. We imp[orted a new sensor and fitted it but it made absolutely no difference. The problem is still there.

Do you have any ideas at all please? Feel free to ask any question, we are becoming very intimate with the truck and the way it all works. Thanks, I hope you can help as i am a very patient guy, but this story is almost over for me.

Wingnut · January 4, 2007

The new sensor could be faulty.

Or you could be having a problem with your fuel delivery.

GM-Tech · January 4, 2007

The new sensor could be faulty.

Possible, though unlikely. I would examine the terminals for the crank sensor connector VERY closely. They can be spread slightly causing intermittent loss of connection. Sticking anything into the terminals for testing, like a test light, can easily spread those terminals. You need to use the exact male terminal to test for proper tension. There should be some drag when inserting and removing the male terminal from the female on the connector.

Also, when it won't start and you're cranking the engine over, can you read cranking RPM's on the scanner? If the crank sensor is okay, you should. If it does not indicate the engine is turning, i.e. no cranking rpm signal, then it ain't gonna start or run.

Other wise I sist on the side of the road and pretend to be making a phone call

Is this for your benefit or does it fool the truck into starting?

Flash2004 · January 4, 2007

Other wise I sist on the side of the road and pretend to be making a phone call

Is this for your benefit or does it fool the truck into starting?

<{POST_SNAPBACK}>

A good question, and apologies for the typos. As you might have guessed, this is the only truck of its type in the whole country that I know of anyway. Our population in the province is about 100,000 and the truck is well known. Small boys dream of owning one some day - just like I did when I was young and Dad would drive us in his 56 Belair. I pretend I've stopped to talk on the phone so no one's dreams are shattered at the thought that the coolest and mightiest pickup truck in the country might be unreliable in any way. After 4 years of it passers by must think I have a humungous phone bill as well as the biggest fuel bill at US$4/gallon!

The terminal suggestion is a good one and I'll investigate today. The other possibility that always suggests itself is the security sytem. Its almost like its immobilising. When I stop somewhere and it refuses to start I tell people it has locked me out because it sometimes doesn't like me. They believe it and I'm beginning to as well.

I really appreciate the help I'm getting here. In NZ most mechanics and technicians eyes glaze over when I ask for input. cheers

Flash2004 · January 4, 2007

[Also, when it won't start and you're cranking the engine over, can you read cranking RPM's on the scanner? If the crank sensor is okay, you should. If it does not indicate the engine is turning, i.e. no cranking rpm signal, then it ain't gonna start or run.

This is a valid point and a darn good one, thank you. Unfortunately, now that particular mechanic (with the diagnostic kit) has spent over 100 hours (@$65/hour) trying everything he knows and has been able to find out about the truck, he is like a WWI soldier who has endured a three day artillery bombardment. When he hears me coming, and I'm not kidding about this, he runs and hides. I've never complained to him, we laugh as I pay, but he has really lost his confidence over it.

I am now absolutely on my own with this problem. Luckily I'm a trained diesel fitter from way back and can follow your instructions pretty well but don't have the laptop to check any cranking speed register.

cheers

GM-Tech · January 5, 2007

If you have access to an oscilliscope, you could tie in to the crank sensor and see if you're getting a good signal during cranking and no start. That's just as good as a scanner that's just basically counting the voltage changes. Might even be better at showing a dirty signal.

BTW, the security system can prevent the vehicle from starting, but it cannot cause the vehicle to stall once it's running.

Flash2004 · January 5, 2007

If you have access to an oscilliscope, you could tie in to the crank sensor and see if you're getting a good signal during cranking and no start. That's just as good as a scanner that's just basically counting the voltage changes. Might even be better at showing a dirty signal.

The "refusing to start" is so random I'd have to have the OSC fitted permanently to the truck to ever hope to see the test. As an example, we had a data logger plugged into it for three weeks during normal use and no malfunction. The day after it was removed - bingo - two stalls. They're good ideas, but at this stage I'd prefer a magic wand. I think the most promising idea is to check the CSP Sensor connection for looseness. I had a look down the back of the firewall behind the left head this morning and could only see a very small black hole. I'll have to get underneath it maybe to try and access it?

BTW, the security system can prevent the vehicle from starting, but it cannot cause the vehicle to stall once it's running.

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This is useful to know and maybe I actually have two separate problems. The chance of a refusal to start doubles if I leave the truck unlocked with the right side window open - unless I want it to fail.. Even worse if I leave the left side open even an inch, whether the truck is locked or not. Interestingly, while the horn will sound very weakly when locked with the remote, I have never been able to set the alarm off. The panic function works though.

GM-Tech · January 5, 2007

Interestingly, while the horn will sound very weakly when locked with the remote, I have never been able to set the alarm off. The panic function works though.

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You may not have the factory "alarm" (Content theft deterrent or CTD). CTD is a whole different thing than VTD (Vehicle Theft Deterrent). VTD is standard. CTD is an option.

VTD is what keeps the vehicle from starting and CTD is for detecting doors being opened. The "panic" button is on every remote whether the vehicle is equipped with CTD or not so that's no indication of your equipment.