O/T one for Beav

[AutoTchr]

99 Suburban 7.4L. It is one of our test vehicles donated from the local dealer. Only 17 miles.

Here is the deal......

After changing the Crankshaft Position Sensor the vehicle no longer starts. It will sputter and backflash, but will not continue to run. The sensor is fully seated. I can not run the CKS learn verification from the scan tool because the vehicle needs to be started. All symptoms suggest a crank sensor that is out of synch because of the sputtering. This is the only change that was made. Interesting note.....according to the Graphing meter, the pattern is NOT normal. Both sensors, new and old, produce the same pattern. No codes.

|\_/\_/|
| |
_______| |______
Or something to that affect.

Any help you might give would be appreciated.

Mike

[AutoTchr]

looks like the pattern got jumbled. Try again



|\_/\_/|
| |
______| |______

[Beav]

That's a good attempt to reproduce the waveform, but...

Anyway, just to make sure I'm reading this right, the truck was running well to begin with, the only item changed was the CKP and now it won't run with new or old sensors. If so, working from that fact alone and disregarding the waveform, something else is amiss, whether it's a bad connector or maybe the new sensor is slightly different and caused some unseen damage, or possibly something else was changed inadvertently, such as a plug wire left off and spiking back (I had a bad coil on a Ford that caused me similar grief.)

I'm in a hurry to get out the door right now, but I'll try to get back ASAP. Bear in mind that I haven't heard what my social duties are this afternoon or evening.

In the meantime you can pick up a free thirty day trial of AES Wave software at their website in order to save and transmit waveforms.

[Beav]

File In Section: 06 - Engine/Propulsion System

Bulletin No.: 00-06-04-014

Date: April, 2000

TECHNICAL

Subject:
No, Hard, or Slow Start, Backfire or "Kickback" During
Crank/Start, "Grinding" or Unusual Noises During Crank, DTC P0338
(Replace Crankshaft Position Sensor)

Models:
1999-2000 Cadillac Escalade
1995-2000 Chevrolet and GMC SIT Models
1996-2000 Chevrolet and GMC C/K, M/L, G, P Models
1996-2000 Oldsmobile Bravada
with 4.3 L, 5.0 L, 5.7 L or 7.4 L Engine
(VINs W, X, M, R, J RPOs L35, LF6, L30, L31, L29)

Condition:
Some customers may comment on one or more of the following conditions:

^ Backfire during crank/start
^ "Kickback" during crank/start
^ "No" start
^ "Slow" or "hard" start/crank
^ "Grinding" or unusual noises during crank/start
^ Cracked or broken engine block at the starter boss
^ Broken starter drive housing
^ Broken starter ring gear on flywheel
^ Any combination of the above

Cause:
A condition may exist that allows the crankshaft position sensor to command up to 50 extra degrees of spark advance during engine cranking only. This in turn exposes the engine to higher than normal cylinder pressures which may result in an inoperative condition to the starter drive housing, the engine flywheel starter ring gear, or the engine block at the outside edge of the starter boss.

Correction:
Inspect for a stored powertrain DTC code P0338. This DTC will NOT illuminate the "Service Engine Soon" light. If this code is stored, the Crankshaft Position Sensor, P/N 10456607, MUST be replaced and the remaining components inspected for damage (engine block at the starter boss, the starter drive housing, and the engine flywheel starter ring gear).

Notice : When DTC code P0338 is set, failure to replace the Crankshaft Position Sensor could result in repeated inoperative conditions of the starter or flywheel.

Important : Some flywheel wear is normal; broken or missing teeth and/or cracks, are not normal.

Parts Information

Parts are currently available from GMSPO.
-----------------------------------------------------

Also from big brother:

The relation between the crankshaft position sensor and the target wheel is crucial. The sensor must be exactly perpendicular to the target wheel with the correct air gap.

IMPORTANT: When installing or removing a crankshaft position (CKP) sensor make sure the sensor is fully seated and held stationary in the front cover before torquing the hold down bolt into the front cover. A sensor which is not seated may result in erratic operation and lead to the setting of false codes.

This is one of the reasons I don't like GMs, they come up with some terribly insipid engineering, either it's cheap, ridiculous or both. Why a crank sensor for a V8 with only four reluctor teeth at a 60° spread? There's a heckuva a lot simpler ways to accomplish the job. Maybe it's because they can't count that fast...

If this didn't help let me know and we'll do some more brainstorming. I just like to check the TSBs first when dealing with weird probs.

[AutoTchr]

I did read through those TSBs while researching the problem. I wonder if I left it lose and wiggled it while cranking......carefully of course.

Maybe if I give you the whole story......my principal also has a 99 with over 100k on it. He is having hard start issues very similar to what the TSB describes. So we borrowed the CKP out of the schools 99 to see if that would fix the problem. It did, and so I ordered a new CKP with the principals money of course, to replace the one in the schools suburban. Now neither sensor will work.

it is amazing how GM can write a TSB explaining how critical the alignment of the CKP is, and then manufacture the thing with enough slop that you can wiggle it while it is in the bore.

Ford used the CKP to moniter misfires and their system was too sensitive. When vehicles equiped with manual transmissions would "trailer", a code would be set. maybe this is why GM uses only four on V8s and 3 on V6s

Any further input appreciated,
Merry Christmas

Mike

[Beav]

Originally posted by AutoTchr

it is amazing how GM can write a TSB explaining how critical the alignment of the CKP is, and then manufacture the thing with enough slop that you can wiggle it while it is in the bore.



Mike

Don't forget they're also the only OEM (that I can recall anyway) that ever had to shim their starters. Great production tolerances, engineering, bean-counters, whatever... "It don't have to be right, it just has to be on there!"

I'm just wondering if it has a threshold, like a 4.0 Jeep. They absolutley won't start until the CKP produces 500mv, 499mv won't cut it. To help them out you need to slot the CKP bracket and get it as close to the wheel as possible. You might scan the principal's output voltage and check it against the problem child's.

It's late and maybe I'm not thinking clearly, but is your waveform depicting a spike on the first square? The Ford I mentioned had a secondary leak in a coil that fed back through the primary to the ICM. I saw the spike and didn't think much of it at first until I noticed I was on a 5kv scale. I realize that a variable resistance CKP can produce some impressive voltage, but 5k is a bit too high. I increased the range and found the spike topping at 30k so I quickly realized the only thing that could produce that kind of voltage was the coil - problem solved.

[AutoTchr]

No spike but there is a variance, on the top of the waveform. The waveform is not flat across the top. I can not tell you the max voltage that the sensor is producing, I forgot to check. I will add it to the list.

Thanks, Time to go play Santa