I watched with great interest as the tow truck brought the latest casualty in; a 1995 Buick 3800 with a crank-no-start complaint. Surely I will get the job, I’ve been dispatched nothing but warranty work all morning while my partner did CP job after CP job…
Darn! I didn’t get it. Doesn’t seem fair but I guess that’s just the way it goes. I watched the other tech fiddle with the Buick out of the corner of my eye as I finished up yet another fuel injector replacement job on a late model Chevrolet truck.
He scanned the data stream and then went looking for a service manual. Bummer, I missed out on a good one…. Once back with the book, he spent over an hour following the trouble charts. Yep, he’s asking for trouble all right <g>
As I passed by his stall on my way to pick up the next warranty job, I saw that he had the book opened up to the P0341 trouble code chart. P0341 is a DTC for a CMP (cam position) sensor circuit performance error. The engine will start and run just fine without the CMP sensor input. Feeling a bit cheated over not getting the job; I snickered to myself and walked away. He’ll be there awhile so its time to kick into high gear and suck up all the other jobs while he’s stuck on that one. Darn, this flat-rate business seems to bring out the worst in me at times…
A couple hours later he pushed the job out of his stall and went on to what little work was left. A week later the customer authorized repairs and the vehicle was pushed into his stall where the tech replaced the crankshaft sensor. Oh it was so hard not to grin when I heard the engine crank and crank and not start…. Out came the service manual again… An hour later the tech told me that the book had been wrong (trouble chart) and that there was no power to the ignition module. Fine, now he was going to look for a blown fuse. An hour passes…. No luck… Another hour and then another…. meanwhile I’m slurping up all the work. You see what flatrate has done to me? I used to be such a nice person…
This Buick has fuses located in three places and I watched as the tech looked everywhere except where he should have; under the dash on the passenger side. He would have known that if he knew how to navigate the service manual. After a bit he found the blown fuse and replaced it but the engine still would not start. At this point the engine had spark but no fuel pressure. There was power to the fuel pump, but it was inoperative. Guess what other circuit that fuse fed? Yep, it not only fed the ignition module but the fuel pump as well.
Authorization for a new fuel pump was obtained and the engine started right up with the new pump installed. This isn’t right…. The tech at this point was paid for both the actual repair and his “guess” while I busily cranked out warranty job after warranty job.
And then the fun began all over again…. As he backed the vehicle out of
his stall, it started missing. After staring at the engine for 20 minutes or
so, he finally rolled our Sun scope over and scoped the secondary ignition. By
now pretty much all of the tune-up work in the shop was gone, so I walked over
and helped him capture the secondary waveform as he loaded the engine up.
An odd pattern wouldn’t you say? Cylinders 2 and 4 are on different coils; this being a waste spark system (DIS). At first it would only misfire under load, but after a few minutes it started to misfire and mistrigger the coils when revved up to about 1500 RPM with no load. So now I know we are not dealing with a secondary issue, but a primary issue. And now my dilemma, do I stay and help him, or do I go see if the dispatcher has more work that I can do? Curiosity gets the best of me and I stay and help.
The first thing we need to do is to understand how this ignition/fuel system works. The 3800 has a dual Hall-Effect CKP (crankshaft sensor) which produces an evenly spaced 18x signal and an unevenly spaced 3x Sync signal. The ICM (ignition control module) supplies power, ground, and signal reference to the CKP sensor so, obviously, both signals go to the ICM. The CMP (cam sensor) also receives its power and ground from the ICM and the CMP signal is fed to the ICM.
The CKP Sync signal is used for cylinder ID by the ICM to determine coil-firing sequence. The Sync signal is also used as a filter for the other signals going out to the PCM. The ICM has two CKP outputs to the PCM; an 18x and a 3x FC (fuel control) signal. The 18x signal (to PCM) is based on the CKP 18x signal to the ICM and mirrors it exactly. The ICM also takes the 18x CKP signal and divides it by 6 to produce the 3x FC signal to the PCM. If the 3x Sync signal is lost, the ICM will not output the 18x or the CMP signal to the PCM. The 3x FC signal however, will be sent to the PCM and the engine will continue to run.
Here is a normal CKP waveform to the ICM. On top is the 3x Sync signal
and below is the 18x signal. Take notice specifically of the 3x Sync signal.
Now here is a normal 18x (to PCM) and 3x FC signal
Now I don’t usually find a need to use a scope for diagnostic purposes, but it makes perfect sense for this one. Here is a scope shot of the 18x (to ICM) and the 3x Sync signal while the engine running poorly with the ignition system mistriggering:
Of note here is that I am losing BOTH the Sync and 18x signal…. Just for interest, here is a shot of the 18x (to PCM) and the 3x FC signals while it is running poorly:
Now the CKP sensor has already been replaced; could we have a defective new part?
I instructed the tech to inspect the PCM harness connectors to the CKP. Sure enough, the terminals had been flattened by a careless tech previously and at certain speeds or loads the connection would fail. Doing some research, I found that the vehicle had been bought from another dealer and their technicians had been trying to fix it. After their technician hacked away at it for a while, it was sent to us. It’s hard to get good help these days, don’t you think?
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