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  Tech to Tech

'95 Avenger
All About 'Timing'

Posted 4/17/2006
By Jeff Bach

“ If you've got a head scratcher on one of these 2.0 engines, check the timing belt. ”

I've talked to three technicians this winter who have traded in their automotive technician status for the chance to start new careers in other fields that are less challenging in some areas and more challenging in others. One is installing hardwood floors, another is installing cable TV while another is running a carpet-cleaning service. All three sold their tools as if to say, "Been there, done that and not going back."


Figure 1.
I had the chance to talk to two of the guys to see why they were giving up on the industry. One was a dealer-trained tech who said he got tired of the politics and the lack of comradeship in the environment where he was working. The other had had his share of struggles with diagnostic work and was throwing in the towel. He said, "They are making these cars so you can't fix them unless you are in the dealership."

We all know that statement is not true. There is a lot of easily accessible and good diagnostic information out there. In addition, there are many proper scan tool scopes and software available to the independent technician today. Anyone will tell you that if you think they are deliberately making these cars tougher than necessary to diagnose, you're just plain paranoid.


Figure 2.
Take this 1995 Avenger I have in the shop, for example. This car was towed in as a no-start with a list of "tried parts" from a fellow who had given his all to this car before he decided to give up on it. The parts list included a new cam sensor and pigtail, a new crankshaft sensor and pigtail, a new coil, plugs and wires and a new coil pigtail. It also had one used and two new powertrain control modules (PCMs). I began the diagnostic process with some research into this model's history and a bulletin search for "weirdness" before I actually broke out the scope.

I did find that this engine has a history of taking out PCMs during jump starts, crank sensor wires melting together and wires chafing and rubbing through on the alternator bracket.

Cranking the engine with a spark tester revealed a good spark and an attempt to start but it wasn't quite getting there. I checked for fuel and found an injector pulse. The compression was good, and the exhaust was not restricted. I went for the scope and current probe and checked the coil primary (Figure 1).


Figure 3.
This one threw me. I started to have questions now about proper coil pigtail wiring or possibly a current leach from one of those wiring harness problems to which these cars are so prone. I figured I better take a look at the coil primary voltage signal and compare it to what I was seeing on the current probe's signal (Figure 2).

I was expecting to see the coils' primary voltage signal go up about where the current steps up, indicating that an external source was causing the additional current but was surprised to see that the PCM was holding No. 2 coils' signal grounded even through No. 1 coils' firing event.

Next, I figured I'd take a look at the crank sensor signal, hoping to see some trash, which would cause these weird firing events - but none were evident (Figure 3).

The crank sensor signal looked fairly normal, and the primary looks like it's firing at regular intervals at least. Next, I took a look at both coils' primary voltage signals and got the image in Figure 4.


Figure 4.
This was starting to make less sense the more information I got. These coils are directly controlled by the PCM and look like they are firing at regular intervals. I knew they had to be based on the cam and crank sensor signals' relationship. The cam sensor signals now became of interest, and in an attempt to see the logic behind the coils' firing time, I figured I'd look at the cam sensor, crank sensor and the primary current all at the same time. To do this, I needed to sync two scopes to the same signal and set both triggers and time bases to the same settings. I triggered both scopes on the crank sensor signal on their A channels. Then I connected the B channel on one scope to the current probe, monitoring both coils' primary circuits through the feed wire while hooking up the other scope's B channel to the cam sensor signal (see Figure 5).


Figure 5.
I couldn't find anything in the book on the alignment for these signals or their logic in firing. But the image I got in Figure 5 brought to mind some of the engines I've seen in the past that disallow the spark when the cam signal occurs outside the wide crank sensor signal pulse for self preservation due to the fact they are interference engines.

I'll never forget the first "no spark" Audi Quatro I diagnosed using my old tectronics analog scope with a 2-inch screen.

It was obvious to me now that I'd been looking at the wrong area of diagnostics.

This problem should have been picked up during basic testing as a timing belt issue.


Figure 6.
I pulled the inspection cover, and sure enough the timing was off by two teeth. Even though the belt was tight, one of the idler pulleys had a bad bearing and must have grabbed the belt and let the crankshaft jump ahead a little. Once the new belt and pulleys were installed, the signal alignment now all made perfect sense. Hindsight is a wonderful thing.

The customer was so tickled with all these waveforms he couldn't get enough of the pictures to take with him. I get the feeling that there may not have been a mystery tech working on this car for him, and he needed to show some technical information to someone to get him out of the doghouse.

I think if I learned anything from this little diagnostic adventure it's that the easiest way to check to see if the timing belt is aligned on one of these 2.0 engines is to look for a nice, regular and evenly spaced primary current signal (Figure 6). If you get a head scratcher, be sure to check the timing belt.

Jeff Bach Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is johntjeff22@gmail.com and his Web site is www.currentprobe.com.


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