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Did I Do That?Posted 3/16/1999By Jeff Knowlen
I got a call about a no-start on a 1991 Dodge Spirit with a 2.2L turbo III engine. The customer drove the car in to have the clutch replaced, and then couldn't drive it out because the car wouldn't start. The technician couldn't find any wiring that was disconnected or looked damaged but felt sure that he must have damaged something - somewhere - somehow. This vehicle had an uncommon engine package: 4 cylinder, turbo charged, multiport fuel injection package with DIS and a head made by Lotus. There weren't very many of these made, so, not surprisingly, the technician really didn't know where to start. He said there was spark, but only sometimes. The same was true of injector pulse; it would come and go. My first thought was to check the crank sensor and cam sensor patterns with a labscope, but since he didn't have one, I asked him to check for power to the coil pack while cranking. That was OK. Next, when the outputs were checked using a scan tool, I found that the PCM could actuate each of the ignition coils and create good spark during an actuator test mode (ATM) and that the PCM was able to control each of the injectors. Why wouldn't the PCM give consistent spark and injector pulse while cranking? Looking at the inputs again, the scan tool displayed "yes" to both the crank sensor and the cam sensor during cranking. This only shows that the PCM sees some kind of signal - it doesn't prove the signal is correct. The crank sensor signal should be a 5-volt square wave signal. For every revolution of the flywheel, the hall effect type sensor reads two groups of four slots, resulting in a scope pattern that shows the signals coming across the screen in groups of four "highs." The cam sensor is also a hall-effect type of sensor that creates a 5-volt square wave signal, but its pattern is different. The cam gear contains four sets of slots - a single slot, double slot, triple slot and a quadruple slot. On a scope this comes across the screen as a single "high" 5-volt square wave followed by a set of two highs, then three highs, and then four for every cam revolution. The technician rotated the engine by hand while watching the voltmeter switch from 5 volts to 0 volts, but still that didn't prove the signal was correct, only that it was present. From previous experience, I had a strong hunch that there was a problem with either the cam sensor signal or crank sensor signal. Logic suggested that since the cam sensor hadn't been disturbed, it was probably OK and that the crank sensor signal was probably the culprit. Since the sensor itself seemed to work, was there a problem with the slots that the crank sensor was reading? It turned out that the clutch pressure plate was not the correct application, even though it bolted on OK. The metal flange around the outer edge came too close to the cut-out slots in the flywheel. This interfered with the magnetic flux that acts on the sensor so that it did not "read" some of the slots. The PCM was so confused, it didn't know what to do when. It would begin to turn one of the coil packs on and off but then quit because it couldn't figure out where top dead center was. The next start attempt would do the same thing. After the correct clutch pressure plate was installed, the engine started and ran beautifully. It's just too bad the incorrect pressure plate physically bolted up and worked OK. Sometimes we're too quick to blame ourselves. It wasn't the tech's fault after all.
2.5 Liter GM R/U EnginePosted 3/16/1999By Ralph Dahlen
COMPLAINT: Start/stall on hot engine. If the injector tests OK with an ohmmeter, try starting the engine with the alternator regulator connector disconnected. If the engine now starts and stays running, replace the injector after verifying the alternator diode pattern on a scope by running the engine with an alternate fuel source, like carb spray. An injector may test good with an ohmmeter, yet the solenoid winding can break down with heat and expansion caused by normal current flow through it. The PCM's injector drive circuit will shut down either partially or completely with above normal current flow through the injector. Disconnecting the alternator reduces the system voltage available. Because of a drop in available voltage, the current flow in the injector circuit will also drop. In most cases, an injector with low resistance may now operate normally with the alternator disabled because the PCM's current flow is closer to normal.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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