February 2008 Articles & Features

Launch Tech
Advance Auto


adjust text size: Driveability Corner

By Mark Warren | February 2008

In this installment, we find Mark a few steps closer to determining the cause of an intermittent stall/no start problem on a Dodge van. More screen captures have been made and clues are piling up.

This is part two of the stall and NO BUS problem on a 2000 Dodge van with the 5.9L engine. In the last column we examined waveforms revealing ground issues and missing injector events prior to stall. The missing injector events point to a possible intermittent failure of the crankshaft sensor.

The crankshaft position sensor (CKP) is a 5-volt Hall effect sensor mounted on the top of the bellhousing. The flexplate has eight evenly spaced slots that correspond to each of the eight cylinders. Looking at Fig. 1 below, you can see the CKP waveform (brown trace). The injector (orange trace) fires every other revolution of the crankshaft (720°). So there are 16 evenly spaced CKP 5-volt events between the two injector firings. Fig. 1 was recorded while the engine was idling normally. The CKP waveform shows no abnormal or missing events.

The green traces in Figs. 1, 2 and 3 are from a current probe looped on a fused wire in place of the Auto-Shut-Down (ASD) relay 30-amp fuse. We’re now monitoring the amperage (current) to all of the circuits supplied by the ASD relay. A short or overamperage condition in any of these circuits could cause the NO BUS condition, according to the factory diagnostic procedures.

In Fig. 1, the largest current event peaks at 1 volt or 10 amps; this is the ignition coil charge. Note that the ignition event occurs every other CKP event, or eight times every two revolutions. The CKP, ASD-switched current, injector No. 7 and CCD bus signals all look okay in Fig. 1. The reference voltage is still slightly high (as in the previous column), as the grounds have not yet been cleaned.

Fig. 2 (top right) was captured at the beginning of the stall event. Note that the CKP signal remains stable, with no missing events or excessive noise. The ignition current events disappear between 15.85 and 15.90 seconds. The No. 7 injection events continue normally, then the ignition comes back on for way too long.

Fig. 3 shows the complete stall event, including the content of both Figs. 1 and 2. It shows the complete ignition current and CKP events, from running well to complete stall. In Fig. 3, the CKP and No. 7 injection events seem fairly normal.

The powertrain control module (PCM) controls the switching (to ground) of the ignition circuit. Why is the ignition control freaking out while the CKP is stable? Why were injection events missing in the waveforms shown in the previous column? Is there an issue with the camshaft sensor waveform? Why does this problem end with a NO BUS display? Is the PCM defective? Are the grounds the source of the defects?

These are good questions. In part three of this saga you’ll have all the answers. See you then.