Tech Tip 1
This month’s tip is a case study involving a 1991 Chevy Pickup C1500 with a 5.0l throttle body injected engine.
Symptoms are as follows: Intermittently runs very poorly exhibiting low power and surges badly. With a timing light installed while it is running badly the timing marks appear to be moving around the harmonic balancer.
With the black/tan BYPASS (timing set) connector disconnected it runs better.
Many parts had been thrown at this truck by the time I got to it. My first instinct when hearing this truck running badly was a fuel system problem but after a few quick checks found the fuel system to be operating according to the ECM's command.
Next step was to put timing light on it. While setting up for the timing check before disconnecting the BYPASS (black/tan) connector I noticed that the timing marks appeared to be rotating around the balancer. After the BYPASS (black/tan) connector was disconnected the truck started running better and when the timing was checked it was in specs and functioning normally.
After shutting off the truck and reconnecting the timing connecter the truck ran well. Timing advance was normal and so was idle speed and power was good also. Hmmmmmmmmm!
After letting the truck run for a few minutes in the bay it started to surge again. At this point I decided to hook up my lab scope to the Distributor Reference (D/REF) signal, which is the RPM signal for the ECM, and to the EST signal. The results are in Figure 1.
Fig. 1
Note how the EST is dropping out and the D/REF is staying steady this indicates
that the pickup is good and the A/D (analog to digital) converter in the module is good as well.
This would indicate either the ECM or wiring to the ECM could be the problem.
(Assuming the Ignition Module is good since it is the 3rd one in the truck)
Now for a little background on the timing control system. Link to wiring diagram at the end of this page.
When the BYPASS connector (black/tan) is disconnected the ignition module pulls the EST signal low and fixes the timing to base specs. (see figure 2) When the Ignition Module receives the 5v signal from the ECM on the Bypass circuit (black/tan) it allows the ECM to control the timing. This is done basically by a "relay" in the ignition module. (see figure 3)
Fig. 2
This is a normal EST and BYPASS signal with timing connector open. ECM is in base timing mode.
Fig. 3
This is a normal EST and BYPASS signal with the ECM in timing control.
As you can see in figure 4 the 5v BYPASS was dropping out in the computer (I was back probed at the computer to rule out wiring problems) and putting the timing to base specs at random. This was the cause of the surge and low power.
Fig. 4
Note that when EST goes low the 5v BYPASS also drops out. The ECM is loosing its 5v output to the BYPASS
line intermittently and the timing is going from controlled to base very rapidly and causing surging and poor running.
EST Wiring Diagram