Car Gets New Life
Back to the car. Armed with a wiring diagram that was actually a three-part image, lab scope, a current probe, a couple of my favorite "pokeaholers," headlight, glasses and knee pad, I settled in next to the passenger side of the car and peeled back the damp carpet to expose and access the electronic control unit (ECU). The first signals I went for (shown in Figure 1) proved wrong my hunch that the crank sensor was bad.
I centered the reference signal, hoping to see if there might be a sync signal with which it needed to align for the ECU to trigger the coil primary circuit. The result is shown in Figure 2.
The absence of the appearance of anything notable became instantly obvious. With the sync issue no longer in question, I focused on the coil primary control circuit. The ECU's control circuit for the coil ground is shown during cranking in Figure 3.
Seeing both the crankshaft position (CKP) reference and engine speed signals present and no primary switching going on had me now suspicious of the ECU. Next, I needed to verify that it had the necessary power and grounds. The diagram I have to work with made this task more difficult than it should have been. There were a few mislabeled connectors and a misdrawn relay diagram.
Believe it or not, the best circuit diagram I could find came from the Haynes manual. I tested the pertinent ECU circuits and determined that there were power supplies absent from the ECU stemming from the main relay also known as the DME relay. This relay is actually a dual relay with one of the contact sets controlling fuel pump current when energized by the ECU and the other feeding power to several circuits of the ECU itself when the coil is energized by "M39."
This all-important coordinate had no corresponding connection to anything that either myself or the information source at the library could find in the wiring diagrams. I did find a reference in the Haynes manual alluding to a circuit on the theft module that read "to DME relay." The elusiveness of this circuit makes some sense in the context that this car was made before the design of pass-key systems or radio frequency chips or multiplexed digital security modules. It relied on simple inputs from the door, hood, hatch and top switches to be satisfied before allowing ignition feed to the DME relay. Testing further, I verified that the alarm input switches were all functioning properly and concluded that the module was defective. I pulled the cover off and revealed the problem shown in Figure 4.
Moisture had apparently invaded the passenger compartment where the module rested and had been allowed to sit long enough to oxidize the discrete components soldered to the board. At the owner's request, we were able to bypass the alarm module that powered the DME circuit. There's a particularly satisfying feeling when that initial "sputtering into life again" occurs amidst the smell of stale fuel and a cloud of dust and debris from cobwebs, dislodged mud dauber nests and pulley surface rust. A smiling customer now has a running car again.
|Jeff Bach is the owner of CRT Auto Electronics in Batavia, Ohio. He can be reached at firstname.lastname@example.org.|
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