Need Help! Electrical problem?

[Kristi]

We just installed a new engine in our '86 T-Type and the car is not running right. According to the Scanmaster, the O2's hang out in the 100's, the cross counts are very lazy, and the TPS reading does not change when the throttle is moved. The car runs, but idles poorly.
We've changed the TPS (and verified its output with a voltmeter), the chip, and the ECM. All sensors (including the AC Delco O2 sensor) are new.
What could cause this? A faulty ground?

 

[Steve Wood]

May be more than one problem. The tps seems easiest to address first (but I am probably wrong ). Is the tps arm engaged properly with the tb so that the arm moves as the tb butterfly opens?

If you probe the tps connector with a volt meter just the meter show the normal voltage variation as the throttle is opened?

 

[Kristi]

TPS Output

We did probe the TPS with a voltmeter and confirmed the proper output. The Scanmaster and ECM are not receiving this data as it updates, however.

 

[Steve Wood]

Okay...when you probed the tps sensor, and found it to be working, did you probe the blue wire and the black wire? Or, did you ground the meter lead and only probe the blue wire?

The sensor has three wires...if you probe gray and black you should have 5 volts. If you probe blue and black, you should have the tps voltage for whatever the throttle position is.

The black wire serves as a return line for the tps, the coolant temp sensor, and the mat sensor.

I am wondering if there is a problem in the black wire so that the ground side does not get back to pin D12 on the ecm.

You should be able to probe the ecm connector at D12 and C13 and read the tps voltage.

 

[Kristi]

Thanks Steve

Your help is very much appreciated. We did probe the ECM connection under the driver's side of the dash (I'm not certain which terminals we used) and confirmed that the same no-update problem shown on the scanmaster was evident at that ECM connector.
If the TPS wiring was the problem, would it cause the slow update seen in the cross counts and the O2 sensor? When the car is idling, the TPS reads .42, which should be within range for the car to idle well. But the CC's and O2's are still off, and the engine doesn't seem happy.


Someone please remind me why I love these cars so much...

 

[Steve Wood]

I chose the tps as it seems more straight forward! As I said, if you go to the ecm and put your meter on D12 and C13 and move the throttle, you should see the same voltages as you did under the hood if you probed blue and black.

Now, The o2 sensor has a purple wire that runs to terminal D7 on the ecm. The sensor is grounded into the uppipe and the ground circuit comes off the back of the passenger side head and goes to D1 on the ecm. You can probe D1 with your meter lead and touch the other lead to something that is grounded under the dash...I often use the screws that hold the door jamb molding down to verify that the ecm has a good ground...ohms scale for resistance.

 

[Steve Wood]

Must have good grounds on the back of the head...or whereever those grounds might have been located to.