1987 3.2l unusual issue with new speed/timing sensors

[Rant6]

Just finished an engine rebuild, installed the engine in the car, and after several false attempts I corrected the few issues and got everything working. The engine started perfectly, purred like never before, and worked well on a short initial drive around the neighborhood. I went back to my house and let the engine idle for a bit while I tried to top off the oil (Bently manual is wrong - 10 liters is not enough!). I started and stopped the engine a few times, and then I let it idle for maybe 15 minutes to insure it was fully warm, and after it idled for a bit I noticed a rapid but regular miss that had not been there initially. I tried to ignore it and I shut down the car and went in for the night feeling pretty good.

The next day I tried to start the car, and it would not start. It would turn over and fire for about a second and then shut down. Exasperated, I started troubleshooting. I replaced the fuel pump relay, checked for fuel pressure, and decided that the fuel system was not the problem. I searched the forums and decided that I would start by testing the two flywheel sensors EVEN THOUGH THEY WERE BRAND NEW.

I started at the DME connector (pins 8 to 27 and 25 to 26) under the seat, and I could not get any continuity through either of the two sensors. I then disconnected one of the sensors at the engine compartment bracket and tested it with an ohm meter, and it tested good. Must be a cable issue. So I jumpered the two lower connectors in the wire between the DME and the sensor cable connector and tested continuity from pin 25, through the cable, past the jumper, and back to pin 26, and I found that I had good continuity. Everything seemed good, so I plugged the cable from the DME back into the sensor cable, and again I got no continuity at the DME connector. Bad connector? I checked the connector carefully and bent the wires around a bit to see if there were breaks at the connector without finding any issues. I checked the entire electrical path a couple of times at the DME connector, but I could never see anything other than no continuity. I then tested the sensor again at the engine bracket and initially it looked like the sensor was now fully open, but as I moved my probes around a bit I was finally able to get a proper reading, so I'm not sure if it was me or the sensor that was causing the open condition to show. So I moved my meter back to the DME connector and placed it so that I could see it from behind the car. The cable from the DME was still unplugged at the engine, so the meter showed fully open, and I watched while I plugged it into the freshly verified sensor again, and for the first time I saw a proper reading (about .9 Mohms) at the DME connector. So I carefully plugged the DME connector into the DME, and the engine started right up and ran beautifully.

What could cause both sensors to read fully open at the DME, and then both sensors to suddenly start working? I WAS fidgeting around with the cables at the engine connector while troubleshooting, but I could not see any issue with them. It would also be strange for two connectors to stop/start working at the same time. Until I understand this I'm afraid to drive the car. Any ideas? Thanks in advance.

[RobFrost]

Corroded connectors
Loose connector
Probing wrong connector
Glitching break in wire
Faulty earth
Wrong setting on voltmeter

Sent from my SM-G988B using Tapatalk

[Flat6pac]

Did you use Bosch product flywheel sensors?
The other brands are questionable.
Bruce

[Rant6]

RobFrost,
Your suggestions were all valid, but I checked and double checked these things when I was troubleshooting.

Flat6pac,
Yes I did use Bosch sensors. Thanks.

I'm sure this is caused by something silly, but I can't figure it out.

MM