Grounding O2 Lead Gives Odd Readings

[Ron]

I'm getting what seems to be a conflict with the manual. It says disconnecting and grounding the O2 lead should give a dwell reading of "87° minimum (steady reading)". When I disconnect it, I get a steady 44° ( manual says 40-50, so OK), but when I ground it, the needle raises slowly up to ~76° (takes 3-5 seconds) then slowly falls right back down to ~44° and holds -- when I remove the ground, the needle flicks toward zero and instantly goes back to ~44°. I tried this with a spare ECU and just tried it on Alex's D, which runs well, and get the same results.
ALL other Dwell specs are OK.

WTH???

[DMCMW Dave]

The manual is correct.

This is going to sound familiar - "Check Your Grounds". Especially the one on the back of the passenger intake runner with the red tag on it. This is the reference ground for the O2 sensor so it is pretty critical. Also check it's feedthrough at the bulkhead connector.

[Ron]

OK, check me out here:
The engine is grounded because it can handle the starter current and the one on the passenger horn is new and clean. To be sure, I ran a jumper cable straight off of the battery to the block and from there I ran a large wire to the phillips bolt and nut holding the ground wire at the ECU, another to the one on the runner, and yet another to ground to the O2 harness plug.

I cleaned all of the bulkhead connections paying particular attention to the two black wires in the Blue plug (22&23...the chart says warmup regulator and diagnostic grounds. Or, did you mean something else?)

...No difference.

FWIW, I noticed that, if I'm leaning on the fender and touch the plug, even the ground through my body will cause the Dwell to act the same way.

Any other related grounds...or ideas?

Thanks!

[DMCMW Dave]

OK, check me out here:
The engine is grounded because it can handle the starter current and the one on the passenger horn is new and clean. To be sure, I ran a jumper cable straight off of the battery to the block and from there I ran a large wire to the phillips bolt and nut holding the ground wire at the ECU, another to the one on the runner, and yet another to ground to the O2 harness plug.

I cleaned all of the bulkhead connections paying particular attention to the two black wires in the Blue plug (22&23...the chart says warmup regulator and diagnostic grounds. Or, did you mean something else?)

...No difference.

FWIW, I noticed that, if I'm leaning on the fender and touch the plug, even the ground through my body will cause the Dwell to act the same way.

Any other related grounds...or ideas?

Thanks!

You didn't mention the ECU Ground on the intake runner that I mentioned. This is a completely separate ground reference for the ECU that does not tie to the other grounds. It is commonly attached at the forward end of the passenger side runner, next to or with the same bolt as the spark plug wire loom. OEM it has a piece of red shrink-tube on it. If that ground is bad or flaky, it completely confuses the ECU. This appears on the Lambda ECU as pin 5 or 16, I don't recall which. One of those is the screw you mentioned, this is the other. It is not called out on the Delman diagram. But I'm guessing that he has one of the black grounds on the blue plug mis-identified.

[Ron]

A single black wire held with a 10mm head screw...goes straight forward and into the harness running across the firewall?

[DMCMW Dave]

A single black wire held with a 10mm head screw...goes straight forward and into the harness running across the firewall?

Yep.

[Ron]

At red arrow?
10983

Edit: Ya beat me back lol

When I said I grounded the runner, I should of said wire on the runner.

I'll remove it and see if it has continuity back to pin 5 or 16 or ??.
THANKS!

[Ron]

I get .40 Ω from runner lug (disconnected) to pin 5 on ECU. No change with wiggle test.