Pin 13 OL Water Temperature Sensor

[Lunacie]

So I've been having a low rough idle situation, adjusting the idle screw does not a whole lot. The idle wont go higher than about 600-700.

I've gone around and checked for vacuum leaks and there don't seem to be any. So I've moved on to looking at the water temperature sensor.

I read in this topic here: https://www.classiczcars.com/forums/topic/63285-280z-78-wont-idle-running-rich-out-of-ideas/

where Zed Head talks about testing the ECU at pin 13 to ground for resistance, so I started there.

When I touch to pin 13 and touch to chassis I get a big fat 0L. When I touch to the ground pins, 17 or 35 and to chassis there is continuity, but direct from pin 13 there's nothing.
I removed the plug from the water temperature sensor and tested it to ground, one pin has continuity to ground, the other doesn't, the one that doesn't DOES have continuity to pin 13, which makes me believe the plug and everything between it and the ECU connector is fine?

I also tested the resistance of the water temperature sensor itself, and it has continuity. It had about 2.5 resistance with a cold engine, California weather. Which seems to be about in spec.

However, when the engine warmed up and the gauge in the car was about 120 a little above, the engine started FREAKING out. It seemed like it had sprung a vacuum leak, idle shot up to about 1200 rpm and misfiring like crazy. It was very sudden. I shut it off and checked the resistance of the sensor again, and this time it was about .64 which doesn't seem to be too far off the ball, as the FSM says at 120 it should be about .74 to .94 and I believe it was a bit above 120.

So, TL;DR I believe the problem is the 0L at the 13. It should read resistance? 

[Lunacie]

It's gotta be not making a good connection at the plug to sensor right?

if there's continuity between the sensor connector and the ecu connector and all the grounds seem solid there's really no other possibility? Or am I overlooking something.

[Lunacie]

Yep, answered my own question. Went out and sprayed it with electronic cleaner and pushed it on and off a handful of times and then did it again and now I'm showing a reading. Engine is still slightly warm from earlier, showing about 1.5 resistance at ecu now. It's too late to start it and see how it runs but I'll try tomorrow!

[Captain Obvious]

What is the range on the meter set on when you are taking these readings?

9 hours ago, Lunacie said:

I also tested the resistance of the water temperature sensor itself, and it has continuity. It had about 2.5 resistance with a cold engine,

However, when the engine warmed up and the gauge in the car was about 120 a little, I shut it off and checked the resistance of the sensor again, and this time it was about .64 which doesn't seem to be too far off the ball, as the FSM says at 120 it should be about .74 to .94 and I believe it was a bit above 120.

 

[Zed Head]

12 hours ago, Lunacie said:

when the engine warmed up and the gauge in the car was about 120 a little above, the engine started FREAKING out. It seemed like it had sprung a vacuum leak, idle shot up to about 1200 rpm and misfiring like crazy. It was very sudden.

Not clear what year car you're working on, and if it's California or Federal.  Some have two pickups in the distributor so that timing can be changed at a certain coolant temperature, for emissions.  Kind of sounds like that might have happened.  Might also be combined with a bad ignition module, one circuit might be bad.  If it does it again, you can disable one side and let it run on the good circuit.

[Lunacie]

Sorry! I totally meant to put that it's a '77 manual 280z. I do live in CA and believe it is a CA model.

Also, sidenote, when it freaked out, at that time the water temp sensor was sending 0L no info to the ecu. Wouldn't that affect the two pickups you describe, as the computer wasn't receiving the water temp info?

I intend to go through the entire 35 pin ecu connector to make sure it's getting all the info it should from everything next. Cleaning up the blackened spark plugs that came from the wts problem and yeah, we'll see if it does it again!

Edited March 25, 2021 by Lunacie

[Lunacie]

9 hours ago, Captain Obvious said:

What is the range on the meter set on when you are taking these readings?

 

My multimeter just has the one all around ohm setting. It was reading in kOhms