78 Bus FI testing temp sensor 2 (TS2)

[locoqueso]

In the process of troubleshooting a "bucking/hesitation" issue that results in the bus spewing black soot, I’m trying to test the resistance of my temp sensor II (78 2.0L FI).

The sensor is still installed and per the Bentley manual, I have disconnected the TS2 wire for testing. I’ve hooked up the red multimeter probe to the sensor’s wire and grounded the black probe. I just can’t get any any Ohms level to appear. The digital display remains at “1”. I’ve reconnected the multimeter a few times but I can’t get anything to register. I even tried a differnt multimeter, but got the same results.

I’ve never used a multimeter to test resistance before, although looking at the illustration in the Bentley and Bosch FI manual, I’m fairly certain that I have it connected properly. Does anyone have any suggestions or even photos to ensure I’m testing it correctly? Is that reading expected if the sensor is bad?

http://www.ratwell.com/mirror/potts/pic ... ual019.jpg

[dtrumbo]

You're doing it right. Two meters say so. It would seem you should doubt your TSII instead of your meters and your new-found skill! To double-check, connect the red and black leads of the meter together. The '1' (infinite resistance) should change to close to 0.00 (no or little resistance). This will prove your meter is working and your technique is sound.

TSII's are (relatively) cheap. Pick up a new one, install it and see what your reading is. Worst case, you'll have a spare.

[Amskeptic]

In the process of troubleshooting a "bucking/hesitation" issue that results in the bus spewing black soot, I’m trying to test the resistance of my temp sensor II (78 2.0L FI).

The sensor is still installed and per the Bentley manual, I have disconnected the TS2 wire for testing. I’ve hooked up the red multimeter probe to the sensor’s wire and grounded the black probe. I just can’t get any any Ohms level to appear. The digital display remains at “1”. I’ve reconnected the multimeter a few times but I can’t get anything to register. I even tried a differnt multimeter, but got the same results.

I’ve never used a multimeter to test resistance before, although looking at the illustration in the Bentley and Bosch FI manual, I’m fairly certain that I have it connected properly. Does anyone have any suggestions or even photos to ensure I’m testing it correctly? Is that reading expected if the sensor is bad?

The only thing I would add, is that multimeters have different scales, and if you are on a clumsy scale it will be stupid to read.
Select rX100 and your reading should be around 25 to 30.
Select rX10 and your reading should show 250 to 300.
Select r X 1000 and it will be a decimaled 2.5 to 3.0
Colin

[locoqueso]

OK, thanks for the quick reply. I just tried again... I touched leads together and saw it zero out. I think I may have had a bad ground duinrg my first round of testing (oops).

My second meter has alligator clamps so I clamped the black to the engine tin and the red onto the wire and I finally got a reading from the temp sensor. With the meter at the 20k Ohms setting, the reading floats between 3.17 and 3.21. The engine compartment temp is about 66 degrees right now.

[locoqueso]

I've been messing with it a little more this afternoon. I finally got a reading as I mentioned in my last post. At 66 degrees, it was about 3.18 (20k setting) more often than not. It just wouldn't consistantly give me a ready. I could disconnect the red lead, immediately recoonect the alligator clamp onto the TS2 wire, and it would read go back to a readying of "1". The wire looks clean and I wiped down the engine tin where the black lead was connected, but I just couldn't get it to give me a reading every time.

There are a few other parts I need to pick up, so I plan to add a new temp sensor to the list. Like dtrumbo said, worst case I'll have a spare. Maybe this will fix the "bucking bronco" syndrom I've been experiencing the last couple years. Colin, I don't know if you remember, but we completely removed the idle screw last year and the bus still ran. I think you said you had never seen that before. Could a bad temp sensor cause it to run so rich that it will run without the idel screw (or am I confusing the issue)?

[Amskeptic]

Colin, I don't know if you remember, but we completely removed the idle screw last year and the bus still ran. I think you said you had never seen that before. Could a bad temp sensor cause it to run so rich that it will run without the idel screw (or am I confusing the issue)?

There are many possible causes to that little stunt. Did we go into the AFM?
Temp Sensors *change* a mixture over a range of temps. WE set the mixture to where we like at whatever temperature we are at. Then the temps sensor screws us up later when the temps change.

Do you see what I am saying? Ifd your temp sensor was at X I know we would have set the mixture to At That Moment Perfect. Later, the temp sensor would change its contribution as it warmed up or cooled down, and if it was incorrectly changing, you'd have driveability issues.

If I was pulling idle speed screw out of the throttle body, then I'd think from this later vantage point, that we had an idle speed that would not pick up?
Colin
(do not ground the black lead to the tin. Just put it on the sensor itself where the wrench goes on it. You want FEWER variables here )

[Gypsie]

My added 2 cents is to test the unit itself as well as it's effectiveness in place . Ie test the lead and TSII base (sensor) (poke that meter probe into the metal of the base gut'nhard. Then test with the lead and a good ground on the engine block.

If you are having some interference in the sensor contacting the metal on the case there may be an impact on the readings. Maybe the threads and other mating surfaces are cruddy and won't let electricity pass. Just a thought.

Remember that this is the sensor that gives a dickens of a time with fuel mix if it comes loose. Gut'ntight means Gut'Connection.

[locoqueso]

There are many possible causes to that little stunt. Did we go into the AFM?
Temp Sensors *change* a mixture over a range of temps. WE set the mixture to where we like at whatever temperature we are at. Then the temps sensor screws us up later when the temps change...

Yes, we adjusted the AFM. If I'm not mistaken, we leaned it out a bit. It ran better than ever right after that. A couple months later I took it on a 3 hour tour. About half way to the destination, it started to buck on acceleration. I decided to push forward and it only got worse. On the way home, it ran like crap the entire time. I had trouble accelerating fast enough to get it into fourth gear and couldn't get it into fourth if I was going up any type of incline. The harder I pressed the pedal, the slower I'd go. The bus would hesitate and buck as I tried to accelerate from 1st to 2nd, and from 2nd to 3rd gear. It would also die while idling unless I revved the engine. The back of the bus was covered in soot by the time I got home.

I've been chasing this same problem for the last couple years and it always happens after I've been driving for a while (45-90 mimutes) and the engine is up to operating temps.

...If I was pulling idle speed screw out of the throttle body, then I'd think from this later vantage point, that we had an idle speed that would not pick up?

Sorry, I'm don't understand the question. I can't recall exactly why we removed the screw. It was on our first test drive and I don't think you were happy with the way it was running. We were tweaking the AFM and I remember you unscrewing the idle screw without the expected results. Eventually you removed the screw and commented that you never saw a bus run without the screw before so that stuck in my mind.

...do not ground the black lead to the tin. Just put it on the sensor itself where the wrench goes on it. You want FEWER variables here...

Very good, that makes sense. I should get a chance to test it again on Thursday.

Thanks to everyone for the feedback. I appreicate the knowledge!

[Psucamper]

Your problem with TS2 is reminiscent of one I had last week. My 70 FI 2.0 would start and run at cold start but quickly degenerate to rough idle and "loping' after the FI cold start devices did their thing. This engine had not been given a decent electrical workup in about 63k miles. Pulled the distributor and installed new points and condenser. Points had severe pitting with the usual "tit" and crater. Reinstalled distributor and hoped my fix would work. No joy..same loping. Tested all hoses for vac leaks and they were tight. Back to reality. Got out the Fluke and started ohms and volts testing.
Power in the right places with no anomalies. Pulled the TS2 wire off the FI harness and ohmed it. Guess what? Our friendly TS2 reads 25K ohms! Just a tad high. Cooled engine and pulled TS2 out and remeasured. Now ~2k at 70 degrees. Damned corrosion! Cleaned engine block,TS2 hole
and installed new Bosch TS2 with correct washer and tightened to specs. Started and ran engine.
Good fast start up with correct slowdown to idle speed. No "loping". Case closed (for now).
Saved old TS2 for spare.
RDL