All this work to get it right and it's still a work in progress....What's next?

[1998_K1500_Sub]

according to the service manual, the VSS being faulty should throw a code 24. It's doesn't have that code stored.

Those old ECUs were pretty crude… great for the time but nothing like those deployed for OBD2… and they’ve continued to improve thereafter.

They’ll mis-report faults or simply fail to report them… which is why I encourage you to test every sensor and actuator to your best ability, because you can’t count on the ECU to report on every problem.

But, I agree, Why would I not be getting a zero reading while stationary?

See the link I posted just prior.

 

[Dajain]

See the link I posted just prior.

The link talks about the speedometer doing funny stuff and my speedometer works as it should. I replaced the cluster due to a faulty fuel gauge so it's a few mph off, but that's a different issue and not really concerning.


But that was just an initial trial run after going thru the old girl. Maybe speedometer issues will show up after I drive her a little bit. But, the speedometer doesn't show any speed while stationary, just the ECM does, which I think is a big part of the shifting issue.

Is there a way to reset/calibrate the ECM to get it back to a "base" zero mph? I know the pins on the back adjust the MPH reading but can the ecm reading be set to zero while stationary?

 

[1998_K1500_Sub]

The link talks about the speedometer doing funny stuff and my speedometer works as it should.

The link talks about testing the VSS and, in particular, one guy's problem with the connector.

Here is it again, for convenience:

Post in thread '700r4 VSS and speedometer'
https://www.gmt400.com/threads/700r4-vss-and-speedometer.65554/post-1374569

Is there a way to reset/calibrate the ECM to get it back to a "base" zero mph? I know the pins on the back adjust the MPH reading but can the ecm reading be set to zero while stationary?

AFAIK that's not an option.

 

[Dajain]

The link talks about testing the VSS and, in particular, one guy's problem with the connector.

Here is it again, for convenience:

Post in thread '700r4 VSS and speedometer'
https://www.gmt400.com/threads/700r4-vss-and-speedometer.65554/post-1374569




AFAIK that's not an option.

I did see the multimeter set on volts test, but will have to look farther into the thread with a little more depth.

I would love to have it running yesterday, but it's a "Tinker" project until it gets running right.

I will also be going back thru the posts and making more notes. I already have the TPS and VSS trouble shooting charts printed and ready to go.

 

[Schurkey]

OK, put the scan tool on it and here are most of the results at operating temp, at idle and in park. I see one issue is the vehicle speed.

Can this scan tool display these readings as graphs? Some tools can...some can't.

Seeing a graph of some of these readouts rather than a single number might be helpful.

O2 - bouncing between 350-400

I'd expect it to bounce between .100 and .900, likely between .200 and .800--ish.

Open/closed loop - Open

Not sure why it's in open-loop under those conditions--engine is up to temperature, but maybe it hasn't run long enough for the open-loop timer to run out.

Exhaust O2 - bouncing between rich & lean

Good.

Throttle % - Changes with acceleration. I seen 30% at 2500 rpm

As long as the percentage or the voltage changes smoothly and steadily from idle to WFO. As said...about 1/2 volt to about 4 1/2 volts, more-or-less.

Desired Idle - 750 rpm

Not sure why the computer can't control idle speed better than that. Desired idle is 750, actual idle is 800. Seems a little off, but not crucially so.

O2 cross counts - 25

Better than I'd have expected.

Baro (inHg) - 25.3
Baro (volts) 4.08

Might be low, but depends on weather and especially altitude.

Air/fuel learned - bouncing between yes & no

That seems odd.

Vehicle speed (mph) - 14 thru 30 depending in throttle position. (I was in park and stationary!)

THAT is a problem.

Spark Advance - 20°

Seems normal.

Battery 13.6

I'd have expected 14+, but not horribly low.

Also looked up the codes and here is what it is currently showing:
22 - TPS too low
42 - EST Problem
87 - Shift solenoid B stuck off
86 - Shift solenoid B stuck on
85 - Gear ratio problem

Clear the codes, see what comes back.

What I see is a vehicle speed sensor that is faulty.

Yeah, that seems to be the most obvious issue.

Disconnect the battery for thirty seconds, reconnect, re-test.

 

[Dajain]

Can this scan tool display these readings as graphs? Some tools can...some can't.

Seeing a graph of some of these readouts rather than a single number might be helpful.

I do think this tool does have the graphs. Never played with those. Just look for spikes or above average if it does the graphs?

I'd expect it to bounce between .100 and .900, likely between .200 and .800--ish.

It probably is .350 - .400. It is an older MODIS and the screen is hard to see.

Not sure why it's in open-loop under those conditions--engine is up to temperature, but maybe it hasn't run long enough for the open-loop timer to run out.

It ran about 15 - 20 minutes to warm up. I would think that is long enough. ??

Good.


As long as the percentage or the voltage changes smoothly and steadily from idle to WFO. As said...about 1/2 volt to about 4 1/2 volts, more-or-less.

The TPS doesn't start to read until the pedal is pushed about 1/2" down. Then tops out at 88% at "to the floor" condition. Aren't these supposed to be self zeroing?

Not sure why the computer can't control idle speed better than that. Desired idle is 750, actual idle is 800. Seems a little off, but not crucially so.

Considering all the jerry rigging I've found in this truck, the PO may have just cranked up the idle to keep it running at one point. I figure the idle should be done after the miss is taken care of for a more accurate setting.

Better than I'd have expected.

Thumbs up!

Might be low, but depends on weather and especially altitude.

Storms moving in and 4300' elevation.

That seems odd.


THAT is a problem.

That's where I think the real problem lies. Focusing on that right now and hoping things will fall into place

Seems normal.


I'd have expected 14+, but not horribly low.

The truck has 4 batteries and not all charged right now.

Clear the codes, see what comes back.

Have the batteries disconnected for the night. I'll get back to it in a few days

Yeah, that seems to be the most obvious issue.

Disconnect the battery for thirty seconds, reconnect, re-test.

 

[Dajain]

Just trying to brainstorm while laying bed this morning.

I haven't looked at the transmission speed sensors yet.
So my question is, if the transmission has a bad input or output speed sensor, would that possibly give me a false speed in the ECM?
Since I am still getting a speed reading even when the VSS is disconnected, that value has to be coming from some other source.

Correct me if I'm wrong but I am thinking the input and output sensors on the 4L80e are the only other sources where that number (mph) can be coming. More specifically, the output, because the input will change with engine rpm's.

Is my logic going in the right direction with this?

Or, will a shorted wire for the speed sensor give me a false reading? My logic tell me no on this, because a "shorted" condition would just be a steady reading and not a variable mph reading which corresponds with the engine rpm.

 

[NickTransmissions]

Just trying to brainstorm while laying bed this morning.

I haven't looked at the transmission speed sensors yet.
So my question is, if the transmission has a bad input or output speed sensor, would that possibly give me a false speed in the ECM?
Since I am still getting a speed reading even when the VSS is disconnected, that value has to be coming from some other source.

Correct me if I'm wrong but I am thinking the input and output sensors on the 4L80e are the only other sources where that number (mph) can be coming. More specifically, the output, because the input will change with engine rpm's.

Is my logic going in the right direction with this?

Or, will a shorted wire for the speed sensor give me a false reading? My logic tell me no on this, because a "shorted" condition would just be a steady reading and not a variable mph reading which corresponds with the engine rpm.

Malfunctioning speed sensors or shorts in the wiring harness and/or connectors can cause all sorts of transmission functional issues. Test them by simply swapping their positions (put the front one in the rear position and vise versa). If something changes, you've found the (or "a") cause to at least one of your problems.

 

[Schurkey]

If you're still getting a MPH signal after disconnecting the sensor(s), the problem is clearly not with the sensor(s), it's in the wire harness and/or computer.

I'm leaning towards RFI and unshielded wires, but I've been wrong before.

 

[1998_K1500_Sub]

I'm leaning towards RFI and unshielded wires, but I've been wrong before.

^^^Yes, to all.

If I was me, I would disconnect from the VSS and then insert a jumper across the connector leading to the ECU so as to short the wires together... and then see what happens on the scan tool. Otherwise, when open circuited, it leaves that circuit susceptible to noise... and I suspect the VCC circuit in the ECU is fairly sensitive, i.e., "susceptible".

The service manual should be a guiding light in this moment of darkness. I know the '91 manual is on-line, I downloaded it just the other day...