OEM or aftermarket pedal?

 

OEM Pedal replacement. But regardless I tested both the TPS and IVS at the pedal and both seem to be working correctly.

 

Using ForScan, monitor TPS values to see if the PCM sees the pedal changing, as you depress it, as it should.

Assuming that you have an Auto Trans,,, I would go have a look, under the column cover, where the OD sw wire can commonly chafe and cause problem with the TPS signal can get confused.


Good .pdf on this;

http://www.thedieselstop.com/forums/attachments/f24/16354d1360555963-starts-idles-no-dash-throttle-control-transmission-shifter-lever-chafe-repair-v-2.pdf

 

Thanks I will check this right now. I should have mentioned it's a manual trans.


When I checked with the engine running with Forscan and have the pedal depressed all the way this is the readings I have:

IVS - Off Idle
TPMODE - PT
TP_F - NO
TP RATE, G - -1.00

 

Any other ideas?

 

Key ON, without the engine running, as you push the pedal down and let it up again, do you see a clear and even change in TPS value? Does it look consistent and "make sense"?

Right now, if you start the truck, will it drive & rev as it should or does this happen intermittently? (the time to Test is, of course, only when it's acting-up...)

 

With key on engine off.

When I watch those parameters in Forscan and press the pedal the only change is the IVS shows off idle. But none of the TPS items show any change.

However when I test the gray wire at the pedal TPS connector with a meter it does show a change.

Does that make sense?

 

Sounds like a breakdown in communication between the pedal and the PCM to me.

 

O.k., update. Been busy at work so I couldn't get to the truck.

With EOKO using Forscan I see the following:

IVS - Off Idle
TPMODE - PT
TP_F - NO
TP RATE, G - -1.00

With the key on I checked with a meter the connector at the TPS and observed the following:

Brn/Wht - 5v
Yellow/Wht - Grnd
Gry/Wht - When pressing the pedal it changes from .5v to 3.7v.

Assuming there must be something in the wiring I pulled the connector from the PCM and testing continuity between PIN 24 (Yellow/Wht TPS) and the connector at the pedal, and there is continuity. I then tested PIN 89 (Gry/Wht TPS) and the pedal, and there is continuity.

I've tried clearing the P0122 code with Forscan but it just keeps coming back. So clearly there is something wrong, but I am not at all sure where to go from here.

Any ideas?

 

Well, not just continuity, but rather do you see that the change is the same At the pedal as well as, At the PCM's plug?

Compare the resistance measurement change at the pedal (not voltage, which would require that it's all plugged-in) and what those same wires are showing the going into the PCM. This is a wiring check between the two...

 

What about from the PCM to the rest of the harness. Where does the PCM send the signal to tell the engine to accelerate?

 

Also, what am I measuring resistance between?

 

We're only concerned about From the pedal To the PCM because you say you don't see the change in TPS value (that's a resistance value) in ForScan. So, whatever resistance there is in the pedal changing should be reflected in TPS position too.

Off hand, I don't know which wires or where in the plug you will find this change and what those values should be. I would just try the 3 (there's probably a Common, a trigger for the IVS sw, and the other for that resistance change).
You can't hurt anything, put your meter in Ohms measurements, pick 2 wires (pins in the plug going To the pedal) and find the pair that moving the pedal up & down "swings" resistance value.

Go to the PCM plug and find those same 2 wires and see if you get the same readings.

When the PCM wants to increase RPM it sends changes to the IDM (but, again, we want to know why the TPS value that the PCM uses isn't changing with pedal movement - as in your post #8)

 

O.k, so I found a service manual on another post and I've been going through the troubleshooting for the P0122 code.

One step states:

DD7 CHECK VREF CIRCUIT VOLTAGE  Measure voltage between VREF circuit and ground.  Key off. Was voltage 5 V ± 0.5?

Yes - GO to DD8 .

No - REPAIR open in VREF Circuit 351 (BR/W). RESTORE vehicle. CLEAR DTCs and RETEST.

I have 5V with the Key On, but not with the Key off. So I'm suspecting there could be an open in this circuit?

I can't seem to find any diagrams for where VREF circuit 351 (Brn/Wht) routes so I can check it.

Also seems strange that this is needed with Key off.

 

No, that 5v (this is what we call the reference voltage that sensors mostly use) is Only going to be found with the key ON. The PCM sends out the 5v, and ya know if the keys off the PCM isn't running, and as that voltage goes thru the potentiometer on the pedal (that variable resistor we're watching to see a change from as the pedal is depressed and released) it "deflects" from 5v (the .5v to 3.7v you saw). That's how the PCM determines how far down you've pushed the pedal.

Can you see Why I want you to measure the resistance change? (not gonna be able to do that with Voltage once it's unplugged from the PCM) and then see if the PCM has that information available At the connector?

This is all because you say that when you're monitoring TPS position in ForScan and depressing the pedal, you don't get a change in the value. Your truck won't "Rev" AND the problem looks like it's because the PCM isn't seeing the pedal being pushed.

So either the PCM has an internal problem reading the pedal OR the information isn't available to it because the wires aren't bringing the information to it because of a connection problem (if it changes At the pedal but Not in the same wires it reads from).

Not sure why you're fighting the method I've laid-out for you....

 

Sorry, no fighting at all, I just got excited when I found the detailed testing in the service manual.

So the three wires at the TPS plug are:

Yellow/Wht. - Ground
Brn/Wht - 5v VREF
Gry/Wht - Signal

So I'm assuming with the PCM plug out, you are saying testing the resistance between the Brn/Wht and Gry/Wht as I depress the pedal and see what it reads at the PCM plug?

 

Again,,, I don't want to guess which two because it's pretty irrelevant I would unplug the pedal and use a meter set to the Ohms scale to find the pair of wires that vary resistance as the pedal is depressed. Take note of those colors and go to the PCM plug and test for a repeat of what you found At the pedal (with the pedal plugged back In of course)

Reread as much of what I've already said if you need, I think I've gone to great lengths to explain it all! ~hehe

 

Awesome, I am off tomorrow, so I will test then and report back.

 

Wheres the TPS fuse lasted on 99,there's two fuse boxes under dash