Throttle Position Sensor-wrong voltage

[Solar1]

OK folks, here i go again with my lastest issue...I just lost the overdrive on my 96 12v autotrans truck.

A couple months back i replaced the TPS becuase I found it to be bad.

All has been good untill a couple day ago. Started truck up for a run to town and no overdrive whatsoever.
Yes, I did let it warm up to temp.

I supected (and was hoping) it was the TPS again and not the trans.

Put my scanner on it and found codes PO122 and PO123 ( low and high)

Checking voltage at the tps Truck on but not running, TPS connected).

PIN 1 (ground) good ground
PIN 2 (orange middle wire): 0.02 vdc
PIN 3 (violet, A17): 0.56

Checking voltage at connector unplugged from TPS:
PIN 1 Good Ground
PIN 2 5.0 vdc
PIN 3 0.62 vdc

Very strange, I have never got voltages like that on those pins.

I tried my spare, brand new in box TPS plugged in same readings, so, I dont think this is a TPS issue.
I almost think, heaven help me, it may be the PCM.

I removed all battery cables and let PCM rest for 20 minutes, then re connected batt cables and rechecked readings...the same.

Im at a loss to figure this out.
I did find a thread online which said to change the a/c relay in the PDC. did that, no change.

???

 

[Solar1]

I think my next step is to remove the connector at the PCM and check the 5vdc output pin. If not 5 volts, can I assume the PCM is bad, or can it be "reflashed" or reprogrammed?

 

[BigPapa]

The PCM supplies the 5V reference from pin A17 to these sensors:

• Throttle Position Sensor (White wire at the Sensor connector)
• Engine Speed Sensor (Violet/White wire)

If you have EGR, it also supplies the 5V reference for the:

• EGR Engine Coolant Temp Sensor (Tan/Black wire)

 

[Solar1]

if im not getting 5 volts at pin A17 without the connector attached to the pcm, does that indicate a bad pcm?

 

[DavidC]

if im not getting 5 volts at pin A17 without the connector attached to the pcm, does that indicate a bad pcm?

Probably not. The pcm most likely need to see at least an input to activate the 5vdc. Being unplugged it will not operate.

 

[Solar1]

thanks. Im just stumped...don't know what to check or where to track the issue.

 

[DavidC]

So, what are the voltages with everything together and the engine running ?.

 

[DavidC]

If Scott is right and I have no doubt he is I would think you would get a code for engine speed sensor, the alternator would not work etc. because it would also lose the 5vdc.

 

[Solar1]

So, what are the voltages with everything together and the engine running ?.

Checking voltage at the TPS with truck running, TPS connected).

PIN 1 (ground) good ground
PIN 2 (orange middle wire): 0.06 vdc
PIN 3 (violet, A17): 0.07 vdc

Only showing codes PO122 and PO123
Alternator working/charging

 

[Solar1]

Im wondering if I need to open the wiring harness loom and separate the TPS wires and inspect them all the way to the PCM.... If the Violet wire (A17 at the PCM) was chaffed, it might account for the low voltage at the TPS.

Any suggestions? Should I open the harness and follow the wires?

 

[DavidC]

I am really surprised the 12v guys have not chimed in here. If I am right here your engine speed sensor is in the front and has a plug on the harness. Guess I would check that for 5vdc as it supposedly is fed from the same place as the TPS. Maybe a splice problem in the harness. Since you get 5vdc when the TPS is unplugged you have removed the load on the wiring which may just be a bad connection in the harness.

 

[DavidC]

First I heard of the chafed wire. How bad ?. Did it go into the actual conductor ?. If you have a wire that has strands cut either where you can see or the strands are broke under the insulation you will get normal voltage until under load then the voltage will fall off.

 

[Solar1]

I don't know of a bad or chafed wire, im just suggesting that as a cause.
The service manual wiring diagram shows 1 splice (S137) between the TPS and the PCM. That splice feeds egr, engine speed sensor, tps from the pcm A17

 

[DavidC]

Check voltage at the speed sensor and verify good or bad. If good it could be the splice. If bad it is between pcm and splice

 

[Solar1]

Check voltage at the speed sensor and verify good or bad. If good it could be the splice. If bad it is between pcm and splice

Thanks DavidC, I will check that in the morning. Im wondering...Does the speed sensor provide the info for the Tachometer? I know the tach is working fine and the RPMs match it on the scanner/code reader. Just curious.

 

[BigPapa]

Yes, the signal from the ESS operates the tach through the PCM.

The wire for pin A17 at the PCM is Violet/White, circuit K6. The Violet wire is pin B1, circuit K54, Trans Temp Sensor Signal.

 

[DavidC]

Sounds like a problem from the splice to the TPS since the tach is working but I would check voltage at the speed sensor just to verify. I have heard Dodges splices were not very good.

 

[BigPapa]

Don’t know if your FSM has this Description and Operation of the TPS or not:

DESCRIPTION
The TPS is a linear potentiometer. The PCM supplies 5 volts to the sensor. TPS output to the PCM varies from approximately 1.0 volt at idle to 3.5 volts at wide open throttle (WOT) .

OPERATION
The Throttle Position Sensor (TPS) provides an input to the Powertrain Control Module (PCM) . It senses how far the throttle is open (past the idle position). The PCM uses the TPS input, along with vehicle speed sensor and engine speed sensor inputs to determine 3-4 upshift (overdrive) and 4-3 downshift. It is also used with the vehicle speed sensor and engine speed sensor inputs to engage and disengage the torque convertor solenoid. This solenoid is used for torque convertor engagement.

CIRCUIT OPERATION
From the Powertrain Control Module (PCM) , circuit K6 supplies 5 Volts to the Throttle Position Sensor (TPS) . Circuit K6 connects to cavity A17 of the PCM.

Circuit K22 delivers the TPS signal to the PCM. Circuit K22 connects to cavity A23 of the PCM. Circuit K22 also splices to cavity 3 of the EGR control module.

The PCM provides a ground for the TPS signal (circuit K22) through circuit K4. Circuit K4 connects to cavity A4 of the PCM.


SO, if you’re not getting 5 volts at the TPS on the Violet/White wire, you have a problem. Check the Violet/White wire at the ESS and see if you have 5 volts there. If you do, then your problem is the Violet/White wire between Splice 137 and the TPS. If not, then you have a problem with the Violet/White wire between the PCM and Splice 137 or a problem with the PCM.

 

[Timd32]

A4 is the Sensor Ground reference on the PCM, you could check A4 to A17 and see if you have +5v there, and follow the above splice info. Try it with the TPS plugged in and out.

TPS Signal Return is on A23.

But as mentioned the TPS missing +5v needs to be a starting point, I personally would try to reference A4 as your black lead if its feasible to get on that point. Might not be able too with what you have to work with lead wise.

And make sure your test leads and meter are working, been there before, had a bad test lead a few times.

 

[Solar1]

Hi Folks,
I found that the was 5 VDC at A17 of the PCM, I did reference A4 as the ground.

I tried to work my way back from the TPS checking the violet/wt line in found it dead as far back as I could reasonably get, as the "Y" split in the harness and the tight quarters at the back of the engine made it quite difficult.

I also followed the A17 line (violet/wt)from the PCM as far as I could as well, until it got too tight to work for this old guy. It had 5 volts as far as I could follow it. Which means that the break in the line is somewhere in between those two areas, which does narrow it down quite a bit. Perhaps when warmer weather (summer) comes back, I will totally unwrap the harness back there and repair it.

In the meantime, I did cheat a little and pulled the 5 vdc off the engine speed sensor line up front. That worked fine, and the TPS is reading as it should, and the overdrive is kicking in as is supposed to at about 45mph.

As a side note, I also pulled my steel vacuum lines for the front axle 4x4 shift motor. I checked the lines for leaks and cleared the plugged line, so now my 4x4 is back working also.

Thanks guys for your help with this latest issue.
This forum is great.