No 5v reference only 1.4v

1999 Chevrolet Suburban K2500, 5.7 (OBS)
Throttle position sensor Code P0122, low voltage circuit "A" keeps recurring for several years now off and on getting irritating. Replaced tps with a new ac Delco all was great for a week. Have always had a 5v reference and a good ground reading and good signal but keeps throwing the p0122 low signal code
This time the 5v reference wire on the TPS only reads 1.4 to 1.5 volts with no change and ground is good Checked map sensor and it had a 5v reference signal.
Only other parts changed in the last few months have been the egr, map sensor and about a year ago the MAF sensor. I'm stumped on this one usually I can figure this out. I'm 67yo and tired of hanging upside down in my engine bay help

Hey Olddog! Let's recheck your readings at the TPS. Key on engine off, what are your voltage readings on pins A, B and C? Sensor connected AND sensor unplugged.

I've attached a section of the wiring diagram:



A reading of 1.4-1.5V on pin C suggests a possible high resistance or poor contact problem between the TPS connector and the PCM. We'll need to do a few more checks to be sure.

First thing this morning the non attached power wire read 5.1 volts I thought yea
Now doing reading like you said

KO attached to sensor
1.36v
0.0 ground (ground continuity is good)
0.13 v sensor wire

KO sensor unplugged
1.36v
0.0 ground
0.03 sensor wire

Love it.

For me, the next step is to find pin 12 of C3 at the PCM and take another voltage reading. If the voltage is closer to 5V, then you're looking at high resistance. If the voltage is still low, then I'd suspect a poor PCM connection or a failed PCM.

Before you go to all that trouble, try this - backprobe a 5V reference wire at a different 5V sensor. The MAP, or the EGR. Something easily accessible. NOT a thermistor. Run a jumper between that and pin A at the TPS. Does the sensor now work correctly?

Did the jumper yesterday from map sensor that read 5.1v before connecting jumper. After connecting jumper it did the same thing, 0.0 reading on scanner and voltage dropped to 1.4 at both connections upon reading them.
Not sure about pin and C3?

Ok probed C3 pin 12 (grey wire) did continuity from pin 12 to sensor power and that checked out
KO Back probed pin 12 got 1.30v
Probed sensor power wire again and got 1.29v

Did the jumper yesterday from map sensor that read 5.1v before connecting jumper. After connecting jumper it did the same thing, 0.0 reading on scanner and voltage dropped to 1.4 at both connections upon reading them.

Well isn't that interesting. :ohmy:

Ok probed C3 pin 12 (grey wire) did continuity from pin 12 to sensor power and that checked out
KO Back probed pin 12 got 1.30v
Probed sensor power wire again and got 1.29v

You may be dealing with a resistive short on that 5V reference wire. Disconnect the PCM and the TPS. Probe pin A at the TPS, and measure resistance between pin A and ground.

Probed pin 12, 5v reference, directly with nothing connected and only got 1.30 v.
Found a place that I'm gonna send the pcm to for repair. Have had this issue for several years now. All other 5v reference sensors check out ok just think the one is fried. Good thing is they will check out the whole pcm as well.

Probed pin 12, 5v reference, directly with nothing connected and only got 1.30 v.

You mean, with the PCM disconnected and the TPS disconnected, you saw voltage on the grey wire? :ohmy:

The MAP sensor voltage getting pulled down when you ran a jumper between the MAP and the TPS is a problem, and suggests an issue outside of the PCM. I just want to cross all T's and dot all I's before you send your PCM off.

When jumped from map sensor 5v reference to TPS the original grey wire from pcm to TPS was disconnect
Then disconnected the wire harness from the pcm, white- pin 12 5v reference , KO only showed 1.30 volts at the actual pin itself with nothing connected to it.

This morning had 5v at map and TPS changed out map and so far all is well