So a 1999 ZR2 5speed Blazer I picked up runs like garbage. Idles fine and revs nice.

Put it in gear. Bogs like an EGR issue. Because there is an EGR issue.

KOEO I get the 12v on the red wire. But only o.489v at the gray reference voltage wire. Same at the tps and map sensor.

On the wiring diagram I see that the fuel sending unit reference is spliced into the same wire. My fuel gauge does not work.

Is a possible short in the sending causing such low voltage?

The pcm was rebuilt to try and fix this problem. Did they miss a zero when they replaced the resistor?

I honestly dont have a clue. Thoughts?

Thanks

 

The TPS sensor uses a separate 5V reference wire from the PCM than the MAP/EGR/FTP (fuel tank pressure), but I am pretty sure that both come from the same power bus inside the PCM so a problem affecting one should affect them all and you should get codes related to these items.

Are you getting codes related to any of these items?

How are you sure that there is an EGR issue other than the bogging down of the engine?

 

PO107, PO405, PO122, PO462 Are the current DTC

Tested the components with the oscilloscope and they seem fine. Previous owner replaced all of them trying to chase this issue.

This is really a beating

 

Have you tried monitoring the 5V reference voltage while disconnecting those sensors that use it? With all of the sensors disconnected, do either of the wires at the PCM connector C3 (cavities 27 & 12) show any continuity to ground?

All of these codes are listed in the OBD2 Code Index linked to in my signature below. You can run through the diagnostic charts, completing all of the steps you can with what you have available to you.

 

I have unplugged all of them at the same time, besides the fuel level SU.

Each of them measure within a few mV of 0.48v

I don't recall doing a continuity test between pin 27/12 and ground. Just a resistance check between the gray wires at each sensor.

 

I am wondering if there is a damaged wire somewhere that is grounding out the 5V reference line. The only way to properly test the circuit is to have all of the components that use the disconnected including the fuel tank pressure sensor or fuel level sensor depending on what reference you are looking at. This is the sensor that is mounted in the middle of the fuel pump assembly. The crappy part is that you pretty much have to drop the tank a little bit to get at the connector for this sensor.

 

Yeah. That's what I'm dragging my feet on doing, haha. I have a new fuel pump to put in. May as well bite the bullet and get it over with.

So I'm looking for a short to ground? Nice. That should be fun.

What's your advice on testing the circuit? Or should I try to find a symptom based diagnostic chart...

Thanks for the help, btw.

 

A short to ground is one possibility. With all of the sensors disconnected, if you still do not get 5V at any of the sensor connectors, then there likely is a problem with the wiring that is shorting out the 5V bus in the PCM.

It is also possible that the PCM itself is at fault, but that is quite rare in these trucks. I am not sure if the PCM will power up properly with the C3 connector (clear) disconnected. You could try disconnecting it and probing pins 12 & 27 for +5V. You just have to be careful not to short pins when probing them. Another course of action could be cutting those two wires 6" or so from the connector. I do not like doing something like that, but if the PCM won't power up the 5V bus with C3 disconnected, that may be the only way to isolate the circuit from the rest of the truck. You will then need to splice them back together. I would recommend a 3M shrink wrap butt splice due to the adhesive in the shrink wrap which will seal things up nice & tight.

 

Funny thing... I did cut the wire to pin 27 to try and isolate it. No change. I will try again with pin 12 cut and update when I get around to it.

 

The throttle position sensor is the only sensor that operates off of pin 12. If the TPS is disconnected, Connector C3 at the PCM is disconnected, and the wire shows no continuity to ground, then the PCM has to be at fault.

If you find that the PCM is at fault, I would suggest getting a tuned PCM from either PCM4Less or Black Bear Performance. You can do the swap with them. You would only have to perform the passlock reprogram as well as the Crankshaft Position Relearn.