5v Reference only 2v at PCM

I tested my 5v reference by backprobing the 5v ref wire at the PCM and I am only getting 2 volts. I backprobed the Fuel Rail Pressure/Temperature Sensor and the A/C Pressure Transducer and both also only show 2 volts. I unplugged both sensors and backprobed the 5v ref at the PCM again and it still only shows 2 volts at the PCM. Does this mean my PCM is bad? Should I check something else? I have attached my wiring diagram images.

Thanks,
Tom

Also just to note. I back probed the PCM 5v reference wire by needle probing the wire at the connector with the positive lead and touching a ground screw with the negative lead.

I would re-test, with the ground lead connected to the negative battery post. Then, check ALL powers and grounds to the PCM.

If the results are the same, before I replaced the PCM, I would isolate the 5 volt reference from the harness. Meaning, snip the wire(s) close to the PCM connector (n a place that it will be easy to solder it back together). If you STILL have the same results, it is pretty safe to condemn the PCM.

Just out of curiosity, what voltage reading do you get (Key on) with the negative lead of the volt meter connected to the negative battery cable and the positive lead touching the metal case of the PCM?

Chad wrote:

I would re-test, with the ground lead connected to the negative battery post. Then, check ALL powers and grounds to the PCM.

If the results are the same, before I replaced the PCM, I would isolate the 5 volt reference from the harness. Meaning, snip the wire(s) close to the PCM connector (n a place that it will be easy to solder it back together). If you STILL have the same results, it is pretty safe to condemn the PCM.

Just out of curiosity, what voltage reading do you get (Key on) with the negative lead of the volt meter connected to the negative battery cable and the positive lead touching the metal case of the PCM?

+1

I connected the negative lead (meter) to the negative battery post and now get 0.8v at the 5v ref connection at the PCM. I cut the wire close to the PCM and still have 0.8v.
I also tested the metal case and with the key off I get 0v and with the key on I get 0.3v.
Weird thing is the truck seems to run fine.

At this point you may want to grab another DVOM to verify, just in case ...

Did you check ALL the.powers and grounds to the pcm as advised by Chad?

How do I know which pins are power wires to check? The wiring diagram does not list the pins as power wires. See attached for list of pins for one of the PCM connectors.

Are the power pins the ones listed as fuses/breakers? One of the other PCM connectors has two fuse/breaker pins and all the grounds. See attached.

You would have to check the list of the pins for ALL the pcm connectors.

If there was no information on powers and grounds then you would have to look at the actual wiring diagram and trace the individual circuits to see where they come from and go to.

Usually there are "power distribution" and "ground distribution" sections to be found in service information (SI).

TG53 wrote:

Are the power pins the ones listed as fuses/breakers? One of the other PCM connectors has two fuse/breaker pins and all the grounds. See attached.

Yes, they most probably are. As mentioned have a look at the actual wiring diagram to make sure. You should have at least one constant (B+).power feed and very likely at least one ignition switch powered feed.

I checked all the pins that are listed as fuses and they all have 12v. Tested all grounds and they are good. 5v ref at PCM with wire cut reading .8v.

Did you check the powers and grounds with a load applied to them?

Check wether there is more than one 5V ref circuit coming out of the pcm. If so they internally may be connected. Then cut the other 5V ref circuit (s) as well close to the pcm. If you still don't get 5V the pcm is bad.

this is amazing, i had the same case study on Toyota avensis, i think it should be 1999 to 2000 wagon with 2AZ engine make, with KOEO i have 0.2 volts on the reference side of the circuit..i almost thought i was running nuts lol ...i called it a bad PCM the client not being satisfied with my decision, he towed the car to another garage for reconfirmation i await the technicians report as to what the issue might be....

So I found another ref wire for the Fuel Tank Pressure Sensor called FTPREF. It also had .8v so I cut it at the PCM and now both ref wire are back to 5v. The only sensor I can tell is on the FTPREF wire is the FTP Sensor. How can I check the wire and sensor to tell which is causing the voltage drop?

Excellent finding! You just avoided to to throw fruitlessly a pcm at it.

Disconnect this fuel tank pressure sensor at the connector close to it and reconnect the cut wire on the pcm side. If the sensor is bad you will read 5V on the ref wires now. If the voltage reading goes low again then this particular wire is (almost completely) shorted to ground.

My bet is on the sensor itself.

Thanks for all the reply's. Disconnected the FPT sensor and 5v came back to the PCM. Just ordered new sensor. Thanks again for all the help.

Nice one buddy.