Pcm back feeding 4 volts?

[Andrewjhj]

What would cause voltage to be present on the signal return side of all the sensors? At the tps, iat sensor, ect sensor I'm getting 5.62 volts on the ref side but on the signal return I'm getting 4 volts. Measured from battery negative to connector (unplugged all to make sure a sensor wasn't shorted) any other places to look or test at?

 

[tomw]

The other end of the wire in the connectors should be attached to the ECM/PCM/ECC transistor 'base' where the signal is fed/amplified and compared to ???. The reference voltage is 'modified' by the sensor, and fed back. The receiving end of that feedback has circuitry, likely including capacitors, resistors, etc to process the signal. The connection would definitely not be at ground, given that the computer has battery power applied. OTOH, if the key is OFF, I have to wonder a bit.
As long as it works, I'd not be concerned as I am not an electrical engineer nor designer. The guys that designed the EEC-IV/V systems knew what they were doing.

Is there a specific problem that urged the inspection of the return lines?
tom

 

[RonD]

MAF sensor uses that 5volts as well

The Return voltage is compared to Reference voltage inside the PCM(computer) so unless the sensor is hooked up you can't test Return voltage to a Ground.
Connect sensor and use a sewing needle to pierce Return wire then use meter to ground to see voltage.

The point of using Reference voltage to avoid relying on Grounds or external voltage.
PCM reads Reference and compares that to Return, no Ground involved in the difference although circuit has a common ground, but common to BOTH reference and return so not a "variable" that matters
If reference voltage is 5.0 and return is 4.2 then .8 is the sensor reading
If reference was 5.6 and return was 4.8 then same .8 sensor reading
What "they" are after is the difference, not the actual voltage.
Thats why Reference voltage is used, so regardless of voltage or grounding variables the sensor reading will be the same for each vehicle/computer software

 

[Andrewjhj]

Reason I was tracing wires is because check engine light was on but couldn't pull codes with a wire, but could with a Genesis computer. When I got over voltage codes on all the sensors I was hoping for a short but after opening the pcm found corrosion on one line. Went to the junkyard and found a 93 2.3 5 speed threw that computer in and although I still have codes truck runs 10x better (before it was a sun baked turd now it's a 4 cylinder ranger lol). Now I gotta take the new codes and diag from there.