Circuits sharing 5V reference voltage?

Okay... here we go.....

Background Info
- 1995 Jaguar XJS convertible with a 4.0L engine
- vehicle idles rough and lacks power when driving

Discovered problem
- air intake temp sensor (IAT) and EGR temp sensor (btw, as per my "guided component test component information" in my Verus scan tool, this EGR temp sensor generates a DC voltage signal that is sent to the ECM) both have 2.7 volts (i.e. the signal wire) with the wiring harnesses disconnected. You can see this information on the pic titled "1995_Jaguar XJS_Engine Performance_2 of 3_pg 1_v1" attached to this post.

What I think the problem is:
- because I also have 2.7 V right at pin 1 of the red connector on the ECU (see pic titled 1995_Jaguar XJS_Engine Performance_3 of 3_pg 1_v1), I believe somewhere in the wiring loom the EGR temp sensor (which should have NO voltage with the wiring harness disconnected and KOEO) signal circuit is grabbing voltage from the 5V reference to the IAT sensor signal circuit. Is this good logic? This problem maybe obvious to some, but before I start cutting wires I wanted to make sure I wasn't missing something.

As always - thoughts appreciated and thanks in advance.

Smeter12 wrote: as per my "guided component test component information" in my Verus scan tool, this EGR temp sensor generates a DC voltage signal that is sent to the ECM

That is highly unlikely. It's drawn as a conventional temp sensor circuit which will be supplied current limited voltage by the PCM. Never worked on a Jag but I'd expect 5V open circuit.

The reason EGR and IAT are both at 2.7V unplugged is probably because they're only being supplied 2.7V by the PCM. The 3.9V at the CMP is also suspect. All points to a problem with your 5V reference.

It does say in the component test section of Shop Key that it generates a voltage, but this misinformation.

More correctly, it pulls 5v to ground as temperature increases. As evidenced by circuit test described, KOEO sensor disconnected, voltage should be 5v which is to be expected on just about any temperature sensing circuit.
I agree with Matt, your test results point to a problem with 5v supply from the PCM.

Troubleshooting this kind of circuit with this kind of problem (i.e. 2.7v at both the EGR temp sensor as well as IAT sensor) is new to me. I did replace the original computer with known good ECU and I still had 2.7v at the EGR temp sensor and the IAT sensor. This would tell me problem is with the car. So what could be a diagnostic approach on this problem? Use similar logic as one would when troubleshooting a downed vehicle bus system? Unplug all the 5V reference signal components that run off of the ECU's red connector (see attached pic) and then one by one connect the wiring harnesses of each 5V component while watching my voltage at the unplugged IAT sensor (i.e. some 5V reference component is bringing down the voltage on the 5V reference signal) go down to 2.7V?

Again, thoughts appreciated and thanks in advance.

In effort to update a resolution to this post, I would like to share the following "head up my bum bum" moment. To initially obtain a voltage reading for the 5V reference, I used a LED test light which has a voltage reading feature on it and I choose a poor ground as well as the clamp on this test light does not clamp well. This all lead to a 2.7 V reading. Later, I got out my high impedance volt meter, grounded my com to the battery and made a male connect to fit snuggly into the IAT sensor wiring harness. On my second go around, I got a 4.95V reading for my reference circuit. Hmmm - guess you can wrap this post up as "a clueless" moment from yours truly!! Learn from my mistake.

Might not have been a bad ground on your test light. A low impedance tester like that can pull down a signal circuit. Useful for IDing Vref and signal on 3 wire sensors.