6.5l 2002gm stalling correction (2003 workhorse p42 base model 6.5L diesel)

this is a food truck made by union city body company. it has a non turbo 6.5l diesel engine. the injection pump is electronically controlled. i have replaced the pmd and relocated. this fixed the constant stalling after 2 minutes. but it stalled the other day for the customer. I drove it and ran it for 30 minutes with no stalling. it does stumble here and there.
I put my scope on the pmd and the signal from the computer matched the output from the pmd so I determined the pmd to be good. I want to prove the optical sensor and ECM. there is no trouble code, and the "check gauges" light comes on at start so it seems the ecm is good. I would expect a code if the optical sensor was faulty?

so my question, does anyone have a good schematic?

and. the optical sensor has 6 wires. how should i probe this to prove a good signal?

not sure if these are the right questions but that's what i came up with.
thanks

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I found the problem. it was just a noisy lift pump interfering with the optical sensor high resolution signal. this caused the pcm to deliver erroneous signals to the injector pump pmd.

I just installed a 50uf capacitor in parallel with the lift pump as a low pass filter and engine runs like a champ.
cheers

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I have a 2003 surplus Postal Service Workhorse P42 with rough running issues. I won't waste time with all the details, but I think I need a new lift pump. Overall, glad to know someone else in this world has a naturally aspirated 6.5 diesel. Glad you discovered your problem, fixed the issue and more importantly, shared your experience.