I copied this out of another post as I wanted to start a new thread somebody could find in a search easier.



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<TD class=bold width="20%" background="" bgColor=#f8f7ef><A name=49449></A>Turbine Doc</TD>
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<TD class=smText noWrap>Posted: 03 February 2004 at 2:05pm | IP Logged </TD>
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Diesel Fanatic
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Group: Diesel Fanatic
Joined: 09 September 2003
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Oil press circuit.


Howie you are correct in the single wire from OPS, spent some time in the manual to make better sense of it . I think though the circuit is different between how OBD 1 &amp; 2 handle OPS in regard to fuel powering, in my 98 manual you have to go to 2 sections to maybe make some sense of it. I had to go to vol 1 for body and acessories for the pin out diagram, and vol 3 for fuel controls diagram.


Looking at the OPS connector there is capability for (4) pins only (1) is used pin A (tan), the oil pressure is registered at the gauge via a 0-80 ohm resisitive output, to test if bad GM says put their J tool on it and vary resistance to track displayed pressure 0 ohm is low, and 80 is high. No number as what ohm = what psi I assume 0-80 full scale and 40 ohm would be mid scale.


Now on OBD 2 I assume somewhere the PCM is also monitoring the output of OPS current for the backup command to energize and deenergize lift pump.


Lift pump control description from manual paraphrased; Key on PCM energizes fuel pump relay, lift pump remains on during glow cycle. If engine not started immediately after glow cycle, PCM shuts off lift pump until engine rpm greater than zero. As a back up , the lift pump can also be turned on by the OPS. When pressure reaches 4 psi &amp; the fuel pump relay does not complete the circuit, the OPS will close and complete the circuit to run the lift pump. This is where I get fuzzy as I can't in my minds eye or from the schematics, see how a resistive device can power the lift pump, so my assumption here is the PCM does the switching on or off either via rpm signal or OPS signal.


In my DP reprint book for 99 it shows a schematic with a OPS using 3 pins (1) 12v supply, (1) to gage, and a 3rd to lift pump (the gray wire) which is where they connected the relay control leg, so I think 2 paths in that OPS 1 instrument and 1 control, &amp; its the control contacts that were failing in the earlier models needing auxillary power for long term fix.


Unfortunately I don't have a OBD 1 manual to confirm my theory, but from what I read between the lines and posts Iv'e read over at DP if I don't have it exactly Im pretty close to how it works. Anybody else know more please add input, I'm here to learn myself.</TD></TR></T></TABLE>

I've read the gm page, but Iforget.


is it '96 newer obdII,'94-95obdI


does L65 relate to obdII.

L65 is HD generally non EGR equipped (F) 8th digit in a VIN indicates this; L56 is LD EGR engine (S) 8th digit in VIN can be OBD 1 or 2 OBD 1 or 2 depending on year. OBD (on board diagnostics) is the data link system and engine coding.


OBD 2 was mandated to be standard in the US in 1996 by legislation, before 96 different vehicle mfrs had different connectors, locations, &amp; diagnostic codes. Now all have same connectors and naming conventions, leading numbers are generic a engine, emissions, trans, drive train, etc. has same basic prefix for all mfrs, the ending code is left up to mfr.

thanks for clearing that up....I checked my vin and I'm glad to see the F in the 8th place, makes since for 3500.