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Although discussing '99 gasoline engine, the expertly outlined LPR circuit diagnostic procedure is the same.


DS4 (EFI) DIY'r guys, specially OBD2'rs.. take notes @ 6 minute mark, mr instructor briefly explains "power side switched circuit bias voltage" and eludes to the quad driver modules (PCM guts and how some DTCs are generated @ 8:45) and how "repeated" DTC's eventually damage that DTC(s) quad driver module(s) (@11:00 mark, he is talking bout the QDM's)...
Although he's talking specifically about the LPR Circuit, the same bias can happen to other systems, EGR, WGS, APP, PMD, any system that can gennerate DTCs can be affected by this ghostly voltage, or not..


https://www.youtube.com/watch?v=McnXLcJNVfI
 

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Just did some speed reading....

So basically, to sum it up, get good batteries with the highest CCA and reserve capacity rating for any vehicle. And if possible, make sure your alternator is over 100amp rated so at idle it can have more out put.....

Sounds like decreasing electrical load would help too. Fan motors are a constant, as with power window motors. Granted, light bulbs could be switched to LED (except headlights).
 

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While back, I began rambling 'bout the construction of the "CONTROL MODULE aka ECM/PCM and how its basically a microprocessor ('puter) thats very sensitive to the tiniest voltage changes, drops or spikes..
In order to safely monitor and/or turn ON and OFF all the various "devices (solenoids, relays, sensors, etc) that require more power than the 'puter can handle, it employs an army of quad drivers (using a playing card analogy) that function just like there name implies, one quad driver controls 4 things. Each quad driver joins the next much like a daisy chain but not necessarily in any specific order to form a series of Quad Driver Modules (QDMs) necessary for that task assigned to that driver. Building on the playing cards analogy, think of QDM's as suits clubs, spades, diamonds, hearts that when stacked/shuffled together, make up the deck (control module). Basically a house of cards the slightest breeze can compromise stucture integrity..
Like some card games that require multiple decks, Control Modules may employ several DECKS (as many as needed) in order to perform all its necessary functions.. sorry,, rambling again..

What Im reading is this.. The slightest power mal-fluctuations in operating system that sends a sudden voltage spike or drop can open a can of ghostly milli-wohms in the PCM that IMHO many continue to misidentify as just another passing PMD Goblin..

I NEEDZ ME ANUTTIER PMD..



bookmark Junction Field-Effect Transistor (JFET) pdf
 

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While back, I began rambling 'bout the construction of the "CONTROL MODULE aka ECM/PCM and how its basically a microprocessor ('puter) thats very sensitive to the tiniest voltage changes, drops or spikes..
In order to safely monitor and/or turn ON and OFF all the various "devices (solenoids, relays, sensors, etc) that require more power than the 'puter can handle, it employs an army of quad drivers (using a playing card analogy) that function just like there name implies, one quad driver controls 4 things. Each quad driver joins the next much like a daisy chain but not necessarily in any specific order to form a series of Quad Driver Modules (QDMs) necessary for that task assigned to that driver. Building on the playing cards analogy, think of QDM's as suits clubs, spades, diamonds, hearts that when stacked/shuffled together, make up the deck (control module). Basically a house of cards the slightest breeze can compromise stucture integrity..
Like some card games that require multiple decks, Control Modules may employ several DECKS (as many as needed) in order to perform all its necessary functions.. sorry,, rambling again..

What Im reading is this.. The slightest power mal-fluctuations in operating system that sends a sudden voltage spike or drop can open a can of ghostly milli-wohms in the PCM that IMHO many continue to misidentify as just another passing PMD Goblin..

I NEEDZ ME ANUTTIER PMD..



bookmark Junction Field-Effect Transistor (JFET) pdf
So, when people tell 'newbies' to clean the grounds at the fire wall, back of block, alternator bracket and check the power connections, it's to eliminate these power fluctuations so that "can possibly" affect said engine computer and/or the PMD.

And don't be "cheap" on batteries....as I said. Aside from starting, sounds like a stable power source is needed.

Was the 100amp alternator "stock" for the 94/95 years? Did they go up to a higher one as stock at some point (120amp)?
 

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Discussion Starter #107
Yes, using the G pin won't test a bad relay but if you just want the LP to run it saves removing the cover over the relay and taking the relay apart.
I tried this. It doesn't work. It's because the circuit isn't closed by the OPS. (I think) All I got was a spark and no lift pump. You have to jump the relay to get power to the lift pump.
 

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I tried this. It doesn't work. It's because the circuit isn't closed by the OPS. (I think) All I got was a spark and no lift pump. You have to jump the relay to get power to the lift pump.
If the system is wired correctly, you should be able to power the LP by putting 12V on terminal G, bottom left.

The relay and OPS are in parallel, so can each provide power independently. Terminal G goes to the normally closed contact of the LP relay.

The interesting thing reading though a lot of threads from the past year is that this info was understood before, just a lot of user turnaround or new 6.5 owners, people not remembering that its been discussed a dozen times before.
 

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Discussion Starter #109
Here's an update, possibly premature. I changed the oil which the shop had put in when installing the engine and replaced with Shell Rotella Dino. Problem has not reappeared although I havent driven it enough or hot enough to be totally confident that the original oil was not performing well at hot idle. Off on a long trip next week so I should be able to confirm if the oil was the issue. I did install a manual override switch for the lift pump.
 

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So... how was the trip, any more lift pump voltage fluctuations with engine at normal operating temperatures?

I suspect the OPS lift pump contacts are damaged, likely from faulty ground(s), lose electrical connections and/or a previous worn, tired and/or defective LP..

With or without the LPRelay mod, we're supposed to ALSO replace the OPS and fuel strainer anytime a defective LP is replaced. Reasoning for it is by the time a defective LP (for whatever reason) is typically discovered, most likely the OPS (LP contacts) has already suffered irreparable permanite damage that can adversely affect the new LP's performance if not also shorten its expected service life.

Switch Contact Design see DC arcing....

Selecting the Proper Relay. A relay is basically an overglorified switch, see contact life.

Relay Contact Life
"Depending on the application, material transfer may be quite prevalent with fine silver contacts. Typically, material tends to accumulate in the center of one contact, while the loss of material on the other contact leaves a hole, or “pit.” This pitting may cause premature contact failure."






Ive seen this contact spatter and pitting first hand after dissecting a few of my own defective OPS. One was the OEM factory original where it was obviously manufactured to much higher "standards" than todays frequently failing BWD replacements apparently manufactured to todays extremely low substandards.​




 

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Discussion Starter #111
Hopes were premature. Still happening at hot idle. Used the manual switch when driving thru towns. I'll swap to synthetic oil soon so I'll see if that solves it.
 

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on my 94, I poked a small piece stiff copper wire into the lift pump fuse on the firewall and with alligator clip on other end of wire, bite onto one of the hot studs on firewall next to where the LP fuse hangs.

I have to jump my LP because the LP won't run any other way. Turn key on and glow plugs work but no LP. The fuse is good, I put a new Rock Auto LP relay in (located also up on the firewall, I haven't bolted this relay on the firewall yet...does relay need bolted on firewall to be grounded? It's just laying, but it's plugged in; and not working)
 
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