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Issues with wiring LP to keyed on power and bypass OPS?

F

fastline

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Ignoring the safety aspects of this for now, I have my LP wiring to keyed on power and ignoring the OPS. I understood the system to be like a gasser where when the pumps runs, it flows fuel continuously and any fuel not needed is returned to the tank thus no issues regarding operation.

I had a shop tell me that the LP actually sees reduced voltage during start to reduce the fuel flow to the IP which could cause a problem for the IP if it gets full (5-8psi) pressure during starting. That would mean the OPS is more of a variable sensor than a switch. I am kind of sensing BS on this one but since I cannot get a fuel system diagram from anyone, I really have no idea how everything is designed. I fail to see how the IP would "fight" to overcome this pressure. Should actually help the pump.
 
Your BS sensor is on the money.

The LP primes the system during the glow cycle and then shuts off during cranking until the OPS sees oil pressure and then the LP starts to pump again.

The only thing you would notice is the LP would get lower voltage since the starter is drawing the batteries down a bit during cranking.

Search here or google for OPS relay mod,and install it then let a new AC Delco OPS control the LP through the relay and you will be good to go.
Or buy the ready made kit from Kennedy Diesel.
 
Before '96, the glow cycle does *not* activate the pump. If you have a 1994, your lp is only on when there is oil pressure.

A lot of people will tell you how the pressure-sensor-controlled lp is a big safety feature.

Having said that, safety issues aside - a lp hot-wired to 12v ignition- switched source will work just fine. diesel is not explosive and not as flammable as gasoline.

A relay mod is better.

A mod that combines the relay mod and also turns the lp when the glow plug relay is activated is even better.

But the simple hotwire mod will work too. You make it as complex as you like.
 
I believe the OP truck (94) only powers the lift pump during crank and when there is oil pressure. That is assuming everything is stock, which his is not as it is wired to keyed on power for now.
 
BS is correct, there was nothing reducing the flow or pressure to the IP in the design other then the very poor OEM design.

Mine still is OEM but I assure you the day it chit's the bed it's gonna get wired directly off the ignition, when it's ON my LP is gonna be ON. To be more precise my LP is gonna get the command signal from the FOS wire. The FOS is another useless part only there to strand you when it fails.

Please don't scald me I know I know hug a tree save a bee safety this & safety that yadayaydayaydayda...:eek:
 
There is absolutely no advantage to running the lift pump with key on as far as the fuel system is concerned. You are just bypassing an unreliable OPS that is solved with relay mod suggestions above. If the lift pump running does help you have an air leak that needs correction. No advantage to 'set safety aside' because diesel will burn especially when spilled on red hot shorted battery cables after a wreck. So the disadvantage is a fuel leak being a bigger problem than it has to be.

Fuel just leaks by the IP to be returned to the tank if the engine is not spinning with the lift pump running. Once the engine starts the amount of fuel flow being returned to the tank is considerable. You have to purge the air from the filter as the lift pump running will not do it without the air bleeder open. Because fuel or air is blocked from flowing by the transfer pump in the IP that is not turning. You can jumper wire the pump to run via the ALDL connector or jump the factory relay to assist purging air from a new filter.
 
Im with Don on this. No good reason to not just do a relay mod and keep safety (its simple and cheap). If the worst happens you could save the life of another.
 
Yeah, the LP is specifically powered during crank on OBD1 by the LP relay. A prime was implemented in OBD2 because it helps the DS4 start easier. If I park on a downward incline, nose lower than ass, I have to prime the system before cranking or it will have a hissy fit about half the time.
 
konstan said:
Before '96, the glow cycle does *not* activate the pump. If you have a 1994, your lp is only on when there is oil pressure.

A lot of people will tell you how the pressure-sensor-controlled lp is a big safety feature.

Having said that, safety issues aside - a lp hot-wired to 12v ignition- switched source will work just fine. diesel is not explosive and not as flammable as gasoline.

A relay mod is better.

A mod that combines the relay mod and also turns the lp when the glow plug relay is activated is even better.

But the simple hotwire mod will work too. You make it as complex as you like.
Click to expand...



The Safety issue is not just flamability. Its fuel spillage.

During an accident, loss of oil pressure when an engine stops, automatically stops the fuel pump. If a fuel line ruptures, the engine will stop, this will prevent a fuel pump from pumping a tank dry.

In a short term emergency, the bypass is fine. As a long term fix it is irresponsible and dangerous! ! ! (for the environment, first responders, and yourself)
Spend the few dollars and add the relay to the OPS circuit if your truck has the lift pump and OPS directly connected.
 
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