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Checking lift pump

R

RD400HP

Active Member
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Location
Fowler, Ohio
I finally got my 95 started so I can put in the garage to pull the intake & change the fuel lines. While it was running I checked the voltage on pin F on the DLC it read about 14 volts shut truck off and you can hear the LP running.

Now when I put 12 volts to pin F with the key in the run position I cannot make the LP run am I doing something wrong here?

Mark
 
It should work with the ignition off or in the On/Run position. Are you just going by sound or are you looking at fuel flow or feeling for vibration. When it was running did you check to see if there was fuel pressure out the drain?
 
Correct me if I'm wrong, but isn't 95 the year where the PCM still controlled the LP? Where it won't "pre-pressurize" the fuel system before the truck is started? My 98 "pre-pressurizes" during the glow plug cycle, the 95 does not. Wonder if that has anything to do with it?
 
buddy said:
It should work with the ignition off or in the On/Run position. Are you just going by sound or are you looking at fuel flow or feeling for vibration. When it was running did you check to see if there was fuel pressure out the drain?
Click to expand...


I was going by sound Buddy the LP buzz's pretty loud. And no I did not have a gauge on the drain I need to go out and buy one. Ill have to check and see if I have fuel coming out of the drain line tomorrow.

Mark
 
It could be that you just need to open the drain line to let it pump. If its already primed, it could dead head and not run. It does not make sense that you would get the voltage that the OPS passes but not be able to pass current the other way. There is no diode on the line.
 
Maybe with the way the fuel lines are leaking might have something to do with it Buddy. I'll go over to the garage tomorrow and see what I can get out of the drain thanks.

Mark
 
I was under the impression that 94/95 only activate the LP when in cranking mode,not during glow cycle?
Glow cycle LP operation is OBDII as far as I know.
 
You can use a amp meter to the pin and see if any current is going through it. You sure you have the proper pin? Look at the OBD connector closely and the letters for the pins are cast into it. (on the side etc, I forget where.)

I would not worry about lift pump during glow cycle. It has not worked for me, ever, on two 1993's and one 1995. Designed in or not it simply is INOP.

LP with oil pressure or cranking is all that matters. Cranking uses the fuel pump relay on older stuff and not 100% on the 1995 if it uses the relay as well.

Your best test is voltage to the fuel pump plug. If you can't feel the pump vibrating wired like that it is done. Otherwise test the pump ground. Lift pumps can fail intermittently and become a pain to catch.
 
If you replace the lift pump get one for a 1993 = higher volume.

You can do the WTS/lift pump relays.

There is an excellant schematic here someplace, that some genious drew up.
 
Here is a OPS relay mod with priming during WTS diagram for 94-95 trucks, or older.
LP relay mod.jpg

I have only tested the dual relay setup and it works, the other one should, but someone had trouble implementing it. Something else you can do with a relay is take the 5th terminal (Normally Closed 87a) and use it as a test/diagnostic power port. Either hook a wire up to it and run it into the cab for a push button primer, or a switch/button in the engine bay to make the LP run after changing a filter or draining water, or just checking pressure.
 
Thanks for the relay schematics guys appreciate it if I can figure it out its kind of hard to read the fine print.

Well I started the truck up and opened the drain valve and the pump appears to be working. I filled a quart bottle up, I didn't time it but it seemed like it didn't take to long. I was going to check how many volts I had at the LP with the truck running, when I checked it pegged the meter ,I thought I had the meter on the wrong scale but before I could check again the truck ran out of fuel, it turned out it was on the right scale anyway.

Whats weird I crawled under the truck and put 12 volts to the LP and I could not make the pump run . Is this pump getting ready to go south on me?

On the rubber fuel lines I could not find one that looked like it was bad, the line going to the IP was a little loose so that might have been the problem. I'm going to put all new hose's on it anyway. Where the supply line hook's up to the IP does the nipple fitting seperate from the fitting with the screen or is that one piece? I tried to take it apart but I could not budge it, I didn't try to hard cause I didn't want to destroy it, thanks again.

Mark
 
It was pumping but you ran out of fuel? Are you saying the LP stopped pumping so it died and the filter bowl was empty?

Yes, the nipple does separate, you can get a wrench on the bottom and socket on top of it perhaps. Do you need to take it off or doing the FTB mod?

You may need to pull the filter manager out of the truck and clean it out. Particularly that little last ditch screen. It is post filter so even changing filter wouldnt help. Yet the drain is prefilter so you would have fuel flow. So with a clog the LP may not pump because it can dead head at 10psi and stock running if its primed. Or in that central shaft where the fuel heater is it could be clogged up, seen some real nasty ones on here before and it may seal itself up after some flow or what not. You can try to look down on it from on top with the filter out.

If you PM your email address I can email you the OPS relay mod diagram
 
I"m pretty sure the truck ran out of fuel Buddy like I say it was leaking fuel, the valley underneath the intake was full of fuel. I have the intake and the fuel manager off,it was alittle dirty down inside the manager ,all this will be cleaned. What do you think of the LP not running after I put power to it Buddy?

Mark
 
Sounds like you have an intermittent failing LP and a fuel leak which turns into an air intrustion when the LP stops working. As soon as LP stops pumping that fuel leak would have turned into a sucking chest wound that would kill the IP, truck would die.
 
Wonder if pump not running with power directly to it might be caused by a poor ground , or did you run a separate ground when pwering directly ? Best to run dirctly from battery.
 
buddy said:
Sounds like you have an intermittent failing LP and a fuel leak which turns into an air intrustion when the LP stops working. As soon as LP stops pumping that fuel leak would have turned into a sucking chest wound that would kill the IP, truck would die.
Click to expand...



Well I better get a new LP lined up and might as well get a new OPS too. Hey Buddy where is a good place to pick up these relay's for the OSB mod does Autozone & Advance have these?

Mark
 
Diesel -T said:
Wonder if pump not running with power directly to it might be caused by a poor ground , or did you run a separate ground when pwering directly ? Best to run dirctly from battery.
Click to expand...


Diesel I ran power right to the pump and I tried grounding the pump through the pump case and tried through the harness too so I figured the pump was bad what's wierd is the pump was just working a few minute's before.

Mark
 
Waytek for the relays and sockets - if you want sockets. I got 50 amp relays at the dealer - overkill, but it's what they had & they were surprisingly cheap. About half the price of Napa's 15 amp.

Do they make a special crimper for the connectors that go in the relay sockets?

I have a heck of a time crimping those little dudes.
 
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