Lift pump problems or OPS problems

[Kollin Syverson]

I am stumped. Truck would hiccup under hard acceleration. Thought maybe my OPS was bad. With engine running, I checked voltage at lift pump harness, had 13.7 volts. So I figured lift pump was bad. Spent $93 for a new lift pump and still no go. If I hot wire lift pump to a battery, lift pump works fine (so does the old one :mad: ). As I sit here asking for advice, I think I will go check ground wires. Is it possible for OPS to send 12 volts while not under load and then drop out once load is added? Any ideas?

[More Power]

The oil pressure switch contacts might not be making good enough contact under load. Try using a 12V test lamp at the lift pump connector, with the engine idling. The lamp will load the circuit. If the lamp doesn't light or is weak/flickering, the OPS needs to be replaced.

MP

[Kollin Syverson]

I tried the test light, which is about half brightness. This would explain lift pump not operational while engine is running, but shouldn't the lift pump opererate while cranking? Is there a fuse to the lift pump relay? If so where is this located. Can the lift pump relay be tested? I have been working on this all day and am beginning to pull my hair out. This also concerns me, as I don't have much hair left :D

[dieseldaver]

Hi Kollin,

I'm having the same problem. I came into the office just to send a note about it, and then I saw your post. I had an occasional stall on startup, and the hiccup under heavy load (like when you floor it going up a 12% grade at 70mph - heh, heh, heh). So I figured it was the lift pump too. While idling, I stuck my hand on the pump and found nothing, opened up the valve on the top of the fuel filter, and nothing came out (didn't stall either). I figured the lift pump wasn't working. I disconnected the pump, put a battery to it, and it pumped away. Hmmm, same as you, the pump worked, but wasn't running. I then checked the voltage at the lift pump connector. While idling, it was 12.4 volts, and went to zero when I shut the engine off. If I connected the pump to its connector, nothing would happen. If I connected the pump directly to the battery, it would run. I didn't think of it until I got to the computer and read the posts here, but is it possible that the oil pressure switch is working, sending 12 volts to the pump just as it's supposed to, but not enough amps. Could this just be the switch? I hope so. Oh yeah, by the way, both the switch and the pump were new last February. Can someone help us out.

[Kollin Syverson]

Hi Dave, glad I am not the only one with same problem. Maybe we can get some answers tonight.

I found the fuse and relay. Fuse is good, relay, I don't know. Question I have is, is the relay powered off the sending unit? My concensus is that if the sending unit is bad, it makes the whole lift pump system inoperative, which includes powering the relay. Am I correct in this statement?

[tom.mcinerney]

Kollin, DieselDave-
Read the 3rd post in 6.5L forum, posted by More Power, 10/14/'03[Blast/Past/OPS].
DieselDave-YES, the max power that can flow in a circuit is limited by series resistance. The 'bad connection' at OPS allows plenty voltage for meter, not enough for pump. {Another view is that at power level required by meter, 12.4 volts drop across meter while 1 volt drops across OPS; but when pump in circuit 12 volts drop across OPS, maybe 1 volt across pump}. That is , when meter in circuit, its high resistance(~10Mohm)sees the entire voltage drop. When pump and OPS divide the voltage, the comparatively high resitance of the OPS accounts for the whole voltage drop. In a series circuit the voltage and current divide between components proportional to the resistance presented by each component. This is worth having a feel for if you drive an electronically-controlled vehicle, because each ground&power connection becomes a voltage divider when the connections aren't pristine(or, good enough to carry enough power to satisfy all connected components).
On '96+ trucks the PCM switches power to lift pump, i think via a relay.
On '95{and I'm guessing '94} the lift pump relay only activates the lift pump when the engine cranks to start. Once the key switch returns to 'run' , the only circuit to energize the lift pump is thru the OPS. At idle on a hot/old engine the OPS contacts oft fry if an(aftermarket) relay hasn't been incorporated in the circuit to protect the OPS point contacts from arcing(a capacitor across the points of the OPS would probably help too, if no relay). So low oil pressure, hi miles , clogged filters, bad electrical connections, leaking fuel pipes/connections/filter cannisters ALL conspire to starve the FIP of fuel(and the PMD of cooling), leading to "maintenance" before and/or following component replacement!
Read threads this forum begun by 'jpchris', "Fuel Pressure & Engine Miss"....begun by Gary Page, "Fuel Pressure".

[ 10-18-2003, 09:54 PM: Message edited by: tom mac 95 ]

[Kollin Syverson]

Well, I spliced in a relay in the OPS wire and now lift pump works fine with engine running. Still don't have power to lift pump while cranking engine. I am going to try a new relay there also.

[dieseldaver]

Thanks Tom,

I made it to the local Chevy dealer 2 minutes before they closed on Saturday, and picked up a new Oil Pressure Switch. That was it. Works like a charm, and I even hear that little purr of the lift pump again. I kind of forgot what it was like. Now I've just got to put in that relay as suggested.

Thanks again for the help.

[ucdavis]

Just to confuse the issue w/some added theory-
When testing the OPS circuit w/an VOhm meter, if you have one w/duty cycle, use that scale & retest after reading volts. If the OPS is going bad it is reported to "chatter" open & closed. A voltmeter will read max potential difference, but not % of time the circuit is active. Duty cycle will give a reading <100% if the circuit is open/shut/open/shut/... indicating chatter & a failing switch.
I opened the OPS I took off my rig after the IP fried & I found the LP no working. Bitsy leetle burn spots on the contacts killed the LP activity & I suspect the IP from fuel starvation (inadequate lubrication).
BTW, for testing the fuel relay, the relay schematic is printed or branded right on the relay so you can see which contacts do what.