OPS Lift Pump Relay Mod - Question

[JTodd]

I am installing a relay to power the lift pump, and have a question regarding the wiring from the OPS. First, I found what looks like the OPS under and to the driver's side of the fuel filter bowl. I found a diagram that shows the wire colors to be Tan (or purple), Grey and Orange. I have a tan and orange, but cannot confirm the other, in fact it looks like I have two tans and an orange. I disconnected this and turned the key to run, I can still hear the lift pump. I started the truck, but don't appear to have voltage to the lift pump while running. I assume I am at the right place. I tested the wires, and engine off, key in run, no voltage to any of the three wires. Engine running I have voltage to two of the three wires, and a small amount (9v) on the third.

I would like to get this relay installed, but am not sure what wires to plug into. Help would be appreciated.

[jamesm]

When I researched this before, I found that our later trucks already HAVE a factory "LPM." Check for the relay in the hood electrical panel.

James Marriott
1996 K2500 Suburban 6.5 "F" VIN; license PMD, 3" down and straight pipes, super fuel filter, 190 AC thermostats, duramax fan blade and adjusted fan clutch, Heath TM and "econo"/tow-safe PMD burn, 212 Hp/385 ft-lb at rear tires, 4.11s, 192k
2003 Buick Regal 108k
1983 MB 300D (228k, frau Auto)
www.engineeringworks.biz (http://www.engineeringworks.biz)
1982 MB 300SD junker, 265k

[JTodd]

Yes, a fuel pump relay is in the panel. Is that the lift pump or IP? I am having severe fuel delivery problems as things heat up. LP makes normal noises and does its job while cold. After a while on the road, it slows and fails to deliver adequate fuel. I have a new pump, but want to rule out electrical issues as well

[DmaxMaverick]

OBD II trucks have a relayed lift pump, OEM.

Strange, that you only now have issues with "heating up". Wide temperature swings during this time of year can cause a number of issues. One is overcoming the ability of a worn-out fuel filler cap vent. Run with it loose, or just replace it (they're cheap, but make sure you get a "Diesel Only" cap). Another is electrical connections, especially grounds. Start with the cheap/easy stuff, then on to the less common, more expensive items.

[benaustic]

I have a 1996 K2500. While driving the other day, the truck quit like you'd turned the key off. It restarted after a few seconds, then quit again. After that, all electrical power kicked out almost immediately after turning the key. After hunting for a while, I found that the fuel pump relay in the fuse panel tripped every time. So, I unplugged the lift pump and hit the key (didn't start the engine) and the power stayed on, no problems. After unplugging and cooling a bit, I plugged the pump back in and drove myself home.

My truck is not original... it's on its third engine (I had nothing to do with the first two), so I'm not sure what is factory and what is not. So I have a few questions:

Are these the symptoms of a failing fuel pump? Or simply a weak relay? Or, I have read that the oil pressure sensor at the back of the block can cause LP failure... thoughts?

[Kennedy]

A 6.5 will typically function fine under light load with a dead lift pump. When I hear of stalling, the first thing I think of is the PMD failing.

That said, there could be something to your relay/shorting theory too.

The lift pump should make a pulsating/rattling sound at key on. If not I'd start by replacing it with a genuine AC Delco pump like this:

http://www.kennedydiesel.com/detail.cfm?ID=29

[benaustic]

Whatever it is, it is definitely electrical. Pump clicks and works fine when it works, lots of pressure at the bleeder. Just was kicking out ALL electrical power when the relay clicks out... no dash, no nothing. That's why I'm thinking either a short (although it's been working fine for 1000 mi- since engine install) or a pump problem. Shouldn't be PMD... it was new with the engine, mounted on cooler all with less than 1000 mi.

I was just wanting to make sure I shouldn't be chasing OPS issues or something else. May try relay first.

One thing that all this brings to my attention, however, is that my pump does not look like yours. It has hose barbs and clamps. Yours has threaded fittings. I yanked a 6.2 out from under the hood of this thing... you reckon the last guy put the 6.2 lift pump in also?

[DmaxMaverick]

The 6.2L lift pumps are mechanical. It's possible what you have is a "universal" pump, and perhaps one for a gasser. I suggest replacing it with the correct pump at your soonest convenience, if that's the case.

Regarding your power outage, even if you have a dead short to ground, it shouldn't cause a system-wide outage (the relay should prevent that), and the relay isn't a circuit breaker. Once a relay quits due to overload, they usually stay quit (burn up). More likely suspects are grounds (which cause circuits to complete through other system components), and/or the ignition switch. Neither are unusual for your year model.

[benaustic]

That's awesome (not). As to the complete power outage... my first thought was a fuse link or something! I didn't think that one relay would do that, but I guess I was hopeful it would be that simple... especially since it all seemed fine when the pump was unplugged. I'll replace the pump and start looking for other electrical issues.

It could be so many things on this truck...

Thanks for the help!

[Kennedy]

We also offer a lift pump control harness that will make the lift pump operation completely stand alone. This plus a new proper pump should eliminate any question as to the LP circuit.

[benaustic]

I checked out your website... your pump is a little high... about the same price as local O'Reilly but much higher than SSDiesel... can you sell me on why? Also, If I replace the lift pump, I will need to get from pipe thread to hose barb since that's what I have. While that's not a big deal, it's an extra trip to Farm and Home. If you can hook me up it would definitely entice...

Another question... If I spend $240 to update the stock pump and wiring, wouldn't I be better off spending $220 to buy your Duramax-style pump?

[Kennedy]

I do not know what SS Diesel is currently selling. Mine is AC Delco. I've seen too many aftermarket units fail prematurely. Also it is I who is involved with helping diagnose the issue not SS.


As for the lines, these pumps use an oring boss type fitting just like the throttle body gas trucks use for their fuel filters. If this is gone, you could source a repair kit from your local parts house. Likely under the HELP! line of parts.


The Kennedy Lift pump is a VERY good pump and very reliable. Comes with the same ORB fittings as the OE pump. The concern that I have with using this pump is that it requires a very good leak free fuel system. Being a centrifugal pump it will not self prime so if there is a major leak that can drain back you may have issues. In the past I have has so-so experience with people not wanting to fix their fuel systems so they just went back to the old rattler. I guess I shouldn't downplay the benefits of my pump as the complaints are few and far between, but I guess I just try to steer clear of potential conflict.

[benaustic]

I ordered yours. Your prices are competitive if compared to AC Delco. Thanks... and thanks for helping diagnose.

Since you're here and we're talking, I have another question. My temp gauge is not reading correctly but it does move. It sits at about 8:00 (on the peg) when cold and warms up to just above 9:00. It does move up some when pulled hard.

This is a '95 block with a DS4 pump off of a 2000 run by my factory 96 ECM. Is there a compatibility issue with the '95 temp sensor and the '96 everything else? Or is this a symptom of a faulty temp sensor? Also, does the gauge read from the sensor on the water pump? I read somewhere in a forum that the water pump sensor feeds the computer but the gauge gets its info from a different place... (that doesn't seem to make much sense, but I read it nonetheless).

[Kennedy]

There is a temp sensor in the thermostat housing. This unit drives the ECM. If memory serves, in the drivers side head there should be a temp sending unit that drives the dash gauge.

While the various models are somewhat different the sensors and senders should be the same.