How is Lift Pump supposed to be grounded?

[sbachman]

Hi,
I'm new to this forum and I'm a new owner of a 1993 6.5L TD. I'm trying to determine whether the Lift Pump was installed correctly. It was replaced by the previous owner about 300 miles ago. It does not seem to be working and I question whether it ever worked.
I've ordered the Service Manuals, but haven't received them yet so I don't have the wiring diagram. Of course, I'm too impatient to wait, so I got online and found this site.
Here is my basic question: Where are the wires that attach directly to the Lift Pump supposed to go?
The reason I ask is that I've tried applying 12V directly to the Fuel Pump Fuse (as suggested in another forum) and I've tried jumpering the Fuel Pump Solenoid (as suggested in this forum.) Neither worked. But if I put 12V directly on the Lift Pump, it works. At the pump itself, there is a short ground wire attached to the frame, which has been cleanly cut off. The connector sticking out of the wiring harness has a tan wire going forward and a black wire going back toward the tank. I tried to follow the black wire and I think I found the end of it sticking out of the wiring harness in the air (!) back near the back of the tank, where two other wires appear to go to the fuel gauge sending unit. I'm tempted to just ground that black wire and see if the pump works, but I don't understand why the pump isn't just grounded right to the frame. Is the black wire supposed to go to a low-fuel sensor in the tank? If not, does anyone know why the black wire takes a long trip to the back of the truck to be grounded? Perhaps this is a strange artifact of the Ambulance-specific wiring? I'm inclined to just cut the black wire up near the Lift Pump and splice it to the nice heavy (previously cut off) ground wire that's already attached to the Lift Pump bracket. Any reason not to do this?
Any help is much appreciated. Thanks!
__________________
Steve Bachman
1993 6.5L TD Ambulance (Collins?); Silverado 3500 dually chassis; Former Fire Dept. vehicle, demoted to work truck.

[JohnC]

Do a search on the lift pump. It has been discussed here many times. In a nutshell:

When the starter is cranking the lift pump relay powers the lift pump. When the engine is running the oil pressure switch powers the lift pump. The other side of the circuit must be grounded. The oil pressure switch is a common failure point.

[sbachman]

Thanks JohnC. Yes, I've read quite a bit about the relay and the OPS! Ironically, while these may also cause me trouble, I clearly have a ground problem.
From your response and everything else I've read about the system, I'm inferring that there is nothing "downstream" from the Lift Pump in the circuit except the ground. Is this inference correct? I know it seems obvious, but the wiring in my truck is odd enough that it raises questions. Do standard 3500 pickups ground the Lift Pump immediately to the frame bracket, or do they also send a black wire through the plastic harness, way back behind the tank, to a splice (mine broke) with a white ground wire? I'm guessing I've found a weak point in the ambulance wiring scheme. I can't think of any reason not to ground right at the Lift Pump. Thanks again for responding.
Steve

[JohnC]

Yes, to ground.

In the diagram I have it joins up with a black/white wire and comes forward to ground G104, which is just aft of the center of the intake manifold where it mates the passenger's side head. I don't see anything farther back than the pump on the same circuit.

[sbachman]

Thanks. I appreciate your effort. This afternoon I went ahead and temporarily grounded that orphan black wire with an alligator clip. All systems go -- Lift Pump, Relay and OPS are all happily playing their designated roles.
Your check of the diagram gives me the confidence to go ahead and ground the Lift Pump right to the frame, and abandon the black wire that heads rearward. I'm still confused about why it was grounded that way, and why there is an unused ground wire right at the pump. It doesn't make sense, since neither matches what your diagram shows. But the world is full of mysteries and I'm ready to move on to the next one!
BTW, this forum has prepared me well for when the Relay and/or OPS eventually do give out.
Steve

[JohnC]

My diagram is for the '95. Your '93 is probably simpler due to lack of electronics. Grounding it to the frame should be fine.

[sbachman]

Thanks JohnC. After repairing the ground wire, my Lift Pump is working well...perhaps too well. Drip, drip, drip goes the diesel fuel onto the driveway. I guess my fuel lines preferred suction to pressure. First I thought I had fouled up the seal on the fuel filter, or hadn't tightened the ring nut enough. However, it turns out that the fuel was coming from the junction between the rubber and metal fuel lines right below the fire wall and behind the OPS. This is a nice spot to work (insert sarcastic emoticon here.)
It seemed like it would be simple -- just tighten that little hose clamp. Nope. That little prick was determined to piss out fuel. I cut off the end of the rubber line (thinking it was too rotten), jammed it back on, tightened the hose clamp, did the Filter Purge Shuffle. Drip, drip, drip. @#%@#$^!! (insert tool throwing emoticon here.) Off to the hardware store for more 3/8" rubber fuel line and non-stripped hose clamps. After a couple hours of cursing and dry humping the 6.5L, I was finally able to coax that short rubber fuel line onto both the metal line and the fuel filter housing. What fun!!
Anyway, the diesel drips are coming slower now. I've convinced myself that it's just residual fuel dripping off the engine from my previous adventures. However, if I find a puddle of diesel in the driveway tomorrow, I may just throw a @#$% match on it!
Good night and thanks for the help!

[jeffscarstrucks]

The positive side of your leak is that it wasn't an air leak to the pump yet. I was having an intermitant run quality issue that I could not find. I didn't even know that my lift pump wasn't working until I found it while trying to figure out the other problem. A new lift pump made the line leak REALLY obvious. Fixed it and all was good. I still did my share of cussing all the way! Good Luck, Jeff