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Lift Pumps & OPS operation and troubleshooting

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This needs to be expanded for the OBD I trucks. This specific info appears to be overlooked. Specifically I have no power to the relay coil wire during cranking. I am supposed to. I suspect wire damage, but, I need to know where this wire from the under hood relay runs.

The specific info would be how the relay is used by year: Prime function yes/no. 1993 is no - crank only.

Also, the ALDL diagnostic connector fuel pump pin controls/kicks in the fuel pump relay on 1993 models - not direct to the fuel pump.

Further under lift pump tests a test for OPS would be pulling the relay plug and checking the voltage on the gray wire that is connected to the lift pump and OPS. Lower than battery voltage while engine is running is a dead OPS. You are testing under the lift pump load where pulling the lift pump connector gives you full voltage as you removed the load - voltage ok just no current on a burned OPS.
 
OBD I LP Relay - The old stuff..

As asked above here are some pics of the relay. Used during cranking only on 1993.

To test the OPS start the engine with this relay removed. Probe the gray wire in the relay plug and measure voltage to ground. If the OPS is good you will have full system voltage 13.5+. (This gray wire goes to the OPS and lift pump.)

If the OPS is burned you will have like 7v. In other words there is a connection, but, not amps going through the OPS. So lift pump can't work.

To test the relay jump the fuel pump wire on the ALDL to +12V. This turns on the relay and then the lift pump.

Note the fuse for the fuel pump under this cover.

Note the fusible links here as well. This includes a fusible link to the computer. Keep this in mind as age will start to break these links without burning them out.
 

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--Thanks for the pix of circuitboard in LP, TurbineDoc. I've been curious for years...

--Awfully Clean Machine -your desert ride there W-W. Looks AbiNormal contrasted with a roadsalted truck ... dare say i'm a tad jealous !
 
Good tread.
I may add IMO,Best mod one can make for on board monitoring of fuel supply or lack thereoff is to install a permanent in cab fuel pressure gauge.with an add on OPS to LP bypass switch fuel supply probs are easy diagnosed at a glance,and may well save one from being stranded.

my 2 c
 
on the subject of gauges i tried searching and cant find it. some one found a fuel pressure gauge that had an electronic sending unit. not to keen on pumpin fuel into the cab
 
WarWagon said:
As asked above here are some pics of the relay. Used during cranking only on 1993.

To test the OPS start the engine with this relay removed. Probe the gray wire in the relay plug and measure voltage to ground. If the OPS is good you will have full system voltage 13.5+. (This gray wire goes to the OPS and lift pump.)

If the OPS is burned you will have like 7v. In other words there is a connection, but, not amps going through the OPS. So lift pump can't work.

To test the relay jump the fuel pump wire on the ALDL to +12V. This turns on the relay and then the lift pump.

Note the fuse for the fuel pump under this cover.

Note the fusible links here as well. This includes a fusible link to the computer. Keep this in mind as age will start to break these links without burning them out.
Click to expand...

Good stuff, although I would like to expand and try to make this more accurate for OBDIs. This is a picture of a 94, 95 OBDI is different with the fuse center. 94 is like the older 93.

Also, the OPS test, you dont have to unplug the relay, that wire comes out to the OBDI ALDL/DLC connector pin G (bottom left). You can check the voltage during operation at any time by probing that pin on the DLC. That works on 93-95. If 92 is also OBDI and this way it will all apply.

For the power during cranking, in 1994 and 93 the fuse center under the steering column has a CRK fuse, for crank, and it provides power to the LP relay while the ignition is in the START position. The fuse under the hood is additional.

So 93 is also unique, in that the LP ran during WTS, during crank, and operation. During WTS was removed in 94-95, and brought back in OBDII for 96+. So in 93 the LP runs during WTS off of the Glo-plug controller, purple and white wire splits and goes to the controller to power the LP relay.

Using these pictures, the easiest way to power the LP for diagnostics is to unclip the underhood fuse out of its cover and press it against the battery junction posts next to it. There is a slit in the top of the fuse that you can press up against the edge of the posts. This doesnt apply for 95 since it is different, but you can still power the LP through the DLC connector pin.
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Turbine Doc said:
Okay power portion, Lift pump gets power differently depending on vintage, OBD-I's get constant power during crank as mentioned above, but if PCM does not see oil pressure at OPS power, it shuts off, this is why OBD-I trucks hear lift run until oil pressure bleeds off OPS contacts.

This is not really accurate, because PCM does not monitor LP power, the OPS just powers the LP with no PCM feedback, so if there is no oil pressure it doesnt pass power to LP, PCM has no say in OBDI for any LP power. For power during crank it comes from Ignition switch in START position through a Crank fuse under the steering column to the LP relay trigger contact.

OBD-IIs get power from 2 sources as commanded by PCM at key on (pump runs briefly with key on, PCM is looking for rotation signal from CPS and will shut down without it), and backup from OPS like OBD-I, (I have not seen confirmation but this feature came in 96 I think to help combat failed PMDs which are passively cooled by fuel flow in IP, dead OPD & delivery flow to IP suffers and IP temps go up which would mean hot driver & early PMD death); back to the lift pump power for OBD-II truck you have to lose both the PCM & OPS to lose power to lift.

This is the PCM signal to the LP RELAY in the fuse center, or a failed RELAY itself.

Now also common to both OBD-I & II is the gnd and the pump relay located in the underhood fuse center, issues to these cause power problems

As already pointed out, this is only true of the 95 OBDI, because in 92-94 OBDI the relay and fuse are located on passenger side firewall, no fuse center

To test pump for running with engine shut down/also expedient way to prime a fuel filter manager after filter change turn ignition to run, then on OBD-I pin G to +12v will make pump run, on OBD-II +12v to underhood test jumper will make lift run indefinately until power is removed, also a good way to empty last of the fuel from fuel tank, (not recommended for fully emptying tank, remember only 15 gph)

There is no need to turn ignition to On/Run (at least not in OBDI), and in 92-93 OBDI LP is powered during WTS with Ignition to On/Run. Also, another method for powering LP in 95+ with truck off put it in gear and turn Ignition to Start (it wont crank but LP will run). Another easy method for 92-94 is just putting underhood fuse up to battery junction post next to it, as described in previous post.
Click to expand...

Update for Technical clarity
 
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A 95 manual and some members with 95s work the LP off of pin F of the OBD DLC port. can other with 95s also confirm?

This is that the normally closed contact of the LP relay goes to the DLC connector, pin G on 92-93 and pin F on 95s.
 
I can confirm that on my '95 K2500 Suburban, the fuel lift pump works on "F" of the OBD DLC port, not "G".
 
buddy said:
A 95 manual and some members with 95s work the LP off of pin F of the OBD DLC port. can other with 95s also confirm?
Click to expand...
Confirmed. 1995 L65 Suburban, DLC pin F jumps LP.

ALSO: Easy nearby source of +12VDC is in the elec. panel on footwell behind emergency brake pedal. Lower LH corner, there are 3 small jacks immediately above the marking "DRK GRN." The middle of the 3 supplies +12VDC.
 
OBD1 OPS relay mod plus WTS priming

I figured this was the best place for this drawing I made. This is for OBD1 trucks, because OBD2 trucks do not depend on the OPS alone for power to the LP, and OBD2 trucks already prime during WTS.

The coil supression circuist I list as optional, because it would only be protecting the relay itself and the LP, and no overly sensitive PCM electronics.

Enjoy! The site cut it down in resolution, so if you want the full file just PM me. If you save it from its zoomed picture you can read it on your computer better.
 

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