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Lift pump voltage before OPS

SnowDrift

SnowDrift

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I read through this thread, but didn't see anything here about it.

http://www.thetruckstop.us/forum/sh...t-Pumps-amp-OPS-operation-and-troubleshooting

In testing to see if I have power to the lift pump, I discovered I do not. I checked the out of the wire harness that plugs into the OPS. When the engine is running, I have +13v on one terminal, 9.65v on another and I assume the other is the ground. I didn't see anything that noted this to be sufficient voltage in the thread above. Can anyone clarify this for me? Should there be a voltage drop across the pigtail? Either way, I think it's safe to say my OPS is bad, since I have no power to it at all.

Second question - I can power the LP by removing the relay and crossing the necessary terminals like when I prime the fuel filter. Should I still be able to do this if the OPS is bad?
 
In the mean time, I've swapped out relays and found that I have power at the LP (and the LP is working). I have +13v at the LP with the engine running. Is there something on the system that might give power intermittently when it's beginning to fail? Original relay re-installed and still have power.
 
Replace the OPS. The relay is used for cranking only. You need to measure voltage at the lift pump with the lift pump plugged in by back probing the connector. You can get like 7v on a bad OPS and no lift pump. With the lift pump removed a bad OPS can have 12v looking 'good' due to no load.

Lift pump relay mode is a cake walk to do.
 
SnowDrift said:
Is the consensus still for the OEM OPS? Just wanted to be sure before I go see my local dealer Monday. Thanks, guys
Click to expand...

Yes, ACDelco ONLY.

The aftermarket is still erratic in quality.

Also, do the relay mod to help OPS pass on current and voltage to the LP.
 
I looked at that schematic, but I'm still unclear on what needs to be done with it for the circuit, if you're talking about operating the lift pump during the wait to start phase.
 
The OPS is not sufficient to carry the current (amp) to run the LP that is the reason why it fails all the time.

The relay will relieve the OPS ability to carry the current to the LP, hence, you have a better system without failing too often.
The secondary consequences is that the relay can also be used to prime the system during start up.

By the way, failing OPS caused the non-operational LP which cause the IP to suck fuel from the tank which cause the IP to fail also due to heat.
 
Thanks for the explanation. I still don't understand the schematic though. Am I supposed to just cut the gray wire and by-pass the relay by connecting a jumper between the cut gray wire at the OPS and the gray wire that goes directly to the lift pump?
 
Think of it this way and maybe it will help.

You have a big load to run called the lift pump.
You are going to use a relay to run that load.
You are no longer using the OPS to run the lift pump.
You are going to use the OPS to turn on the relay.
You will cut the grey wire so the OPS doesn't run the lift pump anymore - it only runs a relay.
So when the OPS switch closes the grey wire turns on the relay coil only. The other side of the grey wire that powers the lift pump gets it's current from the relay contacts and battery.

This said cut the grey wire.
The 12v from the OPS side of the grey wire goes to the relay coil.
The other side of the coil is grounded.
The lift pump side of the grey wire goes to the relay contact that is 'on' when the relay is 'on'.
You have a fused battery (+12v) going to the other relay contact that will power the lift pump when the relay is 'on'.

When oil pressure comes up the OPS switch closes. This turns on the relay. The relay contacts close and then the lift pump starts.
 
and to add to that if you want pre prime during WTS you connect a wire from the glow plug relay (the one from the PCM) and tie it into the gray wire that will be signaling the new relay. you will need a diode so it can't back feed the PCM.
 
Aside from the schematic Buddy posted, does anyone have some pictures of this? I'd like a better visual on what it looks like.
 
Use Google, you can find how relay works to understand schematics.

Also, check the Technical Library section, I am sure there is a photos on how this is installed.

Now, if you really don't have time, Kennedy Diesel have a ready made harness that you can use without cutting.
You have to install a tee on the OPS.
 
ak diesel driver said:
and to add to that if you want pre prime during WTS you connect a wire from the glow plug relay (the one from the PCM) and tie it into the gray wire that will be signaling the new relay. you will need a diode so it can't back feed the PCM.
Click to expand...

Schematic has a protection diode in it, possibly in the relay, as noted. Is that for protecting the PCM?

I was having a hard time following this because I thought I was supposed to use two existing relays in the fuse block. Now, that I understand I'm adding two new ones, I am getting my head around this.

My parts list will be the following, if I'm correct:

1) Relay w/socket harness - 2 pcs.
2) Fuseable Link - 1 pc.
3) Misc. wire

If so, then what amp rating on the relay and what size fuseable link am I looking for?

I had forgotten about Kennedy. I called about his harness and I think I'd rather do this one here with the relays if I can understand if better, instead of buying that. It looks like a quality piece, but the wiring idea, here, sounds simpler.
 
you can do it with one extra relay using 2 diff sources to signal the relay (OPS, glow controller). yes the diode is to protect the PCM. think of it as a one way check valve.
 
IIRC, it is about 20-40A. I think, it has the Amp rating stated on the relay package.

Which after efficiency factor is still a lot more than 2-3A needed by the LP.

As a reference, the properly made OPS should only have 1.6A.
At least that is according to the stuff read here and other forum.
 
I suppose it would make sense to have the same relays that are already in the fuse panel, since we already know they work.

AK, if I use the diagram with the two relays, will I still need the diode? If I'll need the diode, either way, then is that going to be a Radio Shack type item, or is that an automotive store item?
 
If you use the relays that have a built in diode then I don't believe you do. Most aftermarket relays don't have a built in diode. I bought myself a few online because they're pretty hard to find ,at least reasonably priced. the diode by itself is an electronics store item.
 
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