Ground terminal at I/P

[signguy1010]

The other day I posted How my PMD had been moved to the Air Intake Plenum and was advised to move it to a cool place and take the small ground wire mounted to the heat sink and put it back on the I/P. The first was easy to understand but the second I had to give some thought for it to sink in because I thought of it as a conventional ground. His words of "not having that ground on the pump opens up a whole lot of other issues". I kept that in mind as I was paying close attention to my truck today. Here is what i have come up with.
That ground is a signal wire to the ECM. At start up resistance is 0 and ECM sends out instructions for a rich fuel mixture. As the engine warms up one of the resistors in the FSD (I guess it is now)increases resistance in the signal wire and fuel is reduced accordingly. Should that resistor fail the ECM receives no signal and fuel is cut off (when I removed the ground while running the engine quit). Right now my ground signal is permanently 0 resistance and does not change as it is attached to the engine like a conventional ground.
Thus the condition of the truck running great at start up, until it gets warm and then starting to blow soot from rich fuel. Runs pretty good at high speeds but after sitting at a light I take off and get the black soot smoke and under load. Everything changes except for the fuel being supplied.

I am thinking that the newer limp home modules are like a heater fan resistor that leaves you with high speed ( 0 resistance) should the resistor fail.

I believe that instead of burying the connector to the extension harness(I have ordered) under the intake when hooking up that ground wire. I should extend the ground wire so that should that resistor in the FSD fail I can easily make a ground and limp on home with a rich fuel mixture.

For all I know I could hook up that ground to the I/P and find out that the resistor in the FSD has been bad all along. I sure hope not.

This is all new to me so if anyone can confirm that this is how that ground wire works I would appreciate if you let me know or please make some additions or corrections.

It will be a few days before I get the harness and while I have it apart doing the ground I want to set up a limp home mode if possible should the other resistor fail.

If anyone knows of a manual that has the 2000 engine wiring diagram? That is usually the first thing I buy with the money saved from the first repair but c/K 3500 manuals are all gasoline engines that I have seen. Mine is 3500HD but from what I see is just has bigger springs and such. My owner's manual doesn't even refer to Diesel engines at all.

I do appreciate any help given and am thankful for the text book of information on this website. I have found myself clicking posts and reading for the shear knowledge. When I am on the other side of the state it adds comfort knowing that I am prepared for as much as I can be.Plus, knowledge is so much easier to load on the truck than a generator and a battery charger.
Thanks again, Dwayne

[racer55]

I would say that you are reading way too much into the function of that ground wire.
It is the ground for the fuel solenoid and is used to complete it's circuit.
The reason I said to move it back to the IP is because the fuel solenoid is located there and it is the best place to get a quality 0 resistance ground.

When mounted to the heatsink there are just too many places to get high resistance,dissimilar metals corrosion,the fact that most heatsinks when mounted to the intake are supported by nylon insulators and the ground path must be completed by light friction between the heatsink and the fasteners then through the intake manifold and it's fasteners(dissimilar metals corrosion again).

The resistor is for calibration of the IP only for emission purposes and they very seldom fail,only some trucks have a no start condition when no resistor is present,others default to a #1 resistor value.
With OBDII,the resistor value(and timeset command) is only sampled every 50 key starts so it is unlikely you would notice any grief.

Diagram for 2000 3500HD:

[racer55]

Originally posted by GMCTD:
PMD wiring connections:

A - Fuel Inject control - grn - to PCM

B - FSOL+ - lg red - to Fuel Solenoid

C - Closure ground - sm black - to PCM

D - +12v power - pink\blk - to ESO solenoid and IGN

E - Fuel inject signal - small red - to PCM

F - FSOL ground - lg black - to Fuel Solenoid and IP ground

FSD connections to 4-wire IP harness connector from PCM\IGN -

FSD A green to...........A IP connector

FSD C small black to....B IP conn

FSD D pink\black to.....C IP conn

FSD E small red to.......D IP conn

FSD B and F go to Fuel Solenoid on IP

Black ground wire attached to top of IP must be attached to top of IP


It looks to me that the resistor occupies pins D and E in the PMD,but I dont have a harness or PMD to verify at the moment.

[Robyn]

Yesssssss
you are reading way too much into this equation.

One thing to remember about a diesel is, there is no "RICH" or "Lean" fuel mixture

A diesel has air at a constant, or nearly so and it is "white" hot as the lingo goes and the fuel is injected into the cylinder.
This fuel is burned as it is injected, a large amount makes a large pop and a small amount makes a little pop.

The amount of fuel relates directly to the speed at which the engine runs.
RICH/LEAN does not apply at all as it does in a gasoline engine where the mixture has to be somewhere in the 13.5 to 14:1 ratio for it to ignite with the spark plug.

In the diesel engine the fuel is simply varied in amount to control speed and power.
You can reach a point under heavy load where you get heavy black smoke that one could say the thing was too rich, but this is really not quite the right terminology.
Only so much fuel can burn with the amount of air that can be packed into the cylinder at any one time.

The Limp home mode will set up a range of operations such as max fuel delivery (usually fairly low) timing and other parameters that will allow the engine to operate safely, but definately not at peak levels.

The limp mode basically uses a very low fuel delivery range and fixed timing parameters to protect the engine if there is a problem with one of several systems. (turbo boost, timing etc)

The ground wire is simply part of the circuitry for the PMD and it needs, as mentioned, a good ground.

When you fish the connector cable out from under the manifold to add the extender you can simply add a length of wire to the ground if need be so it will reach the ground screw on the IP.

This is the best way to deal with this issue.

Locations for the PMD have run the gamut, from inside the bumper, inside the splash pan under the radiator, behind the LH battery and a host of other places.

I have even seen the PMD located in the passenger compartment below the dash with a computer fan blowing air on the finned heat sink.

I recommend that you keep the location so you can get to it in the event of a failure out on the road.

I carry a spare PMD mounted to a manifold top hat type heat sink along with a wrench to allow a quick bolt up, then swap the plug to the spare and off and running again.

Wherever you mount your PMD, make sure its out of harms way and where it does not get hit with road spray, especially if you live in an area that uses SALT

Any introduction of salt spray onto the connector can result in issues at some time.


Good luck

Missy

[signguy1010]

Thanks again for the responses. The truck was donated to our branch of the company during the winter and came with a warning to be prepared to put a lot of starters in it. The driver was relocated in 2010 as the date was marked on the new one. During the winter it ran fine as engine temp never got past 200.
As long as the engine temp is 200 or below it starts right up and runs relatively quiet and restarts with no problem. As soon as it gets around 205 it seems like the computer goes into another mode but the fuel settings don't change with the program and it blows sooty exhaust and diesels loud like when first started in the morning. If I shut it off and try to restart it while in that mode it just does not want to start and requires a lot of cranking.
Because this problems started happening after the PMD was relocated along with that ground it led me to believe that the resistance in that ground varied based on engine temperature. Reducing fuel delivery similar to how a light bulb will burn dim with a weaker ground.
So as that ground is Ok where it is at and not on the I/P then I suppose the next step would be to replace the temp sender and sensor just to make sure that they are functioning properly. Thanks again.

[DmaxMaverick]

If that ground isn't on the pump, you'll likely have issues. If you insist, I'll go into the "why" (it isn't all about the resistance). If you don't move it, you aren't helping your cause any. The temp sender, good or bad, won't cause the problems you are seeing. If bad, it can cause long starts, but not the way you describe it.

[signguy1010]

Thanks DMax. If it were solely up to me it would have been done long ago. As the service department will be doing the operation it was like pulling teeth to get the go ahead. They were saying it was not necessary and who am I to challenge their intelligence.
Belt tensioner pulley broke the other day on the road and "The Boss" stopped on his way home to check up. We were discussing the wire and Tech said it was fine and I told him that it goes on pump for a reason. Even the mounting bolt for the driver would be sufficient if it were just a ground. GM would not have put it on the pump if the block was sufficient and would have had it going thru the harness to a ground. Boss agreed and said to move it back to where it should be.
I appreciate the offer to go into detail DMax and my curious nature wants me to say sure go ahead but I will leave that up to you.
I also just saw the diagram that someone has posted and wanted to say Thank you for that. The labels for each wires purpose explains much more than this wire goes here and that goes there. Once again thanks for taking the time to help out. Dwayne