Calling all CTD experts,

I posted about this problem previously and many things have changed with my truck with the exception of the problem. Here it goes.

This problem ONLY happens when the engine is hot.

I start my truck up and it idles for a few seconds. Fuel pressure is at 13 psi, solid. As soon as I put it either into drive or reverse the pressure drops to zero and bounces back to 10, then to zero and then back to full 13 psi.
I put my digital meter on the harness and checked to see what was happening. As the truck is shifted into D or R the voltage fluctuates from 13v to 0, back up to 13v and then to 0 and then becomes steady. It can do this between one and four times.
I can hear the engine stumbling from the lack of fuel pressure and I am sure this problem will kill my injector pump.
A point of interest is that while it is still running I can shift between the gears and it doesn't do it again and I can shut the engine down and immediately restart it and it won't do it but if I let it sit for a few minutes it will do it again.
My fuel pressure is strong and consistent at all times while driving.
I know the core problem is that the lift pump is losing voltage for some reason when it is shifted into gear. I spoke to Bill at DTT and he thought it was a wiring harness rubbing on a shift linkage or something. I checked everything that moves and no harnesses are touching the linkages. I checked all the harnesses and could not find any areas of concern.

I would like to find the pins on the computer that control the lift pump and see what I am getting at the CPU. I think that will tell me what is happening. Can anybody advise what the wire colors and pins are that control the lift pump at the CPU? Any and all suggestions are appreciated.

Hottrodjim

 

There's a discussion involving symptoms exactly like you described.

I'll look for it whilst I browse and post a link to it.

Pretty sure your lift pump is bad, as that's the consensus in that thread


Found the thread.
https://www.dieseltruckresource.com/...threadid=52007


phox

 

Phox,

Thanks for the reply. My problem is a little different. I am losing voltage for a second or two when the truck is shifted into either D or R when the engine is hot. It is a definate voltage loss. It doesn't do it if I shut the engine down and immediately restart and put it into gear. It will do it if I let it sit for a few minutes. While driving the pressure is normal and on WOT drops to about 9 psi.
I would like to put my meter on the connector at the computer and see if the voltage drop is coming directly from the CPU. Do you know if the factory manual will show the pinout?

I also swapped known good pumps and the problem persisted.

Jim

 

Jim,
Have you checked to make sure that all your electrical 'grounds' are good!

Resistence in connections can change thru heat expansion/contraction cycles.

 

SBishop,

Thanks for your reply.
That was the first thing I checked. All I found was a loose manifold heater cable which was causing my lights to dim on cold mornings as it cycled. The entire harness looks good and has no cuts or wear marks.

Jim

 

There are only two choices here. The wiring harness or the PCM. The lift pump feed comes directly from the PCM. Check the plug at the pump for cleanliness, check the plugs at the back top of the engine (2) as they can do this if loose or bad contact. Check the three plugs at the PCM.

You might try and find out if it is losing power or ground. The ground wire from the PCM controls the voltage level while the other wire carries the switched 12 volts.

 

Haulin,

I am thinking the same thing you are about the CPU. The harness looks real good and all the grounds are perfect. Would you by chance happen to know what pins and what connector control the lift pump? I would greatly appreciate it.

 

I loaned my service manual to a friend. I can get it back and look, but I would pull all three of them, passenger side of cowel, make sure they are clean and use some dielectric greese on them. Also the two large plugs at the back of the valve cover which are the main harness plugs. I had a charging problem where it would stop charging on pull, was the main harness plug not seated good. That sparked my interest in yours because the engine would buck going into gear.

 

Haulin,

When you get your manual I would really appreciate the info. I pulled off all the connectors on the PCM and checked them, they are all perfect. I didn't disconnect the two connectors on the back cowl but I will do that just for grins and giggles.
I put my digital meter on the lift pump connector and wiggled all the harnesses looking for a bad connection. No fluctuations at all. The only thing I can think to do is check right at the PCM and see what I am getting coming out of it.
I don't know a whole bunch of diesel guys here in San Diego so I can't swap PCM's and see if the problem persists. I do have a friend with a 2000, do you think the PCM's are the same?
I think this weekend I am going to disconnect all the electrical connections on the transmission, shift it into gear and see if the problem persists. If it goes away I will reconnect the sensors one at a time to identify the connection causing the problem. Bill Kondalay says there is no way the tranny can cause the problem but the problem is triggered by shifting it into gear so there is some correlation. Thank you for all your assistance.

 

It still sounds to me that a plug or wire is getting moved when you put it in gear. On the PCM, I really don't know, I doubt it will be the same. Give me a day or two to get the pump pin outs. I also have some overnight work to do.