I just recently bought a 2000 excursion 7.3 diesel. I'm new to diesel but learning. So far I've installed a new drop in filter in my airbox. Cut out the plastic behind the grill for cold ram air into the intake. I've done the FPR mod and cleaned out the exhaust back pressure tube. And last but not least I installed an Edge Evolution 2 programmer running stage 3 with timing advanced 2 degrees. Nothing else to the custom settings other than what stage 3 provided. Here is my problem. On the freeway at 65-75mph cruising with the cruise control on, the truck starts to shudder. I suspect my TC lockup is going out but not sure. So I wanted to test it buy installing a manual lockup switch. So I installed the switch running a wire from the plug going into the transmission on the purple with yellow wire using a saddle tap to a switch to part of the dash frame under the dash. Went to test out the switch and nothing happened. As y'all and other forum's have described I should feel something and the od light should start flashing. I get nothing. Tried testing the switch and my ground source and everything tests out fine. Tested the saddle tap. Its fine. Tested with the switch on and using a meter to check ohms at the tap to the frame of the truck I'm getting 8ohms. I would think there shouldn't be any resistance because its a dead short but that's what I'm getting. Am I connected to the right spot on the transmission or should I move the tap to the plug under the brake master cylinder. Could my TC lockup solenoid be going out and that's what's causing the shudder and why the manual switch doesn't work. Any info will help. Thank you.

 

So after not getting any response on this issue I decided to try moving my saddle tap to the plug under the brake master cylinder. I also hooked my ground straight to the battery. Wanted to make sure I had a good ground. Now my manual lockup switch works. :-) drove it around and its fun. Took it on the freeway and set cruise control and I think the shudder is gone. There was one spot that it may have shuddered. As I was creating a hill as the load was dropping I felt the possible shudder. It may have been the road but not sure. Does anyone know what this shudder could be? Please someone post something. Even if you don't know. At least I know somebody at least looked at my post. Thanks.

 

Are you sure its in the transmission? Is the shutter constant at 65-75mph?

 

No I'm not sure its in the trans. Just thought it could be. Because it doesn't feel like an engine mis. The shudder only happened with cruise control on and it's random when it occurs. Could be completely flat or as I'm increasing/decreasing load. And it only shudders for a few seconds or if I shut the CC off or if I press the peddle down a little.

 

When you are driving, like on the highway, let off the gas and get back on it. you should see the RPMS drop maybe 200 rpms. If it does your converter is locking. do the rpms kick back up with out your foot coming off the pedal? the converter should stay locked until you left off completely and get back on it, then it will relock. There are some good tranny guys on here, Stewart H has a lot of experience and can usually point you in the right direction.

If it was a constant shutter, id say balance all four wheels first.

if your converter is locking and not unlocking under load, it may be internal.

 

Umm I didn't noticed the rpm dropping or raising. Just stayed around 2k at 70mph. What the shudder feels like (best I can describe) the torque converter is engaging and disengaging several times in a matter of seconds. I dont think its tires as I just replaced them and it did it with both sets.

 

It should. Take it out in the highway. Get her up to 65. Let off the gAs for a second then get back on it normally, like feather the pedal. When u let off the converter will unlock. When u get back on it keep your eye on the TAC. It should drop about 200rpms after your foot is back on the throttle for a second or 2

 

Honestly it sounds like the engine may have a miss.

A dead cylinder cruising with the converter locked up will shutter like that.

 

That was my first thought too until I saw it is a diesel and not a gasser. Wouldn't you notice a dead cylinder in a diesel more than every once and a while,,,

 

Surprisingly many times especially while cruising on the highway a 7.3 with a dead hole feels smooth. That is until the converter locks up. lol

When I pulled my 6.0 into the shop with the number 3 rocker and push rod in my hand you could barely tell anything was wrong.

 

I thought possible the same of a dead cylinder but I don't know. I haven't had a code come up for a misfire or anything like that. And it also only happens with cruise control on. So I am leaning towards it not being a dead cylinder. But could it be that my throttle position and/or cam position sensors are going out causing the shudder?

 

Don't know if this helps too but I know when it first started shuddering. I was driving home on the freeway and pulled up next to my father in-law which also has a diesel excursion 7.3. We were going about 55 at the time and he wanted to race. So we both floored it. (I Won :-) lol) and then continued to drive home. Set CC at 70 and about 10mins later is when the first shudder started. I didn't know what it was and it stopped a few seconds later. Did the same shudder about 7 more times throughout my 200 mile trip home. And has been doing it ever sense ONLY with CC on. No other time. I've been avoiding using CC but then I speed too much and get bad MPG after 70. So if I can fix the problem. That would be great.

 

Huh , Silly diesels

 

yeah they are! lol

 

So any thoughts on my throttle or cam position sensor being bad or going out?