A few weeks ago I noticed what seemed like a "slip" in my automatic transmission. Only happened a couple of times and I thought maybe it was just me imagining it or I didn't have the shift selector lever firmly in "D"rive. Then I went on a 4,000 mile trip pulling about 3,500 lbs. on mostly flat roads but it did include some 7% grades in the hilly parts... the truck performed flawlessly.

This is probably not related but ... after I got home, the truck sat for a few days and then I noticed a coolant puddle under the truck. Traced it down to what appeared to be a slow drip from a bad squeeze clamp on the large, lower rad hose connection. Replaced the squeeze clamp with 2 worm drive clamps and no leaking issue since.

Then last night, the tranny seemed to "slip". My short run across town started out OK. However, when I arrived at my destination, the diesel motor would rev to around 2500 rpm but the truck barely moved forward. It eventually did shift into second but also with high revs. It was the same feeling as if it were a manual tranny and you were giving it lots of throttle but only letting out the clutch a little bit (i.e. riding the clutch). This happened a couple of times last night. Also at a stoplight with the brakes on firmly, the truck was gently and rhythmically "rocking forward" as if the tranny was not fully disengaged.

When I worked through the gears shifting manually, the problem seemed to disappear and the truck responded going through the gears normally. Upon restart about an hour later, tranny shifted fine both in the city and at highways speeds on the way home.

B-I-L commented "sounds like your clutch is slipping". Not exactly sure what he meant as I had to leave. I'm hoping this will be a simple fix/adjustment and not the dreaded mechanical diode failure leading to a replacement tranny. Truck only has about 75,000 miles on it.

I'm suppose to head out on my annual vacation next week carrying a 3,500 lbs. truck camper and pulling a 3,000 lbs. cargo trailer with our gear. Obviously if the truck is sick, this trip can't happen.

I know some of you would dive right in and start taking the tranny apart to repair it ... I'M NOT THAT GUY!

Your thoughts on what is wrong with my transmission and what I should be doing next? Thanks in advance.

 

Step 1 is check the transmission fluid - make sure it's full and clean.

 

You've described the classic symptoms of a failing mechanical diode.

If you want to do some checking first, you could drop the pan. Are there small metal bits of debris in the pan? That's conclusive proof that the mechanical diode is coming apart. Are there no bits of debris in the pan? It's probably still the mechanical diode, but it hasn't dropped enough parts to make it to the pan.

The only adjustable part on the transmission is the TRS (Transmission Range Sensor.) It's located on the driver's side of the transmission. I don't see how it could cause this problem, but it can be adjusted.

 

Great points and ... Yes and yes on both counts. Bright red, no burnt smell and shows where it should on the tranny dipstick.

Thanks for your prompt response Mark. I was afraid you or some other knowledge person would say it was the mechanical diode. What to do now ... drive it to total failure or can it be opened up by the shop and properly diagnosed as the fault being the mechanical diode?

Also, assuming the worse and I need a new tranny, are the HD4R100 transmissions a good choice? I do not do any power pulling or hauling other than what I described above in my initial post.

Also, I'm in Canada so having one of the top recommended tranny houses rebuild mine is not easily accomplished either financially or logistically.

 

Total failure is anywhere from one to 100,000 miles. Probably a lot closer to 1 than to 100,000. I wouldn't drive it, but someone that is less risk averse than I am might. Once it's opened up it is easy to diagnose. The one way clutch will be in pieces.

I've heard the HD4R100 is a good trans. I've never seen nor driven one.

 

Thanks again Mark ... reps sent.

 

I read in one of Mark K.'s other posts that the mechanical diode should not appear after a 10/00 build date. Does anyone know if that just for the gassers equipped 4r100 transmissions? In other words, did they continue putting the mechanical diode in the diesels (due to the 4,000 rpm fail point thinking at that time)?

Anyway, I am going through my options here.

1. Having a John Woods or BTS rebuild tranny is just not logistically or financially possible;
2. I am checking out local tranny shops in my community;
3. Buying and installing a HD4R100 tranny form the dealer is an option. I have read about the much harder shifts and TC lockup harshness if the truck is tuned ... these issues concern me.
4. I've thought about perhaps a used unit from the wreckers but am not sure whether all of the diesel 4r100 trannies had the mechanical diode.

Your ideas are welcome.

 

The gassers were not supposed to ever have the mechanical diode. A few thousand were built in July 2000, with the mechanical diode. Almost every one of them had the transmission changed before leaving the factory. I've heard of a handful in recent years that did have a mechanical diode.

 

So my 2001 7.3 diesel with a build date of 12/00 would still have the mechanical diode right?

 

Almost certainly, unless it's been replaced along the way.

 

I thought I'd close the loop for others reading this thread in the future.

So I went to three reputable local transmission shops as I was heading out on vacation in a few days and I wanted their opinions on the shifting symptoms. One shop (regional multi-location operation) didn't test drive/diagnose anything and the second shop (local owner/operator) only test drove the truck. They both said since the truck is driving as it should right now, I should drive it as is until the symptoms become more regular or something catastrophic happens.

Third shop (60 shop national transmission chain) performed a test drive, pulled a couple of tranny codes (72x IIRC) and said of course you can drive it ... but do you really want a failure when you are 700 miles away from home on vacation? This third shop was also quite familiar with the 4r100 trannies and at one time (back in their day), the owner said he had 2 or 3 rebuilt units sitting on their shelf ready to go because they serviced so many of them.

I approved them opening up my tranny to take a better look and he said he found some of the components had excessive wear although the fluid was pink and not burnt. There were no excessive metal bits inside and the pan had been hit from underneath (I bought it like that) and he recommended a new OEM pan since the hit appeared to be on the drain plug and he said the weld could be compromised although it wasn't leaking now. He also said this transmission had been opened up before (different seal compound than OEM) but there was no way in telling what was done to it previously due to the wear and tear.

We discussed my options (installing a remanufactured tranny vs. rebuilding mine), and based on the overall good condition of mine, my perception they knew about the 4r100 issues (i.e. like the mechanical diode failure) and they had the right experience as they had rebuilt many of them over the years, I decided to have them overhaul mine.

They replaced all of the worn bits like bushings, filters, installed a master overhaul kit with all of the current upgrades, installed a new valve body modification kit, installed an upgraded remanufactured torque converter and swapped in the new OEM tranny fluid pan. He said the kit they installed was a mild performance upgrade over the standard kit and he did this because I carry a truck camper and pull a trailer behind it usually in mild to steep hilly terrain.

When they finished 2 days later, I literally loaded our truck camper (~3500 lbs.), hooked up my cargo trailer (~3,200 lbs.) and headed out on vacation. The truck shifted well and a bit more crisply and we had no issues for our 1,500 mile round trip ... some of it through the small hills of northeastern WI.

Overall, I'm pleased with this shop's work and their customer service approach. For my CDN $3,000, I also received a nationwide 12 month/20,000 km. warranty.