Transmission - Mechanical Diode question
 

Hi all,

So I've been driving a 1984 F250 6.9L IDI diesel for a pretty long time. I've always wanted to get a late model 7.3 powerstroke. My local Ford dealer got in a 2001 F250 automatic 4x4 PSD and it's in pretty good shape. Local one owner truck 175,000 miles.

I was looking through the sticky trying to brush up on what things to look for when buying a used truck. I found something I've never heard of before which is a mechanical diode failure in some of the automatic transmissions in the 2001 year lineup. I checked and sure enough, the truck was built during the mechanical diode timeline. I doubt there's anyway to know if it has been replaced or not. Is this something to worry about? I definitely like the truck but I can always keep looking. Also how long can I expect the tranny to last assuming it has been taken care of. 175k seems like a lot for an auto tranny. Anybody have any experience with this subject? Thanks!

While the mechanical diode is not a desirable feature, it can be "hit or miss" on its longevity. I rebuilt my 2001 transmission at ~ 115k miles. There wasn't anything wrong the mechanical diode at the time, but I upgraded that part to a HD version of the OEM roller type one-way clutch. This also requires replacing the Direct Drum, and adding a thrust washer, because of design differences. Mine had a center support ball bearing failure (not very common) that caused undue wear on other parts. Depending on the parts cost, you are looking at less than $200 for the upgrade to be done at rebuild. There have been reports of mechanical diode transmissions going over 200k-300k miles without failure. Some have may have gone longer. These are only ones I have read about. I, personally, wouldn't consider the diode a deal breaker, but would be prepared to deal with that issue when the time comes.

What Roland said.

Our truck in signature has 224k on Mechanical Diode

So I've mostly just had experience with manual trannys, are you guys talking 200-300k on transmissions that have never been rebuilt? I wasn't aware auto transmissions had that kind of longevity.

I am assuming my 2002 has the mechanical diode as well. If so, I had mine rebuilt at something over 300k with no evidence of issues at that time, just did not want a failure on the road with all my highway miles. I did not make mention of this diode when the rebuild was done though.

I was under the assumption that they were only used in the 2001 model trucks, Mark ?

Yes, we are. That's not uncommon. I've seen them never touched at all for maintenance, and only pulled out of the truck at 369,000 miles because it was wrecked. The trans upon teardown still had a lot of miles left in it.

You're assuming incorrectly. The mechanical diode was only used in the first half of 2001, and only on the diesel models, though I am aware of a handful of gas engine transmissions that snuck out of the factory with one. No 2002 had one from the factory, and I can't imagine why anyone would want to retrofit a known bad design into a later transmission.

Here's some info I've copy and pasted regarding the diode trannys and dates of builds:



Mechanical Diode Tranny Build Dates2001 vehicles equipped with 4R100 transmission, having a transmission build date between
7/27/2000 and 3/23/2001.
Roland (mueckster) 04-15-2008
The trans does have its own build date located on the tag on the trans case (left side ,rear).
4th line down should show BD- #Letter##. This is the build date year/ month/ day.
Example : BD-1D28 1=2001 D=April (A=Jan, B=Feb...12=Dec) 28=Day of Month.
Using his example tranny Build Date is Jan 28, 2001
　
Mark Kovalsky 01-03-2009
The mechanical diode was in the diesels from the start of 2001 model year (July, 2000) and left production in March, 2001.

Cody (cleatus12r) 01-03-2009
If anyone is still interested, the "mechanical diode" was the replacement for the one way roller clutch previously (Intermediate one way clutch) used in the E4OD and 4R100s. After Ford found out that the diode wouldn't work in the gasser trucks (they exploded when shifted at 5300 RPM), they decided that they should stick with the proven and slightly more robust design. It is a mechanical device that allows rotation in one direction but not the other. It is used for the 1-2 shift when in drive range. When the "diode" was used, it replaced the roller clutch with a sprag that is inherently weaker. High power/high RPM 1-2 shifts are what kills the intermediate one way in the diesels.

Mark Kovalsky 01-03-2009
The gas engine trucks blew up the diode at 4600 RPM if the driver lifted his right foot just as the WOT 1-2 shift started, then put it right back on the floor. The person that discovered this was able to do it on 100% of the trucks he tried it on. That's why they left it in the diesels, they couldn't run 4600 RPM.
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