Check your alternators!

[907DAVE]

Recently I have been experiencing some shifting irregularity's and some idle stability issues when shifting between reverse neutral and drive, as well as a randomly flashing O/D light. I was able to retrieve a P0720 (output speed sensor) from the PCM and with the truck in park the speed sensor would fluctuate wildly (viewed from scanner).

First thing I did was measure the charging system for A/C voltage using my DMM and there was around .3vac.

Threw the scope on it to get a visual....

Anything above .2 volts A/C should be considered a problem. So I installed a new rectifier and everything is good to go. Keep this in mind next time your truck is acting up.

 

[Derek@Vision Diesel]

Impressive to think of that right off the bat, good work bud!

 

[907DAVE]

Not the first time I have seen an alternator do something like this. Glad it was an easy fix and I'm not tearing into the tranny right now..lol

Saving that one for later.

 

[samoht517]

I wonder if this is why my tranny has been acting weird and the check engine light freaks out every now and again.

misspelled from a fancy phone

 

[TrailerHauler]

That is good info to keep in mind!

 

[Hotrodtractor]

Yep - I've seen voltage issues do all sorts of fun and entertaining issues that can be a bear to track down. Good info to keep in front of people for sure.

 

[primetime1267]

Ohh hell yeah brotha, I just had this problem last week.. I just installed a BTS valvebody and his new tunes and 2 days later my trans started acting up.. Threw the same codes as you and I fiddled around with everything triple checking things.. Then I checked my scangauge2 voltage, it was around 11.9-12.2.. Went to advanced and had them to do a load test and check alternator... BAD alternator,"excessive ripplings" tossed in a new one, 14.1 volts right off the get go. I even pulled out a fairly new Motocraft alternator..

My trans shifts sooo much better now, I thought I was going to have to rebuild the trans, now I can wait for a bit... Time to turn up the tunes and see what happens, LOL.

ohh yeah, checks those grounding cables as well, I had one that the bolt was only in 2 threads..

 

[PSDEng]

I had the same problems with my truck last summer.

Can anybody explain what exactly is going on, how is the rectifier directly effecting the transmission?

 

[907DAVE]

The rectifier is what converts the AC voltage that the alternator is producing into the DC voltage your truck runs on. When this goes bad you will begin to produce AC voltage.

ALL the speed sensors on your truck produce AC voltage which is then interpreted by the PCM and/or ABS module and then is converted into a signal that can be broadcasted throughout the network for various uses.

The problem is when the entire electrical system has AC voltage, which just so happens to be the range the sensors produce in this situation.

So basically the PCM and/or ABS module are receiving a speed signal from sensors even when you are not moving, or the signal it is receiving when your are moving is incorrect or unstable.

 

[V-Ref]

Good stuff Dave! Thanks for sharing.

 

[PSDEng]

Thanks for the explanation!

 

[99powerstroke]

stupid question but where is the rectifier located?

 

[MossBack]

stupid question but where is the rectifier located?

Back of the alternator on my 97 it has the 3 prong plug not sure on SD. I just lost mine and put in a new OEM as I didn't want to waste time.finding parts locally just wanted her running.

Good info Dave amazing how a little voltage causes so many problems.

Sent from my PC36100 using Tapatalk 2

 

[907DAVE]

It is an internal part of the alternator. It requires a bit of soldier work to replace, but is way cheaper than a new alternator if you dont mind doing the work.

This guy has been great to deal with:

http://www.aspwholesale.com/

 

[Arisley]

Good sight. I will have to remember that one.