I put larger tires (32.8") and changed my diff. ratio from 3.73 to 4.56 on my 6.8L E-350 van.

Now I've been told by two Ford service departments that my PCM can not be re-programed for this. Could this be true?
My factory powertrain service manual has an overview of the procedure, but defers to the scan tools directions for specific instructions. (it also suggested that there might be some limitations)

Best as I can tell, all the E-350/450 and striped chassis that came with the 6.8L use the same PCM, and surely they have different diamemter tires and ratios.

I looked into a TruSpeed, but they don't make one for my van.

The factory setup on this year van is a little different than what some of you guys have. The wheel and axle speed sensors are connected only to the ABS module, and there is no connection at all to the PCM. The PCM gets it's speed signal from the transmision output shaft sensor. The PCM then uses this signal to manage powertrain strategies, and compute the speed signal to the speedo.

Can anybody tell me how to correct the PCM so that the powertrain works well, and my speedo is right? Thanks, Rob

 

If it's actually using the old VSS signal for the vehicle speed,then you should be able to use a Dallas Mustang Speedcal for it. There will not be a specific "kit" for it though,and the guys at Dallas Mustang will not have a clue as to how to make it work for you.

 

http://www.fmdkinc.com/wiz.htm

Lots of Ford guys are using this

 

Thanks for the replies guys. I think you are both recomending a speed pulse conditioner.
If I were to use one of these (or one of several others on the market), I'm not sure if it should go befor the PCM (ie. in the VSS to the PCM) OR after the PCM (to the speedo, et. al.)....(et. al. being the cruise control, and a speed signal wire to the overhead consule.)
I'm also not sure how advanced my PCM is in reguards to Engine and Transmission management strategies. I'm thinking that tricking the PCM into thinking that the transmision output shaft is doing a different RPM (or Pulse(s) per revolution) than actual might not be the right thing to do. But if I modulate the signal After the PCM, I'm only correcting the speedo, and the PCM does not know how fast (or slow) the vehicle is going. Perhaps it doesn't matter.

Anyhoo, I was hopping maybe to hear that: "Heck yes, your PCM can be programed, everything will work out fine. Those fools at the dealership don't have a clue. Of coarse you can program that PCM to that stuff."
That way every thing is on the same page.

A fool is born every minute.

HELP.

 

You seem to have to issue's here. The gear change...and the larger sized tires.

It seems completely logical for the PCM to be programmed for the larger size tire. On the SuperDuty...that can be done by modifying the rotation per mile in the PCM. IMO...ask for the PCM to be programmed correctly at the dealership. Stand and watch as the OBDII port is connected...and verify the shop computer cannot communicate with the E-Series PCM. I believe that will be a false!

The gear change issue should be resolved by changing the transmision output shaft sensor to one that recognizes the 4.56 gears. The dealership should be able to look up the correct output shaft sensor. However, not sure that the 4.56 have surpassed the limitations of the sensor, but maybe the 4.30 sensor will do the job...since it is closer than the 3.73 you had. Again...on the SuperDuty...no changes are needed during gear swaps...because the SSV is located at the rear diff.

Hope this helps some...and maybe more E-series folks will chime in.


biz

 

It needs to go between the VSS and the harness. It's not "tricking" the PCM anymore than changing the OEM VSS gear out wouild be-that's how the PCM can determine the correct vehicle speed for shifts and lockup,and how it's intended to be used. It's possible that you can alter the data in the PCM to make the speedo read properly-what's the PCM catch code(4 digit code on the PCM)? I can check for you.
JL

 

My Powertrain Control Service Manual sez that both tire size and axle ratio can be programmed. But it goes on to say "...the strategy will place range limits on certain items such as tire and axle ratio."

I guess I'm going to have to go back to the dealer again, and get them to show me how/why they can't set these parameters. Rob.

 

Dealers are extremely limited on what they can change-they can't just change the axle reatio or tiresize as simple as you'd think-they can only make a change that's EPA certified and OK'd by Ford as an acceptable OEM-type configuration.
JL

 

Thanks for the offer Johnny. Will I have to remove the PCM to read the catch code?
All I can see of it is the 104pin connector.

 

The catch code is located on the connector-on one side of it,and it's in bold print,alot larger than the other print on the label..should be something like SVB2,FTE1,etc,etc..basically,3 letters and a single number.
JL

 

So are you saying that my PCM may be able to be reprogramed, BUT the dealer may not be allowed to do it?

 

Get underneath and check if you have a speedo gear - that "output shaft speed sensor" might be a regular speedo gear setup like in 4R70W's in my '96 t-bird and '97 cougar.

We've been through this before with a van guy

I'm pretty sure it turned out that he had a regular speedo gear - take it out, count the teeth, and figure out the new tooth count - go down to the ford dealer and get the right one.

I think.

 

OK guys, Thanks for the advice.
I'll check for the catch code, and see if the output shaft sensor is gear driven.

 

That's correct. Unless it's an EPA certified configuration of tire rev/mile and axle ratio-they cannot and will not do it.
JL

 

Looked at my van today, (it's at my shop) and diden't have time to find what I was looking for.
Both, the PCM (and it's catch code), and the OSS sensor are not easley accessible. It's going to take a little disassembly to determine both.
The PCM is tucked up on the fire wall, and will require that the engine air intake be removed, and the OSSS needs the doghouse removed, to provide acccess.
I'll do this as soon as I have a chance. Look for an update to this thread soon, and thanks for all the advice/suggestions.

If any other PCM Gurus want to chime in, Please feel free. Rob.