I have a customer who has a 2006 freestar 3.9. He tells me the car stalls when he brakes. The car starts up perfectly everytime and in neutral and park car runs great. Problem is now when I put the car in drive or reverse the car starts to buck and the rpm a surges and fluctuates. I Tryed to clean out throttle body , replaced the iac and still nothing so I finally replaced the PCM with a new reflashed one and still the same. I am running out of ideas And I've spent a lot of time with this already. Anyone can tell me what to try next or what it may be. I'm lost right now thanks

 

This may sound a little odd ,but it doesn't cost much to do.
Start with checking the ground connections, all that you can find. Just looking at them doesn't do it ,you need to loosen and retighten them.
Then carefully remove the plug on the trans check for any corrosion.
When you are coming to a stop and are getting ready to apply the brake, take it out of drive then step on the brake . Does it still stall?

 

The car does not stall in neutral and park but in reverse and drive and 1 and 3. It's the worst in drive. What plug are you talkin about on the tranny? I will check grounds next. I also am going to check fuel pump and maf sensor

 

Look for the plug that tells the PCM where the trans linkage was shifted to.

 

Hey I haven't had a chance to check grounds but I hooked up my modis and it says there is a fuel pump duty cycle and I'm trying to figure out what that is. With the car running the fuel pump duty cycle is at 0%. Does that sound right? Also when I try and test the fuel pump duty cycle the car runs rough for a couple of seconds and then stalls and dies. Does that sound right? Also new mad sensor didn't do any difference

 

Some Ford fuel systems have a pulse width modulated signal that is generated by the fuel pump driver module based on the pressure signal sent by the fuel line pressure sensor up at the fuel rail. The modulated signal is also called "Duty Cycle" and is monitored by the PCM. If no great amount of fuel is required the duty cycle could be 0 until the pressure drops due to increased demand for fuel.
You get me off on a tangent here,,,,,,,,,,,,, and,,,,,,,,, me thinks it would be better if we stuck with one problem at a time ,What do you think?

I'm trying to follow a train of thought here, It is possible the fuel pump voltage is dropping when the addl demand for current by the stop lites is starving the pump circuit. You could be on to something and verifying the alt output might be a good place to start and the ground for the fuel pump too.

 

The reason I brought up the fuel pump duty cycle % is I thought it may have something to do with the stalling. I am trying to fix the same problem also. What your saying is when I put the car in drive I put more demand on the fuel pump and maybe that's why the car stalls? This might sound dumb but what is the alt output and how do I check it? Thanks

 

Do have an instruction book with your Modis?