Change gear ratio 3:73 to 3:55 in PCM

[Tommy V]

Yea thats what i was telljng u,the pcm gets it signal.for mph from the output shaft speed sensor which is part of the pcm not bcm.So on the 15 and above cars u.dont have to change anything when doing a gear swap or tire size change.Make sure whatever u changed in the bcm using the forscan is back to your specific data.I bet your bcm data u put in is not jiving with your oss speed sensor data and causing some problem in the torque managment putting it in limp mode.It has to be something in that data u changed and maybe some kind of error happened when you tried to change it back.

Sponsored

 

[Tommy V]

U will have to.use forscan to put your old data back in,a reset or relearn will not do it,or a handheld will not do it either.

 

[EastTNMustang]

Yea thats what i was telljng u,the pcm gets it signal.for mph from the output shaft speed sensor which is part of the pcm not bcm.So on the 15 and above cars u.dont have to change anything when doing a gear swap or tire size change.Make sure whatever u changed in the bcm using the forscan is back to your specific data.I bet your bcm data u put in is not jiving with your oss speed sensor data and causing some problem in the torque managment putting it in limp mode.It has to be something in that data u changed and maybe some kind of error happened when you tried to change it back.

So my speedo would be accurate, I changed the settings, thru FORScan, when I changed tire size. I had the same bog-down/no power in 4th gear as OP and Wolverine are talking about...I did the PCM relearn and codes were cleared and I had full power again.

It appears there may be other issues.

 

[Wolverine]

So my speedo would be accurate, I changed the settings, thru FORScan, when I changed tire size. I had the same bog-down/no power in 4th gear as OP and Wolverine are talking about...I did the PCM relearn and codes were cleared and I had full power again.

It appears there may be other issues.

Did you make a change to gear ratio, or just tire size?

 

[EastTNMustang]

Did you make a change to gear ratio, or just tire size?

Just tire size. Maybe try only that change, then PCM relearn and see what you get...leave gear ratio unchanged.

 

[Wolverine]

Just tire size. Maybe try only that change, then PCM relearn and see what you get...leave gear ratio unchanged.

EastTNMustang - Technically speaking, I have done this already...by setting EVERYTHING in FORscan back to factory settings, and relearning the PCM. The DTC (U0422) goes away, but the driving issue remains. Limp-mode in 4th & 5th gears (obvious in those gears anyway, who's to say it isn't occurring in other gears as well to a lesser degree?).

My last hope is that a tune will possibly help here. I am installing a 2018 intake manifold soon, and will get my first tune to go along with that. I have heard that perhaps Lund can do something to defeat this "Limp-Mode" issue.

 

[Glas86]

I found a dirty way to solve the issue and have been living with that since then. Changing the gear ratio didn’t work since the value in the pcm never changes. There is one tire size in the pcm and one in the bcm. So Instead of changing the gear ratio I calculated what kind of tire size would result in the same speed asumin the pcm calculates with engine rpm gear ratio and the pcm tire circumfence and the bcm just using abs signals and tire size. So if you have the original 0858 tire circumfence in the pcm (change it in the bcm then relearn) change the tire circumfence in the bcm to 07F0 and do not relearn. Thats working for 3:73 to 3:55. You will have the Pxxx DTC but everything is working otherwise.

 

[Glas86]

You could also make the tire circumfence in the pcm bigger instead of the bcm one smaler. This works by changing it in the bcm relearn and reset to original but not relearn.

Tire pcm = Tire bcm x 3.73/3.55

In the end as long as the new tire values fullfill this formular for changing from 3:73 to 3:55 your Mustang will drive without limp mode but havin the DTC. It also works for other tire circumfences and gear ratios. Just dont forget to convert from hex to dec before you do the calc and convert the new value to hex before you put it in.

 

[Wolverine]

So my speedo would be accurate, I changed the settings, thru FORScan, when I changed tire size. I had the same bog-down/no power in 4th gear as OP and Wolverine are talking about...I did the PCM relearn and codes were cleared and I had full power again.

Felix, reading your last two posts made me sit back and think about this for a while. Then I went back through this entire thread, post by post. Re-reading the snippet that I quoted above from EastTNMustang, it clicked that you two fixed your limp-mode issues the exact same way. Leave the OEM gear ratio value/numbers in the BCM and update the TIRE SIZE values/numbers only! Even if you have stock tires and swap gears over, changing the tire size in Forscan seemed to work for you two and changing the gear ratio doesn’t.

My theory is that the PCM doesn’t care what the tire size is, and it just lets the BCM take care of it completely, and whatever the BCM tells the PCM is the current tire size the PCM is just like “Alright, cool” and it trusts/uses that data. However, if you update the gear ratio in the BCM, then the PCM throws a fit because it has to validate/verify the ratio is stock but since you changed it, it’s NOT matching what it expects. This conflicting information is a big red flag in the PCM (DTC U0422) and causes nothing but issues (such as limp-mode) if you don’t have the means to force the PCM to relearn the new BCM values. Forscan doesn’t seem to be powerful enough to carry this out, even though it claims it can and has a button which is supposed to do so, it simply can’t.

I’m a little hesitant that it will work because I’m so exhausted from trying everything else but I will absolutely give it a shot either tonight or tomorrow morning.

 

[Wolverine]

@EastTNMustang I have a few questions for you

1. What was your stock gear ratio and tire size? (Basically what did line 726-12-01 in Forscan look like?)

2. What tire size did you put in when you updated it?

3. What tires are you running, are they actually shorter or did you only change that value in Forscan to fix the limp-mode issue?

@2016s550gt I wanted to add your newfound info to this thread too. Can you please post all the details and info you have on how you resolved your limp-mode issue by having Shaun at AED update your tune and disable/remove the Torque Limit code in the PCM?

 

[Glas86]

I feel your pain. Give it a shot. It does work I just put it back to all stock to not have dtcs when I bring her to service. Limp mode was back all dtcs gone. All I did afterwards was changin the tire size to 07F0 in the bcm and did not learn it into the pcm. If you have any isues tomorrow let me know. But I am drivin like this for nearly 6 month without any issues.

 

[Wolverine]

Update: I performed the tire size update that @Glas86 suggested in the past few posts. Leave the gear ratio value alone, and change the tire size value from 0858 to 07F0. The checksum automatically recalculates itself. This will throw a P160A DTC code. Do NOT relearn the PCM. The DTC will remain in the module as a stored code, but no MIL/Check Engine Light will illuminate in the instruments.

Issue (temporarily) resolved!!

This isn’t going to be a long term solution for me if it can be fixed correctly using a tune, by way of the logic/code/feature that could be disabled. I will say I’m pretty relieved to have any kind of solution at all though, so thank you to Glas86!

Immediately following the test drive confirming the limp-mode issue was resolved, I took apart my engine to install a 2018 manifold and JLT CAI. I’m now waiting on the Lund tune to come in so I can safely drive the car again. Can’t wait!

 

[EastTNMustang]

@EastTNMustang I have a few questions for you

1. What was your stock gear ratio and tire size? (Basically what did line 726-12-01 in Forscan look like?)

2. What tire size did you put in when you updated it?

3. What tires are you running, are they actually shorter or did you only change that value in Forscan to fix the limp-mode issue?

@2016s550gt I wanted to add your newfound info to this thread too. Can you please post all the details and info you have on how you resolved your limp-mode issue by having Shaun at AED update your tune and disable/remove the Torque Limit code in the PCM?

Here ya go:

1. 3.73 and 275/40R19 were the stock ratio/tire size

2. When Roush added the Stage 3 package to my car, the tires were changed to 275/35R20...about 1/4" smaller circumference than the stock wheels

3. See #2 above...with this small of a change I had no driveability issues/power loss, but the speedo was off by above .3-.5 MPH at 70 or so (speedo read 70, but GPS said I was doing 69.5)

I changed the tire size in FORScan so my speedo would be spot-on when compared to my GPS unit...over a straight, level, smooth road with the cruise control set and steady.

After the change I had the power-loss/bog down you described...I did the Re-Learn procedure and my power restored and drove great...and my speedo was still spot on from the corrections I made

 

[TexasRebel]

which connecting tool are you folks using to connect FORScan?

 

[Wolverine]

which connecting tool are you folks using to connect FORScan?

OBDLink MX Scan Tool.

About $80 retail price, available on Amazon or anywhere else similar.

Sponsored