"Park Brake Limited Function, Service Required" after truck rolls backward toward a drop-off

[Tbone289]

The parking pawl and the electric parking brake failed to operate properly at the same time... crazy. The odds of that must be exceedingly low. I have no explanation for you.

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[MavStangVa]

The parking brake takes a moment to engage, it may be possible that the service brakes were released before the PB was engaged which over-rode the action, leaving it off and the vehicle able to roll?

My '25 hybrid will apply PB on any slight incline automatically, so it's not something I typically need to futz with.

2024 Ecoboosts do not set it automatically ever. Tried it on a steep side street after another thread about it being automatic. 2025 MY change to the software I'm guessing.

 

[Glen Baker LLC]

2024 Ecoboosts do not set it automatically ever. Tried it on a steep side street after another thread about it being automatic. 2025 MY change to the software I'm guessing.

2023 Hybrids do not set the parking brake automatically. Tried it on a steep side street after another thread about it being automatic.

I also made a video of it not happening. My proof.
Same Hill. Watch the drive indicator as I shift to park. I then open the driver's door. NO automatic parking brake activated.

2025 change to the software?

 

[Dad]

This story reminds me of something I was taught way back in the 1960s as I learned to drive with manual transmissions. We were taught, when parking on slopes, to always turn the wheel so the vehicle would back into something (like a curb or a tree) that would stop it. Because back then, vehicles would pop out of gear, or parking brakes would fail - all kinds of crazy stuff. It was the era of "it's a good day if my car works".

Good advice. I think we're all hoping we've come further along since the '60s.

 

[notfast]

This story reminds me of something I was taught way back in the 1960s as I learned to drive with manual transmissions. We were taught, when parking on slopes, to always turn the wheel so the vehicle would back into something (like a curb or a tree) that would stop it. Because back then, vehicles would pop out of gear, or parking brakes would fail - all kinds of crazy stuff. It was the era of "it's a good day if my car works".

I still do that on vehicles with no front CV axles. On vehicles with front CV axles, I chock the wheels and set the parking brake because I'd rather not keep the CV boots stretched for hours on end.

On that note, I always shift to neutral, set the parking brake, let go of the service brake to be sure the parking brake is holding, then put my foot back on the service brake and shift to park.

 

[Butterscotch]

You stated "The truck was serviced at the Ford dealership from which I bought it, the trailer towing signal issue fix was applied, and it was confirmed to be up to date on all other fixes as of May 22, a week before this trip. "

So the question remains did the person that performed the service run a diagnostic, like they are supposed to, to test new software update ? Did the person do it correctly ?

Just asking...I understand why youwant to go elsewhere but for me I'd still need to know if this has never happened or been noticed by you before.

Very, very glad nobody got hurt. Stuff can be replaced. People can't.

 

[iam2sam]

I just got off the phone with the service manager at the dealership where I bought the truck. It's going in next Wednesday for evaluation. He told me that the Park function on the transmission is a mechanical pawl, but that engaging and disengaging it is controlled 100% electronically. He also agreed with me that the truck could not have rolled backward as it did if it was actually in Park mode without doing major transmission damage. The truck drove normally the rest of my time on site and for the >400 mile trip home; it seems highly unlikely that such damage could have occurred with no symptoms since. Since the parking brake and transmission Park mode are both computer controlled, maybe these weren't really two separate failures after all. Another strange thing is that when I chatted with Ford support on-line yesterday, the rep reported that the truck modem was not communicating. The service manager confirmed that this morning. A couple of weeks before this trip, I took the truck to them to diagnose a check engine light (no problem was found) and I had the update for the trailer light issue applied at the same time. I was told when I picked it up that they had done a scan for other codes and issues at that time. Shouldn't the modem issue have been found then?

 

[Butterscotch]

I just got off the phone with the service manager at the dealership where I bought the truck. It's going in next Wednesday for evaluation. He told me that the Park function on the transmission is a mechanical pawl, but that engaging and disengaging it is controlled 100% electronically. He also agreed with me that the truck could not have rolled backward as it did if it was actually in Park mode without doing major transmission damage. The truck drove normally the rest of my time on site and for the >400 mile trip home; it seems highly unlikely that such damage could have occurred with no symptoms since. Since the parking brake and transmission Park mode are both computer controlled, maybe these weren't really two separate failures after all. Another strange thing is that when I chatted with Ford support on-line yesterday, the rep reported that the truck modem was not communicating. The service manager confirmed that this morning. A couple of weeks before this trip, I took the truck to them to diagnose a check engine light (no problem was found) and I had the update for the trailer light issue applied at the same time. I was told when I picked it up that they had done a scan for other codes and issues at that time. Shouldn't the modem issue have been found then?

Oh yes, it should have been found !!! Scanning for codes is something totally different than scanning and testing the CAN buses which, after an update by a dealer, is supposed to be done. At least my dealer does it. A ten minute update will take almost an hour...which I'm willing to wait.

Please keep us updated.

 

[HeyBales]

I just got off the phone with the service manager at the dealership where I bought the truck. It's going in next Wednesday for evaluation. He told me that the Park function on the transmission is a mechanical pawl, but that engaging and disengaging it is controlled 100% electronically. He also agreed with me that the truck could not have rolled backward as it did if it was actually in Park mode without doing major transmission damage. The truck drove normally the rest of my time on site and for the >400 mile trip home; it seems highly unlikely that such damage could have occurred with no symptoms since. Since the parking brake and transmission Park mode are both computer controlled, maybe these weren't really two separate failures after all. Another strange thing is that when I chatted with Ford support on-line yesterday, the rep reported that the truck modem was not communicating. The service manager confirmed that this morning. A couple of weeks before this trip, I took the truck to them to diagnose a check engine light (no problem was found) and I had the update for the trailer light issue applied at the same time. I was told when I picked it up that they had done a scan for other codes and issues at that time. Shouldn't the modem issue have been found then?

Well - the rep could only state they had not received communication from the truck. They can't discern if there is a modem issue, they are getting nothing from the truck.
So there easily could be no codes about the TCU.

If you disable that system - that happens.
If you never had it setup - that happens.
If you remove a fuse - that happens.
If you have an issue - that happens.

Only 1 of those would create a hidden error during scanner connection.

 

[HeyBales]

I just got off the phone with the service manager at the dealership where I bought the truck. It's going in next Wednesday for evaluation. He told me that the Park function on the transmission is a mechanical pawl, but that engaging and disengaging it is controlled 100% electronically. He also agreed with me that the truck could not have rolled backward as it did if it was actually in Park mode without doing major transmission damage. The truck drove normally the rest of my time on site and for the >400 mile trip home; it seems highly unlikely that such damage could have occurred with no symptoms since. Since the parking brake and transmission Park mode are both computer controlled, maybe these weren't really two separate failures after all. Another strange thing is that when I chatted with Ford support on-line yesterday, the rep reported that the truck modem was not communicating. The service manager confirmed that this morning. A couple of weeks before this trip, I took the truck to them to diagnose a check engine light (no problem was found) and I had the update for the trailer light issue applied at the same time. I was told when I picked it up that they had done a scan for other codes and issues at that time. Shouldn't the modem issue have been found then?

Regarding the failed engagement of the pawl and EPB it appears.

Can you find a slope (this would be good for shop testing too, easier to find the issue if they can cause the problem) that matches where it happened?
If you sit in vehicle after stopping, and just slip to Neutral - does it really roll back at the same rate as you dealt with?

It just seems like from your OP - there was time to react, and it was a decent incline.
In neutral it seems as soon as you stepped out, it would have been noticeable rather easily.
You should mention if it is NOT just neutral basically.

I only mention that because it may assist allowing the techs to be faster on finding the issue.
The only thing I can think of is either the EPB calibration is/was off and you got partial brake hold.
We certainly know that from the days of stick pull PB's that aren't used often.
Or your parking pawl got rounded off and just kept slipping the notchs.

But as commented above - the double effect seems like it would be awfully rare.
Pawl not actually engaged, and EPB not fully set.

 

[Backin15]

Wow - scary stuff! Even if you turn off the ignition before putting it in park it does it for you so this should be impossible. This is why I prefer parking brakes that are mechanical and use a physical cable.
I have a fairly steep driveway and the parking brake has automatically applied itself randomly, I am guessing it has to do with the hill hold feature.

 

[mstrbill]

2024 Ecoboosts do not set it automatically ever. Tried it on a steep side street after another thread about it being automatic. 2025 MY change to the software I'm guessing.

Well my 2024 FX4 does. The slope on my drive is 5% and if I open the driver's door driving up it, the transmission shifts to park and the parking brake activates. Same thing if I open the door at the community mail boxes which are in a relatively flat culdesac.

 

[MavStangVa]

Well my 2024 FX4 does. The slope on my drive is 5% and if I open the driver's door driving up it, the transmission shifts to park and the parking brake activates. Same thing if I open the door at the community mail boxes which are in a relatively flat culdesac.

2 different scenarios. It does that because it is in gear and moving. Yes mine will also prevent me from stepping out of a moving vehicle. When I stop at the end of the drive to get my mail, IF I do not take it out of D before opening the door with my seat belt off it will shift to P but and set the EB. I have tried it multiple times. But if I shift to P prior to opening the door it does NOT set the EB. Now test it after turning the engine off. It will not automatically set the EB. 2025 software change.

 

[HeyBales]

2 different scenarios. It does that because it is in gear and moving. Yes mine will also prevent me from stepping out of a moving vehicle. When I stop at the end of the drive to get my mail, IF I do not take it out of D before opening the door with my seat belt off it will shift to P but and set the EB. I have tried it multiple times. But if I shift to P prior to opening the door it does NOT set the EB. Now test it after turning the engine off. It will not automatically set the EB. 2025 software change.

Ditto - if I have my seat belt done - there is no auto-park on door open.

That's probably the kicker for mstrbill if not noticed yet.

But I'll have to test that incline aspect if that makes the EPB engage.
I've been on enough incline the hill hold engages - but it appears that has a lower level of auto-use.
I can't get the EBP auto-use no matter the incline so far, when I go to Park.
I haven't tested the Auto-Hold feature on a steep incline, where manual says EPB may be used.

 

[iam2sam]

So, my Ford dealer had the truck for 6 hours today, and could not repeat the issue, nor did they find any codes recorded, in spite of the "parking brake function limited - service required" error that was displayed after the truck rolled away. I don't know that the no code behavior isn't just as concerning as the incident itself. Also, the Ford (Pass) app will not run on my phone. I got a new phone that was identical to the one I broke that was running it successfully, including the OS, which is Android 9. Not the latest and greatest, but according to Ford's published spec for the app, should suffice to run it. But Ford app 6.15 downloads from Google Play Store and installs, then fails with o cryptic error when I run it. The Ford service rep had no answer for that after witnessing it, either. Oh, well. I need to wash the truck and clean a few things up this weekend, then early next week I'm off to the Toyota dealer to make a deal on a Tacoma.

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