Permobil F3 20AA tilt error; how I fixed it.

[worserbytheday]

Numotion said it would be a month before they could send a technician; something about a labor shortage. (I suspect it is a salary shortage). After a day of that, I slowly and systematically took all the cover plates off so I could see all the wiring. That is a 2 hour job with a few standard Allen wrenches (standard hex keys). I then took a multi-meter (volt and ohm) and checked the continuity (resistance) of every wire in the system to see if I could find a break.

A good estimate is there are 50 individual wires in about 20 cables, and I checked them all twice. It is a time consuming process, as some of these wires snake about three feet through the chair from one end to the other. Worse, they are all the same color (black). So you cannot just see one end in the front and then immediately find the other end in the back, for example. (Design suggestion -- color code everything.) Still, I have all the time in the world for this. About 8 hours of testing wire ends, I found the offending wire.
Most cables are a 3-wire plug: red, brown, black. Red and brown appear to be the power and neutral lines. Black is probably the signal wire from the controllers. Without a wiring diagram, it's hard to say for sure. This particular failure was in the black wire in the power cable from the lower front controller to the power distribution bus on the front right side (if sitting in the chair) of the seat bottom. This wire snakes about 3 feet around the vertical lifting post. It powers the entire controller distribution bus for all the control functions. As a result, "tilt error 20aa" has nothing to do with the tilting circuit. (Design suggestion -- improve the diagnostic fault detection.)

I think the failure mode is that every time the seat lifts up and down, these wires are bent just so slightly like a coil compressing and extending, and this slowly weakens the copper wires until one of them finally breaks. To repair the break, I cut that black wire 2 inches from the connector ends and spliced in a good wire with a compression fitting, (a few dollars of material at the local hardware store). I snaked the new wire along the outside of the existing cable. It looks to me like a permanent fix. Chair works perfectly now.
I do expect more wire breaks to happen like this because of all the motion the cables take, with arms and legs lifting, chair raising and lowering, and the back and seat tilting over time. At least now I have an approved solution. It's about two days of dedicated work to get to know the wiring of your chair and find an internally broken wire in that haystack of cables. The copper inside is broken, not the plastic covering; you cannot see the break, so you have to test each wire at each end. The failure could also be intermittent -- the wire is not fully broken, or the broken ends can still touch momentarily.

My chair did temporarily work three times while I was testing everything. I first plugged and unplugged every cable one-at-a-time, and shook them a little. It was only when I pushed the seat up button and it shut off did I realize it was probably one of those wires. Any automechanic or electrician can do what I did and fix this problem. The key is to have the confidence that you cannot make the chair worse, as it is already useless at the moment, and systematically go through each connection and wire. It's better than waiting a month for a repair technician, with your ALS victim stuck in bed.

Good luck, and embrace the suck!

 

[affected]

Wonderful!
May I ask, are you the PALS or the CALS as you say your 'victim' is stuck in bed?
Yet your profile says you are the PALS.
Diagnosed 15 years ago?

 

[worserbytheday]

Thanks for the review. Sorry if I messed up my profile -- ultra burnout here. My wife has ALS, and when I filled in the profile the date of diagnosis example was (1/2007) so I just entered that; but, yes, she was coincidentally diagnosed in 2007. By now I forget precisely when. By now I forget precisely a lot of things. But I can still repair electronics!

I thought more about what I wrote. It's actually closer to 70 wires in 20 cables. Some are 4-wire, some are 3-wire. Each battery tests to 12 volts, independently. They are wired in series, so it powers at 24 volts to each motor. Curiously, the control distribution bus runs around 16 volts.

I'm throwing this info out there because if you don't have a wiring diagram, you don't know what is a correct reading. There is a circuit breaker at the center rear bottom, something like you'd find in your house fuse box. Then there is a little push button fuse to the right in a clear plastic box that resets if you push it. Something to play around with when diagnosing system failure. The motors look well made and I doubt they burn out.

Her chair is 5 years old now, and we had a loaner before that had to be 10 years old. Never had motor problems. I think most of the failures everyone gets is related to wires slowly weakening inside all these cables with constant flexing. That is a simple fix, once you find the broken wire, in a haystack of 70 wires, that is. I've got a lot of time on my hands.

There is a saying in prison, I've heard, "you do the time; don't let the time do you." It's been 15 years now and I still have no idea when I come up for parole. I can't even remember anymore if I actually committed any crime that got me here. Oh, yeah, I remember now. I said, "I do."