I haven't really dedicated any troubleshooting time to my battery pack for a few reasons;

1. Temps monitored via forscan (while driving) seem fine....confirmed a few times by touching the top plate (cool) and checking airflow from the cooling vent while waiting for cooldown after wrench light is tripped
2. my 2012 uses cabin air and has neither external window vent nor compressor-based A/C. I don't have pets, so no fur concerns here
3. I have had one battery overtemp event (vent got blocked by cargo shifting) and though I don't have it written down, I recall different codes - the pack and cooling fans are pretty well monitored and I think you get a battery-specific indication if there's a problem there.

i'm not as suspicious of battery health at the moment because of decent voltages showing the last time I had the PA07C error light up, but it still remains a possibility (to my mind) that increased charging demand and/or internal resistance from old cells is putting more load on the charging system and warming it up...and this would explain it becoming an increasingly common issues across aging escapes.

The most frustrating part of this is the lack of specificity in the DTC - "motor electronics" suggests to me the inverters responsible for directly controlling both M/G units so I am first trying to rule out (or confirm) poor coolant flow or contact with everything in that housing atop the transaxle. Still would like a good way to measure the flow rate in the MECS loop....I have the impression that the overflow bottle doesn't get full-force return from that loop, but rather a Y off from it

 

finally getting around to the fluid changes - a few initial notes:

• MECS pump seems to run well, flow easily felt through hoses on both sides of it. Can't really feel much flow coming out of the inverter though (either well-damped by that point or inhibited)

•Not much flow back into the bottle (this is just a y-pipe so not too surprising)

• Picked up a new DY1144 coolant temp sensor for the front of the inverter (easy to get to from below) that I'll change before refilling
*** do NOT buy this part from a dealer, they quoted me over 3x the price ($70) I ended up paying to order the same FoMoCo part online

• Pulled my PCM and it is GROSS.


PCM casing pulled from my 2012 that's lived in CA for the last 10 years


Look at the green!

While certainly concerning, I'm not sure I want to get into this thing...they use a pretty aggressive sealant to close it up, so even with the screws out, it's not budging. and there is nowhere to pry. Am tempted to just clean it up and paint it so it doesn't continue to disintegrate, and come back to it if new fluids don't change anything.

• I'll be changing transaxle fluid and stumbled across THIS SITE reporting measured temperature differences of 10-40º after doing this on an older 1st gen FEH. That gives me a glimmer of hope; I have been suspicious of the motor coils getting insufficient cooling from trans fluid with no heat exchanger anywhere....if this stuff is breaking down and retaining heat, it could explain the number of higher mileage FEH's having this problem pretty well. Especially since.....

• I have reason to believe my PTU fluid is cooked. After a LONG process removing rusty suspension components to address what I thought was a bad wheel bearing, the very-much-still-there growl that modulates with wheel speed is either that PTU or tires that somehow make the same noise on any surface...which I doubt.. I mention this mostly because if I DO find cooked fluid in that, it's more reason to suspect transmission fluid breakdown in the transaxle it's bolted to.

[EDIT: I picked up Valvoline full synthetic 75W-140 gear oil for the PTU mostly because their squeezy pouch will help get into Ford's "no one will ever need to change this fluid" single access port ....I am afraid of what I'm about to find in there]

 

So in regards to what you think may address the P0A7C, you think it's the transaxle (aka "transmission", correct?) fluid replacement that may the solution? As to the PTU fluid, would that be a separate issue that's not causing the P0A7C?

I did a drive from San Jose to Moss Landing last weekend (about 1 hour each way, mostly flat with some small hills) and purposely kept my speed under 70 mph. The wrench light didn't turn on during that day trip. Although not very scientific, it seems from my experience and other posters' stories that driving slower (e.g. in the city) or at lower RPMs at extended periods of time reduces the likelihood of the wrench light turning on. From other people's posts, the light seems to turn on while highway driving and possibly when going uphill. However, I'm wondering if anyone's driven long distances (2, 3, 4, hours) at sub-70 mph and didn't have the wrench light come on.
Interestingly, I found a post that highway driving tends to keep transmission fluid cooler (How hot is transmission fluid supposed to get? | BobIsTheOilGuy) so not sure why the wrench light would turn on during highway driving if the root cause turns out to be transmission fluid...

 

For the first part: Yes the hybrid transaxle is still my primary suspect given the conditions present when I and others have seen the code trip, so degraded transmission fluid could be the problem, or a significant contributing factor

Second: In my observations, this has not been the case for my 2012, which I attribute to 2 non-standard components:
1. Electric motors, producing heat (electrical resistance) that automatic transmission fluid hasn't traditionally had to deal with
2. the absence of a a trans cooler (external heat exchanger)
....and actually maybe a third, given the likely heat soak from exhaust header into unshielded PTU (RIGHT next to each other) and subsequently into the transaxle it is bolted onto.

I have driven all day (800 miles) at highway speeds without seeing a wrench light, only to get it the following day going over a pass.... as recently as beginning of this year. This suggests that the issue is marginal; cooling is able to keep up (barely) with most conditions, but the added load of overcoming high speed drag (FEH has all the aerodynamic finesse of a clay brick) and slope or headwind are enough to push it over a limit somewhere. I have watched TFT rise indefinitely going up the central valley...it takes so long to reach thermal equilibrium that I questioned whether it would so at all.

I would be a little more optimistic about this theory if we could identify the specific location and temperature tripping the DTC, but that continues to evade me (as far as I've seen, MGcoils and phase temps have varied too much on subsequent occurrences for me to identify any obvious thresholds). I think I can rule out the Inverter's coolant temp sensor (that DY1144 mentioned above) as I've got the new one in with no apparent change. In fact unplugging it entirely (while running) will bring the radiator fans to full speed but NOT produce any codes.

I have a distilled water flush in both MECS and Engine coolant loops, and drove around the block to circulate...will drain again and probably try the vacuum fill tonight (not sure my compressor will handle it). Just filling from the bottle, the MECS system seems to be doing a decent job of purging air...had to top it off a few times. I will check the bleeder before draining it again


And yeah - the PTU is a separate issue (change your fluid though!) but I thought worth mentioning because of 2 ways in which I think it may be loosely related:
1. Just as a telltale, confirming the presence of temp-related fluid breakdown
2. added running gear drag due to poor lubrication and/or failure (which I am probably looking at, given the noise)

 

Love this thread and the effort guys.... My skills are nowhere near yours but I'll add my experience. It echoes so many of these stories

2009 FEH with the P0A7C. 127K miles but the wrench light been happening for probably 25K miles. Need a long amount of time and generally highway driving.

ForScan shows the B12B8 Driver Blend Door code as well coupled with the P07AC (and the SAT antenna open circuit). I hastily replaced the blend mode door in the back for the battery initially only to realize I read the code wrong. Seems that actuator was also very common to fail and I didnt pay enough attention to detail.

Just yesterday replaced the correct driver blend door by the gas pedal. But the wrench light came back under a highway test, though now the only code thrown is this P0A7C (and SAT antenna)

What else can I add of value that my vehicle exhibits?

• I also know the CCA on my 12V battery is a little low, measuring 453 instead of 590 rating. Likes to get fairly heavily corroded now matter how much a pretreat it.
• My AC struggles to get ice cold on a hot day.
• I had my PTU replaced at 60Kmi for what its worth. Never had the coolant changed. I tried to once at a Firestone and they said they couldnt do it. Dealer only.

Wish there was a known fix out there.

 

Quick update:

• have transaxle fluid changed (super easy with a $6 hand pump - you can do this without lifting the car)
• Distilled water in the coolant loops has been sitting for a while waiting for me to come back to it...have done 3-4 short drives in the meantime
• initial fill level dropped quite a bit, indicating a lot of air purging (this is good...lessens my concern that an air pocket was the culprit)
• cracked the bleeder screw on the inverter housing and seemed to get fluid right away, but it is admittedly difficult to tell
• drained (significantly pink) water from both loops - lots of residual coolant
• pulled one hose from the inverter housing to fully drain it, and noticed some scale inside the nipple:



This surprises me a bit given how clean the coolant was from both loops. Upon closer inspection there is some crust on the MECS cap too, but not the main coolant cap. I don't know if this is related to the different (yellow) coolant that Ford used when I had them do a fill a couple years ago (they mentioned the OE is no longer made), but it concerns me enough that there might be similar crusties on components inside the inverter housing that I'm going ahead with a chemical flush - using BlueDevil since that's what the nearest shop had. Will check that fitting again afterward

It doesn't look good for being able to vacuum fill, despite the kit I got seeming decent...my little 2 gallon compressor doesn't seem to cut it. I'm getting less than half the vacuum recommended.....so it may be another standard-from-the-bottle fill, in which case I will open that bleeder again.

In any case, I will know tomorrow if any of this made any difference as I have a long, hot drive ahead of me...will check back in here after that (couple days)

 

Apologies for the lag time - busy since arriving. I finished the flush and fill before driving 2 states north - after letting the BlueDevil sit overnight, I got this out of the MECS loop:


This is all the schmutz in the drain pan after pouring off the rest of the coolant flush (water + BlueDevil)

The yellow scale inside the hose fitting (previous post) appeared unchanged. I managed a decidedly non-ideal vacuum fill (my compressor could only pull about .5 bar vacuum, which I had to repeat to get near the cold fill line) but the coolant level did stay pretty much unchanged after running the MECS pump this time. The previous few fills while flushing had required topping off the bottle several times as air purged out and the level dropped, so I took this as a good sign

With that, I headed north over the grapevine with a heat advisory in effect, aaaaand got a wrench light IMMEDIATELY.

So...not solved. Worse, even.... but that's probably the heat and a heavy load (at least 500lbs of cargo plus me). I had about 2 full days of driving with ForScan up on the dash most of the time, and here's where I'm at:

• temps looked largely the same; Engine coolant and trans fluid in the mid 80s, motor and generator coil temps tracking 5-20ºC above trans fluid, depending on load.
• no pattern observed with coil temps in the MANY times I got the wrench light.
• MECT ranged from 37-43ºC
• the Generator Phase temp was ALL OVER THE PLACE *more below
• Motor phase temp stayed a few degrees above MECT
• Battery temps very constant. high temps ranged from 33-41ºC (106ºF as displayed)
• voltage and state-of-charge also stayed very close to 50% and 330ish V - no signs of low voltage/SoC, or any excess charging demand



Monitoring temps en route via ForScan Lite on android


From what I could monitor, the clear standout was Generator Phase temp (G_PHTEMP), which I understand to be the portion of the inverter controlling the Motor/generator on the ENGINE side of the planetary gear in the transaxle...the one frequently sending current directly to the other motor to determine the effective output ratio. Its reported temp would go from mid 40s (just above the coolant temp in the MECS loop) to mid 80, a full 40ºC hotter, VERY quickly under load, then recover almost instantly. I watched it drop from 85-60ºC within 3 seconds of letting off the accelerator. This seems...dubious. Furthermore, that Gen Phase Temp was in the mid 80s every time the wrench lit up, and while it varied some (I saw as low as 83 and as high as 89 at time of DTC) it was almost always exactly 46ºC above the coolant...making me wonder if it isn't a fixed threshold that's tripping the wrench light, but a differential. By the end of day 2, I was curious and pissed off enough to just keep my foot in it to see if that phase temp would rise any farther, and I don't believe I ever saw higher than 90ºC during aggressive climb.


This leaves me with some significant gaps I could really use help filling:

• Is this temp swing normal for the Gen Phase?
• what components are in there (I assume a bunch of beefy FETs) and what temps are they sensitive to/rated for? i.e. What is too hot?
• How is that temp reading determined? is it measured or derived?

If you have the ability to run ForScan (or other monitoring software) while driving, PLEASE take note of the following and post here WHETHER OR NOT you are having this issue:

• MECT (Motor electronics coolant temp)
• G_PHTEMP (generator phase temp) - coasting and under load
• M_PHTEMP (motor phase temp)
• optionally TFT, G_CLTEMP, M_CLTEMP ....both motor/generators and the fluid they're in
•• note, if you can, how quickly your G_PHTEMP value changes
••• if you are able to reproduce the P0A7C code on demand, ABSOLUTELY note these temps as closely as possible to the instant the lamp illuminates, given how quickly the G_PHTEMP value changes

I want to know what the normal ranges are for these, if 85-90ºC is concerning, and if my temps and rate of change are plausible. IF (and that's a big if) this is the responsible component, I see basically 2 possibilities:

1. there is a very particular cooling issue (flow restriction, contact problem) that is allowing specific components to heat up, but cool back down very quickly
2. The temp being reported (and subsequent warning) is false

Or I suppose 3. the reported temp is correct but not enough of a concern to warrant teardown or replacement of the inverter. The only way I can think to establish the nature of a real, physical cooling problem at this point would be to peel the cover off that inverter housing and see WTF is going on in there (i.e. more of that yellow scale, some other blockage, etc).....and I think that's more job than I'm up for.

On the plus side, I managed a trip average of 29.4 mpg with a lot of 70mph+ freeway speed....good for the AWD with AT tires and 3 better than the last time I made this trip. Not sure what did that, but new coolant, trans fluid, PTU gear oil, spark plugs, air filter, recharged A/C....take your pick. Nonetheless, I am well and truly tired of this wrench light. I live in CA; can't be walking on eggshells every time it's above 80º outside

 

Man... you're in deep. Unfortunately, I have nothing direct to offer; however, I think you might be on the right track for some kind of obstruction in the inverter cooling passages. This is not uncommon on high mileage Prius with original inverter coolant.

 

yeah...well I'm kinda straddling the fence between wanting to solve this for all the higher mileage FEHs and just trading for a PHEV already.

The thing I don't like about blaming blockage is the extraordinarily fast recovery time I observed on the way up here. 20 degree drop in 3 seconds seems...great. Doesn't suggest to me degraded cooling performance....and in fact that's a steep enough delta that it does more to make me doubt the accuracy of the PID.

I would really like to know what other people are seeing; having some temp numbers from FEHs with and without this issue would be ideal. All I can say from my own experience is that the same southern OR pass responsible for several wrench lights this trip was apparently no problem for this car 6 years ago with the RPM pegged at 4k because the friend that was driving that stretch just left cruise control on the whole time ....so SOMETHING has changed.

 

​​​​​​Spitballing here .. Should we also look into the catalytic converter as another source that is cooking transmission and ptu? I think it sits in between both trans and ptu. A clogged cat increases in temperature over time and could be another heat source soaking into mec loop.

I to myself have been having similar issues.

Replaced mecs pump, sensor, and coolant flush.



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