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Ford PCM controlled alternator testing

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12 Aug 2017 12:46 - 12 Aug 2017 12:48 #11857 by Tyler
Another thread in the Repair section got me thinking about testing these GENCOM/GENMON systems. I had been digging around in Mitchell over lunch one day, bored, and found a fun little tidbit on how Ford suggests testing the wiring on these systems. :cheer:



For anyone not familiar, here's some theory/operation:



GENCOM is the PCM's command to the alternator on how hard to work. GENMON is the alternators feedback to the PCM on how hard it's working. Normally, the signals look like this on a labscope. Green is GENCOM, yellow is GENMON.



It's fun to look at the detail, but it actually makes more sense if you zoom out. This is a 30 second capture:



The quick GENCOM blips come once every five seconds when there's no load change commands to the alternator. That big green blip in the middle is a commanded load change when I turned a bunch of accessories on (headlights, blower motor, wipers).

There are associated scan data PIDs on the OEM side, which are VERY useful on their own. GEN_MON is what it sounds like, while GENFDC% is what we've been calling GENCOM.



Normally, the GENFDC% is zero, except for when an output change is commanded. Then you get the little spike in the graph, which correlates to the big square wave burst on the green trace in the scope capture. A commanded output change, usually in response to changing battery voltage.

So, you've got a charging problem on one of these systems, but no scope. :-( Or you're not familiar enough with the waveforms to make a call. What do you do? Ohm check the wires to the PCM? Ain't nobody got time for that! :lol:

Straight from Ford: Disconnect the alternator, and short the GENCOM and GENMON together at the connector. :blink: Sounds crazy, right? But according to them, if you now start the engine and see the GEN_MON and GENFDC% PIDs mirror each other, then the wiring is GOOD. I had to try it. :silly:



Sure enough! :cheer: I'm all about scoping signals, but doing this test was almost faster. Would DEFINITELY be faster on applications where the alternator is out in the open, i.e. 4.6/5.4 engines. FYI, this same test can be done with an ELM327 adapter and Forscan Lite on your smartphone. ;-)
Last edit: 12 Aug 2017 12:48 by Tyler.
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12 Aug 2017 21:01 - 12 Aug 2017 21:02 #11892 by Dylan
Replied by Dylan on topic Ford PCM controlled alternator testing
Tyler wrote:

Straight from Ford: Disconnect the alternator, and short the GENCOM and GENMON together at the connector. :blink: Sounds crazy, right? But according to them, if you now start the engine and see the GEN_MON and GENFDC% PIDs mirror each other, then the wiring is GOOD. I had to try it. :silly:


Well I'm glad you tried. Cause I was thinking...what?? :huh: :huh:
Last edit: 12 Aug 2017 21:02 by Dylan.

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12 Aug 2017 22:44 #11899 by Amich28
Replied by Amich28 on topic Ford PCM controlled alternator testing
That will save a lot of time during diag. thanks for pointing that out. Now I want to try the same test on my 13 escape. Let you know. my scope/scanner is at work will have to wait tell Monday. Did you hear the test from Ford hotline? They told me a different way to do a test but it was years ago and never had a case study like that one freestyle before. turns out my gencom wire was bad.

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13 Aug 2017 02:00 #11905 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic Ford PCM controlled alternator testing
I had a good link on these but I lost however I found this from the Hella Link to Hella PDF

The interesting part is in this screen cap

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



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13 Aug 2017 13:50 #11920 by Tyler
Replied by Tyler on topic Ford PCM controlled alternator testing

Well I'm glad you tried. Cause I was thinking...what?? :huh: :huh:


Right? :lol: 'Cause we've been told forever not to short stuff on communication wires.

That will save a lot of time during diag. thanks for pointing that out. Now I want to try the same test on my 13 escape. Let you know. my scope/scanner is at work will have to wait tell Monday. Did you hear the test from Ford hotline? They told me a different way to do a test but it was years ago and never had a case study like that one freestyle before. turns out my gencom wire was bad.


Please do let us know what you find! I got this from Mitchell, after diagnosing an F-150 with a bad alternator that wasn't setting codes or turning the dash light on. :silly: Wanted to figure out why the PCM didn't catch on to the problem, when I found this step in the flow chart.

I'm also interested in doing some more experimenting with this system, as far as creating wiring faults. I can totally see why a fault on the GENCOM wire would cause issues, like on your Freestyle, but what about the GENMON? More experimenting. :cheer:

I had a good link on these but I lost however I found this from the Hella Link to Hella PDF


Nice link, sir. B) Do they have more technical documents like that for other systems?

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13 Aug 2017 18:29 #11928 by cheryl hartkorn
Replied by cheryl hartkorn on topic Ford PCM controlled alternator testing

mr eric o has a good one
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27 Aug 2019 23:28 #33194 by joe.babaian
Replied by joe.babaian on topic Ford PCM controlled alternator testing


mr eric o has a good one


Eric's video helped me a lot to understand some of what I am seeing with my 07 Escape 3.0 charging insanity. I'll have to thank him for his video!

Here's my issue -

I am having this experience with overvoltage output from the alt. Upon an overvolt episode, I can disconnect the B+, run the car on battery for a few minutes, then reconnect and the charging resumes to the normal 13.5-14.2 range. After a while (hours to days), boom, back to > 17v. Rinse and repeat. This is the 3rd alternator. Here is the timeline. The job, as you know, is a BEAST, but hey, I am getting good at it. :)

2006-2017 Original Alternator
Late 2017 Failure with high voltage output >17v. Replaced with AC Delco branded unit.
June 2019 Failure with high voltage output >17v. Replaced with warranty unit (also AC Delco).
Aug 2019 Failure again with high output. I go crazy, being to research. Find that when this occurs, if I pull the B+ wire from the fuse block (effectively disconnecting the alternator output), the alternator output drops to 0 (checked with DVM on the now disconnected B+ wire). I drive the car or just run it for a few minutes, then reconnect the B+ wire. The system then charges at normal rates for 1 to 4 days of normal driving. Then it happens again.

The time frame is odd! Ford does specify a pinpoint test to pull the three-wire plug (including the volt sense, and two PCM comm connections) to see if the alt goes into self-excite mode. After that, the next step is the PCM.

I am tempted to get a 'scope add on for my phone, but as Eric's video showed, I might actually be looking at a "simple" alt failure (again). Based on the background of the good folks here, I am hoping someone already has the insight to say "no way, the PCM wouldn't do that" or something along those lines. Even on my best day, swapping this alt out is a four-hour ordeal on the 07 Escape. I hate to guess and I know someone has some further insight.

I am just searching for any further insight. I have several choices.

- One more new alt (but having three fail with the same symptom seems unlikely, but the internal regulator might be crappy in all)
- New PCM
- Remove harness and use the default mode
- Other

Humble thanks.

Joe

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28 Aug 2019 10:09 #33208 by Tyler
Replied by Tyler on topic Ford PCM controlled alternator testing
Hey Joe! Any trouble codes stored when it start overcharging? P0621 or P0622 are what I usually see with alternator failures on these Fords.

I know it's a huge pain, but I think the smoking gun would be to reach down and unplug the alternator regulator connector while it's overcharging. I've done this job, and I know it's not easy. :( Especially given how the connector latches in. I can never get then unhooked without a pocket screwdriver or pick.

As you noted, the other easy option would be to unplug the alternator and make darn sure it goes into default mode before you take off.

TBH, I've never seen a PCM or wiring fault cause this problem. It's always come down to bad replacement alternators. 100% new Motorcraft is the only reliable option on these, IMO. Everything else is a roll of the dice.

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28 Aug 2019 14:40 #33221 by Landroverman1958
Replied by Landroverman1958 on topic Ford PCM controlled alternator testing
Just a thought from the UK, on single line lin bus systems,disconnect the single wire for the lin,back probe using breakout connector connect to scope,look for a signal coming from the pcm,also check battery voltage should output at a predetermind set voltage which is the alternators default,reconnect connector now backprobed to observe signal from the alternator,lin activity hould vary according to varing load,
This applies to Petrol and Diesel single wire lin bus systems,as recomended by uncle Henry.

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18 Nov 2019 10:18 #35098 by vstar650cl
Replied by vstar650cl on topic Ford PCM controlled alternator testing
I'm a Nissan guy and don't see many Fords to play with, but these alternators are a common issue when guys swap BT6 Cummins engines into E/F series trucks to replace the doggy PowerStrokes. In the top scope shot it seems that the pulse widths are equal when the alternator is in stable operation, so it looks to me as if a box the "echoes" the last commanded pulse width on GENCOM back to the PCM on GENMON should turn off the alternator light. However, your shots don't show if the GENMON pulse widths follow GENCOM after a load change or how quickly they stabilize at a new level. Did you happen to notice either of those things?

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01 Jan 2021 15:36 #45606 by Sadsixty1
Replied by Sadsixty1 on topic Ford PCM controlled alternator testing
I was checking an 07 focus for charging issues, i'm getting battery voltage at the gencom wire with the pigtail plugged into the alternator this seems like a short inside the alternator (remanufactured). as far as i understand the gencom and genmon are signal wires and shouldn't see battery voltage, is that correct?

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02 Jan 2021 09:55 #45613 by Tyler
Replied by Tyler on topic Ford PCM controlled alternator testing

I was checking an 07 focus for charging issues, i'm getting battery voltage at the gencom wire with the pigtail plugged into the alternator this seems like a short inside the alternator (remanufactured). as far as i understand the gencom and genmon are signal wires and shouldn't see battery voltage, is that correct?


GENCOM shouldn't see battery voltage, no. GENMON might show closed to battery voltage if the alternator isn't pulsing the bias to ground like it should.

If you disconnect the alternator and start the engine, what voltages do you see on GENCOM and GENMON?

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02 Jan 2021 11:04 #45614 by Matt T
Replied by Matt T on topic Ford PCM controlled alternator testing

I was checking an 07 focus for charging issues, i'm getting battery voltage at the gencom wire with the pigtail plugged into the alternator this seems like a short inside the alternator (remanufactured). as far as i understand the gencom and genmon are signal wires and shouldn't see battery voltage, is that correct?


Was this KOER? GENCOM and GENMON are both battery voltage signals so that may be normal unless the engine is running. And if Tyler is correct about GENMON being supplied bias by the PCM then the alternator should be supplying bias to GENCOM. So even KOER it doesn't really point to a fault with the alternator.

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05 Jan 2021 09:36 - 05 Jan 2021 09:36 #45709 by Sadsixty1
Replied by Sadsixty1 on topic Ford PCM controlled alternator testing
hello, i was getting bat voltage at the gencom with key off engine off (pigtail plugged in). i checked voltage at the com wires with the pigtail unplugged and engine running and i get 9.3 v on genmon and about .07 volts at gencom and bat. voltage at the red wire. thank you.
Last edit: 05 Jan 2021 09:36 by Sadsixty1.

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05 Jan 2021 13:17 #45713 by Tyler
Replied by Tyler on topic Ford PCM controlled alternator testing

hello, i was getting bat voltage at the gencom with key off engine off (pigtail plugged in). i checked voltage at the com wires with the pigtail unplugged and engine running and i get 9.3 v on genmon and about .07 volts at gencom and bat. voltage at the red wire. thank you.


Those values sound right to me. GENMON has integrity from the PCM, and GENCOM isn't shorted to voltage. Methinks that reman alt sucks, or it's the wrong part for the application. ;)

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15 Jan 2021 19:38 #45909 by bcwrench
Replied by bcwrench on topic Ford PCM controlled alternator testing
Hello all

I'm a newbee and arrived at this thread because I have a wonderful issue with one of these systems on my personal car. The system seems to control with a heavy load of lights, rear defog, wipers but if you turn the blower on (even by itself) it is a no go.

I closed my business in 2000 so my equipment is minimal and my red brick is done at 1999. I did find some cartridges on Ebay but won't have them for a while. Just in case I picked up an ELM327 USB so I can at least look at the As-built data in the PCM to see what the Alt capacity is.

Some of my history goes back to the early days of iATN when I wrote my testing article on using vacuum waveform patterns for diagnostics, that was 1993. I also used to be on For Techs Only forum of Compuserve where John Anello was just a youngin starting out. I also wrote diagnostic articles here in Canada for Service Station & Garage Management in the 90's.

There is lots of good info here and I thought I would add some additional insight if I could. I am fabricating a breakout harness for my 2002 Focus 2.0L so that I can make access a little easier. It will allow me to also switch the 3-in connector in and out of the circuit and has test points for a scope and an AWG.

I found some info from electrical sites that you may find helpful in understanding the systems.

To add to all the good info here, some of the info I found on line can be found here

www.google.com/url?sa=t&rct=j&q=&esrc=s&...R9MOok4WFfCeuJqc9twI

195.125.241.148/support/flyer/fl0406.htm

shop.waiglobal.com.mx/VRC101-31.html

Since these are the people that work directly in the rebuild industry they have some good tips.
Like the fact that since the system uses GENCOM from 5-95% each percentage point is 0.045V.
The field amperage rating is 8A.
The regulator will function in its primary default voltage set point mode (14.03V) if only system reference voltage is present; (when no PCM generated control signal is provided).
The frequency of the GENMON & GENCOM are both 125Hz and the GENMON has an amplitude of 14V and represents the field current but the waveform is actually inverted when viewed with a scope. Since the field current rating is 8A one could do a rough Est if what the field is drawing.

The kicker I like, is they show a tester (VRC101-31) that is available for these systems and from the outside it just appears to be an arbitrary waveform generator with a switch for 4 test settings.

Guess I am going to have fun firing up my 2 DSO's and a 2-chl CRT scope and one of my AWGs and see what happens.

There has to be a day when we get this stuff figured out and ashcan the swagging for good.

Thanks to Tyler for getting my "I gotta know" going.
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05 Oct 2021 14:32 - 06 Oct 2021 12:59 #51882 by DennisH
Replied by DennisH on topic Ford PCM controlled alternator testing
Moved question to Repair Questions section
www.scannerdanner.com/forum/post-your-re...rt-charge-issue.html

 
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