Transmission Destroyed By Python 871xp

My apologies to “Installer_mss.” Even a cursory investigation reveals that it’s unlikely that s/he just entered the arena yesterday to throw a few red herrings (thanks to the12volt.com site features for making the ‘background’ check easy). Yes, some of this discussion is fishy and I’m not ruling out the possibility that you’re running interference, but it’s more likely that this is your independent (perhaps uniformed) opinion. So my apology is both appropriate and sincere.

I’m not a transmission “expert.” Remember that last week I didn’t even know that modern automatic transmissions were electronically controlled. But it seems that I already know quite a bit more about this topic than some of the posters who don’t see how an electronic malfunction in the PCM could cause mechanical damage. I guess this is becoming somewhat frustrating, and when people reassert this argument it looks to me like they are trying to confuse the issue.

OK, repeat after me: “all on, all off, all on, all off; stupid driver depresses accelerator pedal, stupid driver depresses accelerator pedal; snap, crackle, pop.” (See yesterday’s posts for more details if this is not clear.) The engine revs up in lower gear and slams into high gear sending tremendous torque stress through the power train. The process repeats itself while chugging down the street as the driver continues to depress the accelerator pedal thinking it’s an electrical problem and that the higher fuel feed is keeping the engine running which it was but not for the presumed reason. (In actuality, it was increasing the engine’s torque output to contend with the inappropriately low gear ratio when the transmission was in the high-gear state.). Yes, it’s ultimately a “stupid human trick” instigated by an electrical malfunction.

I have a pretty good working hypothesis of how this may have happened. Now, the three probable causes need to be investigated: (1) improper installation, (2) defective alarm control module (AKA “brain”), and (3) spontaneous failure of the PCM. The preliminary analysis suggests that the last cause is very unlikely for two reasons. First, the failure rate of the Ford PCM appears to be very low (but this needs confirmation by examining more data), and second, a spontaneous failure of the PCM is extremely unlikely to correlate with work on the vehicle’s electrical system (The correlation is high even if the causation is not clearly established; if you need help understanding this, please wiki the word “correlation.”).

I find it very difficult to understand how the photographs of the transmission damage in this case could indicate how the damage occurred. The gears are still on their shafts, there are no burn marks around the bearings, and the ATF was very clean. The transmission photographs were simply posted to illustrate the extent of the damage not the probable cause. Sorry “Installer_mss,” but this is where your statement “he said . . . looking at the picks that it was solely mechanical failure probably from a loose/broken part . . .” appears to be BS. I can post a picture of the PCM module if you like, but there is nothing obviously wrong with its outside appearance. Perhaps your friend has x-ray vision that works over the Internet OK, another cheap shot just after I apologized for the first one, but really, the only thing apparent in the photographs is the damage to the gear teeth and the metal fragments found in the transmission pan (and of course, the very clean residual ATF suggesting no pre-existing mechanical problem).

The problem with expecting an obvious “smoking gun” is that the exact cause could have been intermittent or very brief, such as a sudden voltage drop or spike in the tach output which sent the PCM into a spasm. And if the PCM was totally fried (i.e., frozen into a stable state), the transmission should have locked into a single gear not shifted abruptly between gears.

FYI: I suspected the tach line earlier. From my limited knowledge, this appears to be a fairly high voltage (for automotive circuits) AC line. This type of signal is a common cause (from my experience) of interference with 12 VDC digital lines. In my work the EMF alone is enough to reeve havoc with the 12 VDC digital signals, thus requiring shielding of the lower voltage lines. The simpler possible cause involving inadvertently grounding the tach line is much easier to investigate and is perhaps more likely to happen.

The quest for truth and justice continues . . .

The tach signal is NOT an AC signal (the most common installer misconception).  The signal, is in fact, DC, but it is a pulsing DC signal (usually a squarewave).  Because the DC voltage is frequency based, DC voltage meters will NOT read the signal.  If you switch the meter to AC you can detect differences in the voltage potential based on the difference in frequency (the principle behind alternating currrent).  The AC readings you get on your meter while reading a tach signal are NOT accurate by any means, and actually represent absolutely nothing.  All it does is shows the installer that the frequency on the wire changes when the engine revs, thus making it a good choice for a tach signal.

Keep in mind, also, that the tach signal is an INPUT, and most likely a high impedence one.  The alarm module never outputs any kind of signal to this line.  If it is a tach related problem, as stated above, it is most likely related to the tach wire being pinched or somehow grounded out (something that removing the power to the alarm wouldn't fix).

Thanks again for another lesson, Kevin. I am learning by picking up what I can from installation manuals and from online discussions and of course from this discussion. Lots of people appear to have this wrong. I’ve seen reference to 50 to 80 VAC signals, but what they really mean to indicate is that you need to detect the signal change using the AC setting on a DMM. For the record, I was the only one that I recall who mistakenly referred to the tach signal as being an AC signal on this thread. FYI: I have used monophasic cathodal square-wave pulses for my brain stimulation work for years. I measure with an oscilloscope; the pulses are typically 20 to 200 pps (as you probably know, I actually measure pulse period directly not frequency) with a 100 to 300 usec pulse width at 50 to 300 uA. I also measure impedance and have to detect any high frequency noise on the signal in real time in my applications.

I’m willing to bet that the engineers at DSI are real smart people and that they design their devices with high impedance inputs. So, I too am particularly interested in the possibility of an inadvertently grounded tach signal from the installation, but I don’t rule out a component failure with the DSI module. The fact that the alarm is functioning quasi-normally with the ground disconnected indicates that the engineers have not isolated their system components as well might be expected. I’m sure this causes no problems with normal automotive work, but this can create major problems with the circuits that I have used in my research.

Now this brings up another question: I’m guessing that the same tach signal that’s used in the installation also ‘feeds’ the automatic transmission solenoids or that the same tach signal is fed to the PCM. (I could envision designing a system to work either way.) What is the source of the “tach” signal, and does it ‘feed’ to the PCM or to the automatic transmission directly?

Update: I got the van back from AAMCO today. It seems to be running smoothly again. I’m going out to wash the Arizona, New Mexico, Texas, and Oklahoma bugs off it now. Yes, I picked up a few bugs from Missouri, Illinois, Indiana, Ohio, and New York too, but the Southwestern bugs seem to be the juiciest.

no offense taken...and i haven't worked for CC for a few years and don't really care if they get sued to bankruptcy.  i just threw in my 2 cents to try to help solve this problem.  those were my opinions based on past experience from both myself and my mechanic friend, and merely stated what i thought MIGHT have occured.  hopefully ferret nailed it and you can get to the bottom of this.  i DO believe in people being held accountable for their mistakes (even if it were myself).  good luck

P.S.  if you bring it down here to Texas, I'LL take a look at it...lol

My thoughts keep coming back to either tach or possibly a whisker of cable being in the wrong place, but I reject tach because the starter will grind only on remote start. one look by an experienced indie installer or company should give an overview of the installation 's general quality.

If gearbox ECU is fried surely there won't be physical damage because unit (ie mechanical/hydraulic part) is in fail-safe mode?