Something to consider....

Jim Warman · December 24, 2006

Something to consider is the weather.....

I have egg all over my face and I can't help it....

04 F150.... runs bad and stalls.... CEL is "on". P0190, P0191 and P1235.

PC/ED tells us to disregard the first two if we have a P1235... no problem - we start at KB23ish. This has us checking the FPC circuit between the PCM and the FPDM. We start with the continuity test which returned zero ohms on the line in question (did I mention that the connector pin layout at the FPDM is 2 - 3 - 1 - 6 - 4 - 5? Nice going, Ford). Next step is to check for a short to power.... I had .5 volts - I thought this tasted bad but the PC/ED states anything under 1 volt is good. Unplug C3137 (on the frame rail under the drivers door and I got no volts.... I'm worried, but the PC/ED is leading me by the nose, I'm hooked on trouble). A couple more test steps and we get 7 volts returning from the FPDM - spec is 4.5 to 5.5. Replaced the FPDM, all is well and release the truck (to the wash bay).

The customer isn't gone for more than a couple of hours and he's back on the doorstep. P1235 and runs like crap.

Repeat PPT and we have 4K ohms on the FPC circuit. Disconnect C3137 and measure from there to the FPDM - 4K.... just for sh*ts and giggles, I touched the other connectors in the top row of that connector.... and got readings between 2K and 5K on all of them to the one pin at the FPDM. The only difference is that now the harness is wet inside the low point in front of the rear spring..... The harness is undamaged on the outside but opening it up shows some insulation concerns for no apparent reason.... The FPC circuit was hanging by a strand with black wire from nearly one end to the other.

Once again, I have had it driven home that we should decide in our own minds what the PC/ED wants us to test and treat our results as we see fit rather than "listening" to some of the arbitrary numbers that the PC/ED throws at us.

In this case, the truck had sat inside overnight, allowing the harness to dry out.... this skewed my test results as far as I can see. In my mind, I saw a half volt as being too much, but, hey... the PC/ED says that's good to go....

In a similar case, I have an Excursion that has the batteries go dead after 4 or 5 days.... The ICM is sending 900 millivolts to the alternator on the indicator wire and this will turn the alternator on randomly. A new cluster is on the way for that and a new 14405 harness for the 04.

Rule of thumb... (and I should have known better) - look at what the PC/ED wants you to check and act appropriately... both in test methods and in expected results.... The PC/ED is a guide (often, a poor one) and we need to second guess the engineers nearly every step of the way.

Have a safe and merry Xmas....

Northman · December 24, 2006

I try and make it a point to disregard the service manual when it tell me to replace a part rather I use it to get a better understanding of the system I'm working on. early on I realized following the ppt exactly equals Comback.

I also seldom use my ohm meter to test a circuit I found it best to open each end feed power to one side and connect a headlamp to the other. one strand od wire can show little resistance but can't flow the amps needed to light up a highbeam.

I still find myself on occaision following the ppt to the letter then make it to the parts room to order a component and while waiting for the "parts natzi" I get a second burst of what ifs cross my mind. and I go back and test my tests to see if they are even valid tests to perform.

Jim Warman · December 24, 2006

In this case, FPC (as well as FPM) are part of the FPDM "network".... While I am likely going to start load testing these types of circuits in the future, we need to remember that these circuits are designed for reference voltage at very low current levels.

That half volt reading I go should have been my big hook but on went the blinders and the 1 volt allowance took me past something I knew I should have looked closer at (FWIW, we see many concerns involving C3137 and I had separated that and inspected for corrosion before I even set out on the PPT).

What is interesting in this case is the vast difference in readings between a "dry" harness and a "wet" harness. . Not unexpected, in retrospect, given the type of failure... but something, I'm sure, that few of us have considered.

I had driven the unit the previous day and verified the customer concern.... the truck sat in the shop overnight and I proceded with testing the next day *without verifying that the concern still existed*....

Likewise, my "pet" Excursion with the parasitic draw.... There is always 900 millivolts on that one wire but the truck needs to be cold for it to have an effect.... Bring it in the shop and my PD is always 40 milliamps. However, if I freeze my can off testing it outside, I can watch it spike up to 4 amps and then watch it drop to about 170 milliamps (presumably, the 4 amps warms the alternator which "turns it off" and the cycle repeats).

Still a third case.... an 06 Expedition.... left the customer high and dry twice. On a cold start, the alternator would not charge. Of course the heated seats, heater going full blast, rear defogger and all the lights (dawn is about 8AM or so at this time of year) would overtax the battery. Get it into the shop and it charges like a champ (aside from a P0622 in memory). PPTs showed nothing wrong (even checking for spread pins) but I proved it out by apply a heat gun to the alternator (yes, dammit - outside at -30C) and having it start charging almost immediately (or as soon as the PCM turned it on).

We have progressed to the point where replacing some high end audio units has become a PMI procedure... we ain't mechanics any more....

Not only do we have to decide what kind of a test is going to be appropriate but we are at the point where we may need to decide when and where we will conduct the test. I have two that I might never have found the concerns on had I not decided to get cold and another that wouldn't have come back as long as the harness stayed dry....

As it stands, this is the sort of stuff we should all be discussing... what we've seen, what we've had to do to test our system in order to find a concern with it.... Even if someone elses situation is nothing more than food for thought.

Merry Xmas, all.....

Bruce Amacker · December 28, 2006

Something to consider....

I'm wondering if anyone else has seen the weird ones I have- like the electrical insulation becoming conductive on primary wire. I have seen this one three times, all on Fords, and am still baffled by it.

Case #1, F-750 with an Allison WTEC and a repeat VSS code, defaults to 2nd. New VSS, check for a loose reluctor, inspect the harness closely, etc, all OK. Cust reports it only happens in snowy (salty) weather. Replacing the VSS twisted pair to the TCM fixed it, and a close inspection of the wires replaced showed no fault.

Case #2 was when I stopped by at a friend's shop, a mid 90's F-model gasser. He says, "Hey, you wanna see something weird?" I'm always in for weird, so he shows me the O2 wire, which is pulled from it's harness (but not peeled to copper). He wraps one hand around the insulated wire and touches the battery hot and ground while we watch the scan tool, and he can force it rich and lean by doing this. This would be normal if you were touching the O2 terminal, but this was through the insulation.

Case #3 was a '97 F with a 7.3. Cust complains of surging with the 4 ways on, and only when it's hot. I confirm the problem and see a CMP code stored. Install a BOB and put a scope on the CMP line to have it pulse 12v high whenever the 4 ways illuminate, which makes the engine purr, die, purr, die, etc, as the 4 ways blink. I'm thinking it's a problem in the cluster, so I pull the dash to find no difference. I opened the CMP harness from the PCM to the CMP, separated the green wire and wrapped it with a piece of 1/4" polyloom, and that fixed it. My conclusion? When the harness got hot by the engine, it bled voltage from the front turn signal wires to the CMP circuit, interrupting the CMP signal. All I did to fix it was to put polyloom on the CMP wire. I saw that guy regularly for years and he said the problem never returned. In retrospect, it probably should have gotten a new harness.....

I talked to guy who worked at one time in a plant that made primary wire. He told this was no surprise to him at all, that when they were making the mix for the insulation they would sometimes run out of raw material. "Put another bag of clay in it" was what the foreman would say as the recipe was being put together.

Sometimes I feel like I'm walking through a mine field.

Happy New Year!

Jim Warman · December 28, 2006

Interestig point on the insulation, Bruce... When I decided to go for a harness, I had opened the old hraness in what appeared to be the affected area.... One wire had rotted through (IIRC, it was the FPC circuit to the FPDM - Gad, doncha love acronyms)but I couldn't see any damage to other conductors in the immediate vicinity. This is a retail job so I might be able to keep the old harness for further scrutiny....

Jim Warman · December 29, 2006

OK... the 04 F150 is finally gone. Close inspection of the now dry harness indicated no concerns.... I ohmed the wires that, at one point, were "related" to each other through some high resistance readings (indicating some sort of continuity) and we were once again electrically "sound". This adds to Bruces findings on primary wire insulation.

My dance card is extremely full this week so we had an apprentice replace the harness for me. And we immediately had a no start on our hands. On Demand code for an open fuel pump circuit (P1237, IIRC). Apprentice states he had no problems fitting the new harness....

So - off to the PPT for P1237.... (bearing in mind that I have seen replacement harnesses with mis-assigned pins). At one point we are directed to check FPRTN for a short to power at the FPDM plug..... 800mV - WTF? Had the apprentice unplug the fuel pump.... 0V... OK, I'm about to get confused. Had the youngster step aside and did some furbling while I thought about where I need to go...

Hmmm.... fuel pump connector wont seat - can't hear that little "snick" from the lock tab - but it looks all the way in but pulls off easy. Up on top of the tank with a mirror and I see he's managed to push some wet sand (and probably some salt) into the fuel pump connector. Clean it out and get the plug seated satisfactorily (initially I thought that he'd chipped off the little lock tab off the FP connection)... Check FPRTN at the FPDM plug... 0V.... OK, the old ticker can slow down a bit.... plug in the FPDM and turn on the key... we have fuel pump and our no start is fixed....

This concern could have had me tearing open a brand new harness but I have gained a new appreciation for milliVolts - more old notions having to be rethought....

I love this job...

ablokzyl · January 15, 2007

Had an early to mid 90's full size Bronco with a rough running condition. Found the o2 sensor on the right bank reading full rich. Could not drive it lean with a vacuum leak so i decided that it needed an o2 sensor. R&R the sensor, start it up and its still running full rich on bank one. Time to get the PCED out. Follow the PPT and where does it lead me but directly to PCM replacment. It didnt feel right so I did some wire chasing on my own and found that the o2 sensor wires run directly behind the battery and the battery had qutie a bit of corrosin on it. The o2 sensor was pulling stray voltage from the battery acid and it was telling the PCM that it was rich. Cleaned the battery with baking soda and also cleaned out the harness and it ran ok.

Jim Warman · January 23, 2007

I gotta dredge up this old subject - that Excursion turned nasty on me....

After a long wait for the cluster (and another long wait for the time to put it in - used truck...), I still have 900 mV on circuit 904....

We'll keep it short and sweet - here's the "gotcha".

This thing has a remote start - and I removed the module very early in the going. What I didn't consider is that this truck has PATS - but what can go wrong with the PATS bypass, right? The remote start module powers it up when needed and it's output isn't really "coupled" to anything.

As I'm staring at my nemesis, scratching my head and wondering if I should use a gun, a rope or pills to end my sorry existance, I'm probing around with a high impedance test light.... and there is 12 volts at the PATS bypass module!!!!! Some idiot has wired this up direct to B+

Unplug the module and there is 0 volts on ckt 904 - plug it back in and my 900 mV is back. The PATS signal is waking up something in the IC module and that "something" is powering up the alternator.