Before I put a new PCM in this thing..

DamageINC · May 27, 2010

..I just wanna know if anyone else here has seen this sort of thing.

Got a '97 E-350 6.8L, crank-no-start. 208k miles. Turn the key on and the fuel Pump relay cycles about 4 times a second, not a random chatter but a perfectly timed cycle. You can hear the fuel pump in the tank cycling on and off with the relay as well.

Just as a quickite I swap relays, same problem. Go to check for codes - no response from PCM. All fuses good. Startng to think that I have a possible bad ground or weak power supply to the PCM, so I load-test (with a 4amp Fog Light bulb) every single power and ground circuit to and from the PCM. 3 times. Same results every time, all circuits in good shape.

Take a large jumper cable and jump battery power straight to the underhood fuse box, just to make sure that the fuse panel is getting a good solid 12v source - problem still exists. Found a pretty loose ground connection by the drivers side headlight, so I clean it and tighten it up - no change.

For grins, I have a new PCM power diode in my box and stuff it in - no change. Also swapped PCM power relay, with no change.

Basically, everything I've ever learned about electronics is telling me that this PCM is bad. But, as we all know, FORD PCM's don't usually fail. I've *reluctantly* diagnosed 4 bad FORD PCM's in my life, and twice I was incorrect, so I'm kinda nervous about selling this. But I can't seem to think of anything else that could be causing this..

We did have some pretty violent storms recently and I'm wondering if maybe we've had some water intrusion, but the connector appears fine. I've not actually pulled the PCM out yet.

Anyone got any input here?

Dave

DamageINC · May 27, 2010

*UPDATE*

I got lucky and found that we have an identical model year E-van here with a 6.8- both PCM's have the same part number and tear tag. So - A swapping I go!!

...and the clicking is still there. No start. No communication.

WTF.

Going to start disconnecting sensors one-by-one in hopes that I can get something to change here.. gonna be bald by the end of the day, I think.

Dave

Bruce Amacker · May 27, 2010

Do you have a 104 pin breakout box, a DVOM and a schematic? That's where you're heading. You need to check powers and grounds on the PCM while they're active and loaded. In my experience, a clicking relay has always been a weak power supply (Yea, I know) but I wouldn't rule out a shorted sensor/harness pulling the VREF down.

Good Luck!

Posted Image

DamageINC · May 28, 2010

Unfortunately Bruce, the BOB is not something I think I can get my hands on, lol, believe me that's the 1st thing I thought of when I found out that I'd not be needing a PCM for this. I'm not looking forward to backprobing a dozen wires in use at the PCM but it might be what I have to do.

I'm just amazed that my fog light bulb didn't reveal any sign of a problem. The way I checked the power wires was by turning the key on, plugging a bulb terminal (via a wire) into the PCM connectors respective pin/terminal, and then the other bulb terminal would get jumped straight to battery ground. So I was using the supplied 12V from the battery, through the entire circuit to the PCM, and then my own ground. And the grounds were checked the same way, just with the bulb being attached to the + post on the battery instead.

I actually love these kinda problems though, I just wish I had the BOB to make life easier at this point.

I think I'm just going to temporarily overlay B+ straight to the 12v inputs on the PCM and see if the problem continues. If it does, then I can be somewhat confident that the issue isn't a supply voltage/current problem.

Dave

Bruce Amacker · May 28, 2010

You work for a frigging Ford dealer and they don't have a BOB? WTF?

I'll send you one of mine if you pay shipping both ways.

Posted Image

DamageINC · May 28, 2010

Haha, I USED to work for a Ford dealer. Now I'm the lead tech at an indie, hehehe.

(Does this make me a bad person?)

Dave

Jeff_ · May 29, 2010

(Does this make me a bad person?)

It just means Rex is your peer now Posted Image

You're still a good person though, so you have that going for ya.

Mbl35 · May 29, 2010

Did you probe the data bus at the dlc to see voltages? I think each wire should have 2.5 volts or one a little more than the other-i forgot but they should both add up to 5 volts. Can you do a data bus check to see if the other modules show up like srs and abs. What about overlaying the dlc wires to the pcm and see if you get comunication?

Shake the crap out of the harness and see what happens.

Btw i have changed my fair share of processors-escapes for the

ignt coils, plugs and pcm-theres a tsb on that. A few 97 expeditions for crank no start and no comunication, 05 crown vic for throwing random false codes and check engine light and my very favorite lincoln ls for a no pedal issue.

DamageINC · May 29, 2010

OK well the vehicle was a non-priority so I let it sit for a bit before jumping back onto it. The fix was actually found pretty quickly. Even though all the load testing showed no problems, I decided to start fresh. I noticed that with the fuel pump relay out, there was some clicking noise from under the dash on top of the engine.

Pulled the Doghouse to find that the IAC was actually cycling at the same speed as the relay used to be. Unplugged the IAC - teh clicking moved to a solenoid in the trans.

Now I'm thinking that I have a ground issue because every time I disconnect something that's cycling, the cycling moves to another component, telling me that we're searching for a ground because the "intended" ground isn't working anymore.

Sure enough, I find that 6 or 7 of the ground wires for the PCM all tie into Ground 101 (I think it was) which is by the battery near the marker lamp. Found that the nut & ground wire ON TOP were just fine, but the wire that gets grounded by the stud to the frame was corroded and loose. Would have been impossible to see with the battery in place.

Cleaned up and refastened the stud and ground - guess what, everything works now, lol.

I'm amazed that the fog light would turn on so immediately and brightly with the ground in such bad shape like that when testing those circuits. Usually a bad ground results in the bulb slowly, progressively lighting up until it gets fully bright.

Either way - problem solved, and no PCM needed Posted Image

Dave

Jim Warman · May 29, 2010

A few years back I had what everyone lovingly refers to as "the dog food truck"... I might recant that part of the tale some day...

It was an intermittent no start and this is a truck that taught me the difference in odometer readings (blsnk, mileage or dashes).

The year would be 2006ish and the truck was a 2000ish. Under the hood was a siamesed ground that was tucked into the harness... it had never been hooked up... it passed several load tests as I kept going back over the "obvious" causes. Had I used the case ground PID, I might have seen it sooner...

I can only assume that the condition of the ground made my PCM voltage "dirty".

Mbl35 · May 29, 2010

Had an intermitant no crank on a 00 impala. Turned out to be a loose ground at the left fender. It was in a colission and when the body shop put the ground back they did not pay attention to the loosness of the ground wires in the crimp of the eyelit.

Had an 05 towncar with no switching of the modes on the duel climate control. On the pinpoint test all the ohm readings checked out fine-indicating no fault, another store just put in a atc head. I got the ok to pull the dashboard to have a good look and like 4 wires in the harness were sandwiched between the back of the thin metal bracket of the dashboard and the a pillar on the right side.One of wich was a vref circuit for the atc head to a blend door position sensor-halting proper operation to the system. Warrenty turned to cash due to the fact that there was an aftermarket navigation radio installed. But the funny thing is that i recall on the pinpoint test there was no continuity to ground on these sandwiched wires and they all had continuity to each other in the pinpoint test as well as like 4.6 or close to 5 volts on the vref circuit.

Brad Clayton · May 30, 2010

The biggest culprit I am seeing with Ford powers and grounds is the heat shrink glue. They heat shrink all of the eyelets and sometimes the glue will ooze out and block the metal to metal contact. This holds true more on power wires that can get hot.