Thinking Outside the Box
Case Study on a 2004 and later Ford F150 cruise control problem

An 04‘ Ford pickup came into the shop with a non-functioning
cruise control system that had already spent some time at a couple
of other shops. None seemed to give the customer any kind of
answer as to why the cruise control wasn’t working. And, like many
of these types of jobs I get in from those “other” shops, they all
eventually come to the same conclusion as to what’s wrong with the
vehicle. You know, the typical bail out answer for a problem they
couldn’t solve. They’ll tell the customer, “It must be electrical”,
and of course, they don’t do electrical. Seriously, what isn’t
electrical these days?

A lot of times I find the term “It must be electrical” is just an
excuse from these other shops to throw their hands up and send the
customer down the street. They either don’t understand the diagnostic
procedures or have already spent way too much time swapping parts
and components hoping they’ll eventually run across a solution rather
than actually diagnosing the symptoms.

I’m not one to shy away from some “electrical” problem. I’m more than a bit bull headed and stubborn enough to stick it out to the very end.  Even if that means going to the extreme to diagnose a given problem. This one was no different. But, first things first, as always, verify the customer’s complaint.  So, off on a test drive I go.  Sure enough the cruise wouldn’t engage.  There was no green indicator on the dash and no signs of any action taken by the PCM to engage the cruise.  Now, it’s back to the shop and grab the scanner. 

Codes were absolutely no help.  No codes were stored and no history to see.  Which, is probably where these other shops stop testing things and came up with their conclusion, “It must be electrical”.  For me, codes are only step one of many to solve an electrically related problem.  Let’s face it, codes are not the defining answer.  Today’s cars have so many different methods of watching the various components involved with each system that it just makes sense to use the scanner as a tool to aide in diagnosing, and not just simply for reading codes. For this problem using the scanner to look at the PID’s (Parameter Identifications) was going to be more than a bit helpful.

As I’m sitting in the service bay watching every function involved with the cruise (according to the operation description), I did not see anything out of place or giving me incorrect readings.  Everything from the emergency brake signal to the BOO (Brake On-Off) signal were correct.  There didn’t seem to be anything standing out as the culprit, but there had to be something, something that everyone else has overlooked. Sitting in the service bay is not where the cruise control does its job. The vehicle has to be brought up to speed, before you can rule out if all the various components are actually working according to the manufacturer’s specifications. So, it’s back out on the road, but this time with the scanner installed. The safest method is to have a co-pilot watching the laptop screen.  With the truck moving down the road there was only one item that didn’t act the same way it did when the car was stationary, and that’s the BOO signal.  As we drove around the BOO stayed ON all the time.  It never switched from ON to OFF when the brakes were applied.

It’s back to the shop to try this whole thing again.  This time I left the engine running and watched the BOO signal.  As I pushed the brake pedal down, the signal switched back and forth from OFF to ON just as it should.  Now what in the world is going on?  I know I saw a constant ON signal while we were driving, but it shows ON/OFF as we are sitting still. That’s when I reached over and dropped it into drive and allowed the truck to roll forward just a bit. Well what do ya know, the signal never switched anymore. But, in park it worked just fine. I tried the same thing over and over again, and every time I had the same results.  It can’t be the brake switch, I’m not changing anything there. The only thing that’s changed is the gear selector. So it’s got to be something with that. Could it be the TR switch? (Transmission Range) Nope, it’s working perfectly.  So, what else can it be?

I went back to the description and operation page of the service manual, but even after reading it a second time nothing seem to make sense as far as what I was seeing on the scanner. But, there was one thing I thought might be involved that the general description page didn’t mention anything about, and that’s the shift interlock switch.  According to the wiring diagram there is a signal for BOO at the shift interlock, but only briefly mentioned as a possible cause of loss of BOO signal in one of the sub headings regarding the diagnostic procedures for testing the brake switch. Still confused, but willing to go with the “It must be electrical” as the primary cause of the problem, I decided to check further into the shift interlock switch.  This time instead of driving it or spinning the roulette wheel of possible components, I’m going to pull the shift interlock and check it myself. 

From the outside of the little box everything looked great, all the connection are solid and there were no signs of something that might have been spilled into the console. The circuit box was not glued together and could easily be taken apart, and I had a pretty good idea it had to have something to do with the BOO signal going awry, it seemed like the logical thing to do.  After I opened up the box, all I could say was, “Holy cruise controls there’s the problem!” A transistor had a burnt terminal. Now I’m more than confident this is the problem, time to order one.

After installing the new shift interlock I took it down the road for a quick test drive. The green cruise indicator light came on, it accelerated, resumed and functioned just as it should. The shift interlock was definitely the problem.  Of course, just to prove my hypothesis that it was the cause of the entire problem, I had to perform the same test I did earlier by placing it in and out of park and letting the truck roll forward while watching the laptop.  The BOO signal was doing its thing.  ON then OFF just as you’d expect it to do. 

It’s not the first time I’ve run across a diagnostic situation where all the PID’s or information given wasn’t in plain English. Sometimes what you have to do is go that extra step and follow your instincts as to what you believe is the problem. I’m sure another sharp tech would have a completely different way of coming up with the same answer, but in this case, this is how I came up with it, and it worked. That’s what counts in the end.  The customer is happy, I’m elated and you can be sure I’ll be watching out for the same kind of problems in the future, too.

    Even though my diagnostics information didn’t have all the answers laid out with pinpoint accurate details the answers were still there. Ya just had to dig them out from between the pages of the diagnostic manual. As with a lot of today’s electronic mazes, you might find yourself having to solve a problem that wasn’t a problem just a few years earlier. I mean seriously, who would have thought a shift interlock would have something to do with the cruise control 20 years ago? Or for that matter that you could look at so many different sensors or components all at the same time on one tool.

At times it does seem like an uphill battle to keep up with all the changes in the modern mechanics field, but at the same time very gratifying when you overcome a problem that seemed impossible to solve. Sometimes, ya just gotta think outside the box or in this case… open it up and look inside.