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Thinking Outside the Box - Case Study on an 04 F150 cruise control


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Thinking Outside the Box

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.

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I needed to finish up some tech articles for later publication. Thought I'd throw this one out there this weekend...well, kinda busy last week...never got around to writing anything else. LOL


Oh, and I did some research on this setup, seems Ford uses this same BOO signal for a number of years. I'll probably see it again I'm sure.

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Great story Gonzo, That is what separates the field. The ones with the determination to follow through to the end and be able to look at PID's and wiring diagrams etc. to see everything involved and connect the dots. I for one always take all the failed parts apart even if they are glued, a hammer and a screw driver take almost anything apart. I always want to see what inside has failed

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We get Mercedes cruise control issues all the time, talk about weird. How about when the cruise works perfect below 55mph, above 55 it is inop? Huh? Not possible. Different tread designs on the tires can cause this. MB came out with a tsb on the issue. Yup, 3 michelins and one dunlop results in intermittent cruise operation. Sometimes I wish the automakers would go back to chain operated cruise control.

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We get Mercedes cruise control issues all the time, talk about weird. How about when the cruise works perfect below 55mph, above 55 it is inop? Huh? Not possible. Different tread designs on the tires can cause this. MB came out with a tsb on the issue. Yup, 3 michelins and one dunlop results in intermittent cruise operation. Sometimes I wish the automakers would go back to chain operated cruise control.

very interesting.. that would mean that the tire size because of the tread design would be ever so slightly different, more noticeable at higher speeds by the wheel speed sensors.. Hmmm why wouldn't MB make a patch to flash the computer to allow for a slightly bigger margin of error?

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I'm the same way. Sometimes tho it can get you in trouble, and set you back. Sometimes I look back and say I would have made more money had I just let it go and forget about it, but that is hard for me to do. I'm OCD with my business approach and my repairs. The same way I put back every single nut and bolt, bracket, heat shield, etc. I am very thorough with my diagnosis. My associate once tossed a couple of fasteners into my parts tray while I was doing an involved repair. When I was done, I had an extra 10mm and a T25 torx. It was a Mercedes, and those make up like 90% of the fasteners on said vehicle. I was going crazy looking for where those went. Started to wonder if I missed them inside the engine somewhere. My associate comes over and says hey Gabe, relax it was me. I threw those in there for a laugh, but I'm seeing it's affecting your health. Let it go buddy, you got them all. No worries. Same way no matter the car, or customer, I always go through the car bumper to bumper, and offer services to the customer. If they say no, well at least I tried. That I can deal with, and go to sleep at night. But the what if's would drive me mad.


That is just the way I was brought up. It's how I approach everything. My mom would tell me I don't care if you are splitting atoms or cleaning restrooms. You do it to the best of your ability, and leave a good impression. If you are hired to clean the bathrooms, I want that person to say damn! I have never seen a cleaner bathroom in my life. I guess that stuck with me. So all my endeavors, all the projects I take on, I give my 100%. We see the half assery so much in this industry on all levels and positions. Makes me wonder how those ppl get through life. They cannot see much success.

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