First The Basics - Diagnostics starts with powers and grounds NOT with parts and labor

Gonzo · September 30, 2017

First, The basics

Let’s talk diagnostics. Do you follow any kind of diagnostic

procedure, or do you just throw darts on a wall, or play

“pick-a-part” and hope you fix it before you or the customer

runs out of cash. I hope you don’t do that. That might work

some of the time, but it’s not a good way to get to the source

of problems quickly or accurately.

One of the tire shops that I do business with dropped off a

2003 F450 with a 7.3 diesel for me to look at. It’s one of

their service trucks that died on the highway.

These guys are super, I’ve known them for years, and they’ve got a great reputation and excellent work force. In fact, I buy all my tires there, and they do all my alignments. They try to fix their own trucks “in-house” and sometimes, well……the repair/diagnostics are a little out of their comfort zone. This was one of those times.

Now, they don’t try to keep up with the scanning or diagnostics on most cars and trucks. It’s a tire shop that specializes in tires. They stick to what they do best, tires, wheels, and undercarriage stuff. The only “techy” stuff they get into is with the TPM systems. Most generally, when it comes to their vehicles they’ll go with the tried and true…”throw a dart and whatever it hits we’ll change.” Of course they’ll ask around first, but you know, second hand information hardly ever gets the job done these days.

They had it at one of their stores in another town for about 3 weeks trying to solve the problem. When that didn’t work they decided to tow it up to another one of their stores, and see if the guys there had a better dart. Another couple of weeks and several darts later, all they had were holes in the wall and no truck running. Then my phone rang.

“Can you program a PCM on a F450?” the shop asked.

“No, sorry I don’t do those, but I know who does. I’ll call him and see if he can come over and do that for you,” I told them.

A day or two went by and the phone rang again. “Hey, this thing still doesn’t start. The guy that programmed it said it sounded like an electrical problem”. Ok, somehow, I’m getting involved now.

“Sure, bring it over,” I told them.

Well, they towed it over with a strap pulled by an F250 diesel truck. The F250 looked like a toy truck compared to this behemoth. With a push and a shove from the F250 the guys got it lined up and into one of my service bays.

The big concern was the IDM relay, it kept chattering like a machine gun. Instead of checking codes I thought it best just to start with a complete wire to wire check to determine if there was some lost signal that was causing the problem, or a wire that was scraped and grounding out. Removing the inner fender on the driver side I could gain access to the Injector module (IDM) and the PCM (Power control module). Seemed easier to start here than any place else. It didn’t take long before I tracked down a problem. On pin #71 of the (new) PCM there should have been 12 volts from the ignition. No voltage at the terminal.

Tracing the wiring diagram thru its maze it led back to the in-car fuse box on fuse #22. I grabbed my test light and checked the fuse… (Rolling my eyes about now) the fuse,… oh man… the fuse is blown. Good grief… all this for a blown fuse.

Well, I better change the fuse, and see if it starts. Sure enough; it fired right up… sounded great, good throttle response, and no service lights.

Now the big challenge, what blew the fuse in the first place? Following the wiring diagram again…. I traced out all the components on the fuse circuit. There was one that caught my eye as the likely culprit. The brake cut-off switch mounted on the master cylinder. (It’s the one that had the big recall a few years ago.)

The updated replacement piece was in place but somebody forgot to secure the wires. The replacement piece has a newer style connector and an adapter connector to allow you to attach it to the original style fastener. Which makes it a little longer than it originally was from the factory. It was hard to tell where the new wire and connector started, and the old one ended, because the whole thing was lying on the exhaust manifold, and had melted down to a glob of wire and plastic.

Looking around under the hood there were all kinds of new parts installed. The nicest part……they were all installed correctly. There were no other wires out of place, or any signs of scraps or melted wiring. The important thing is that it runs, and the truck can go back to doing what it needs to do. I think the biggest thing that threw everyone on this job was the chattering relay. It sounded bad, sounded expensive… but, all it turned out to be was a loss of proper voltage to the PCM, because a fuse blew from a lead that grounded out. This was due to the improper installation of one small component.

The PCM couldn’t spread enough voltage and ground signals to all the necessary systems when it was missing the voltage it needed. As the relay would engage, the voltage drop was too much to keep the relay engaged. The IDM would pull more signal voltage as the relay would come to life. Then the PCM would have to drop the ground signal to the IDM relay to compensate for the loss of voltage. All this was going on very rapidly … on and off, on and off… making the machine gun sound coming from the IDM relay.

The guys at the tire store were extremely grateful that I got the job done, so they could use the truck again. For me, it’s another day at the shop. I’ve got nothing but good things to say about the guys at the tire shop. Hey they tried, I’ll give them that.

But one thing I wish they would do next time --- CHECK THE BASICS—BEFORE BUYING PARTS! It’s cheaper that way…

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kenk · September 30, 2017