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Wrestling an 'Alligator' on an '85 Dodge PickupPosted 1/17/2006By Jeff Bach We all have a day every now and then when we get to deal with a customer who has been shop hopping by price. You know the ones I'm talking about. These people usually ask you right off, "How much do you charge for a diagnosis on an electrical short?" I consider myself one of the lucky few who gets to do what are referred to in this business as "alligator jobs" on a frequent basis. These are the jobs that many shops steer away from because they know going into them that it could be time consuming. They may risk getting buried so deep that they fear making the fretful phone call to the price shopper that goes something like this: "Well, Mr. Customer, we've reached the max of Y on the 'anywhere from X to Y' estimate, and we don't know where the problem is yet. We're going to need authorization from you for additional funds to continue digging." No one likes to give up on a job like this. I know plenty of shops that would rather let the customer take the car and pay the "so far" bill and leave with a bad taste in their mouth than to continue searching for a problem on the clock and risk making that second phone call. I love it when I get one of these cars in that's been referred to us by another shop and the customer has been "preconditioned" a time or two and the only question left is, "Do you think you can fix it?" I like hearing Vern's (my service writer) answer to this question. He says, "Sir, I have no doubt that we can fix this problem - all I need from you is your patience and your wallet." This is followed by the "Vern chuckle." I remember hearing a successful shop owner and friend of mine once say that the secret to dealing with alligators is to not let them get into the shop. That works for some, but I've grown to like wrestling with these jobs and gotten a reputation over the years of being the guy in my area to take them to.
The story went that the fuse link had been repaired several times and it would periodically burn up, leaving the driver stranded. The truck had been to two other shops that had no luck finding the short. When closing the garage door, I saw that I had more room between the back of the truck and the door than I needed. I could use some space in front so I went to back it up a foot or so and when I did, my breaker popped as soon as I dropped it in reverse.
I got in, started the engine and dropped it back into reverse again and got the short to occur and blew the breaker, which immediately killed the engine. The scope triggered and shot a trace straight up off the screen (Figure 2).
I'm excited about my progress on this one already. The next step is to divide the harness into two approximately equal bundles and grab one of them with the probe to see if I still have the short captured - that is, if I can get it to duplicate again (Figure 3).
I narrowed the short in two more tries to just the red wire (Figure 5). By now I have the short occurring at every engine start so I'm hoping to try to find the problem with minimal disturbance to the harness so as not to lose the short.
The red wire comes from the switch and is hot with the key on. There are no less than six splices in it and it feeds more than a dozen components. I pulled the air cleaner off and looked beneath to where the red wire came out of the harness and went to its first splice.
This truck uses an electric choke heater that you'll remember doesn't want to be hot with the key on to avoid the choke opening unless the engine is actually running. This is accomplished by using a switch that closes the circuit only when the engine has oil pressure. I pulled the harness from behind the manifold that went to the oil-sending switch to inspect it and found what I was looking for (Figure 7).
I explained the whole scenario to the customer who eagerly took the scope images I printed for him and a handful of our cards, which I suspect he will hand out to his accomplices. I like the feeling of giving someone back peace of mind about his vehicle, and you could tell this old truck had sentimental value to this guy. It feels good on the "some days" when "you get the bear."
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Someone did just that recently. The Dodge '85 pickup truck came in on the hook with the fuse link burnt into. Naturally, when I substituted my breaker for the burnt link, the thing started right up, and I pulled it into the shop.
I pulled the wiring diagram for the system and got my breaker and the current probe. The fuse link that was burnt fed three more fuse links and everything under the hood that was ignition hot. I decided to try to catch the short, at least spiking and narrowing it down to either in the vehicle or under the hood. I grabbed the harness coming from inside going to the underhood circuits with the current probe (Figure 1) and set the scope to "wait for trigger."
Exactly what I was looking for in a short to happen. I have the scope set on half a millisecond and the trace is still going straight up off the screen. This lets me know I have the short in the jaws of the probe. Now all I need to do is narrow it down to which wire is carrying the current and then look on the diagram to see what is on that circuit.
I do and it does. I divide the bundle again and this time (Figure 4) the scope doesn't trigger but the breaker did kick and the engine stalled but it happened before I could even get it into reverse.
I went to the wiring diagram to see what I could determine from the information I have gleaned so far. The diagram for this circuit from either of my two information systems left a little to be desired. I happened to still have an old Valley Forge-style wiring book that had this vehicle covered - which was a bit more helpful - but still had me thinking I could physically find the short before I could trace it logically through the diagram.
The sight I saw in Figure 6 made me chuckle a little as I thought to myself: "Ah, yes ... now it all makes sense." Some of you reading this article may be too young to even know what you're looking at here but I suspect the rest of you guys have a pretty good idea as to which circuit I'm going to be taking a good physical look at.
It wound up being the dark blue wire coming from the choke switch that had rubbed barely behind the intake, causing a double intermittent short in the ignition circuit.
