There comes a time in this profession for every technician - no matter how skilled, trained and experienced - to experience some head scratching about a certain car. This usually happens to me when I'm following a procedure dictated by a trouble tree. I'm faced with a choice on which path to take based on the specific results of a test being performed. The options are generally limited to path "A" or "B," and sometimes "C." But what do you do when "Y" pops up?

I've often wondered why trouble trees seem to be geared toward taking you through the longest, most difficult-to-follow trails. I like to believe that trouble trees are written by textually oriented right-brain thinkers as maps to guide technicians to the right conclusion without having to explain the where and how in each step.

Figure 1.

The trouble is, most of the good technicians I know are visually oriented and inquisitive left-brain thinkers and get a better understanding of systems through their hands-on visual experiences than from the text manuals. Like most pilots I know who have to learn to fly in the dark using only instrumentation, they can do it but prefer to "see" where they're going whenever possible.

Last week I received a '99 Volkswagen Jetta diesel from a local car lot that sends me some of my more entertaining jobs. This one was in the shop for a malfunction indicator light (MIL) that was turned on by a code P0380 - a glow plug monitoring circuit malfunction. The circuit diagram showed the glow plug controller having two wires leading to the glow plugs that connect together at the terminal strip on the engine. Basically this is a parallel path from the controller to the glow plugs to eliminate the need for a heavier-gauged wire. The volt ohmeter showed a 12-volt reading at every test point in the circuit and a good engine ground. I assumed from my readings that this seemingly simple circuit was good and that I must have a bad glow plug or two, which caused the low current reading and code to come up.

Figure 2.

The next step in the trouble tree required me to unplug the glow plug connectors and test each glow plug using a LED test light connected to the positive battery. It was about this time that I realized that this testing was leading me down a dead-end trail. I've had enough experience with loaded circuits that I don't waste my time testing them using a tester that won't allow me to see the circuit under its normal working load. I tried to get a circuit diagram of this simple system. But the information source I was using to look up the code and testing procedures had a series of tracts and component numbers to follow that would require more of an attention span than I possessed at the time. Fortunately I also have the other information system that has redrawn the circuit diagrams onto a single page, wire colors and all. Figure 2 is a replica of the circuit.

Figure 3.

Quick testing this circuit with a test light gave me voltage readings at every test point between the fuse and the glow plugs. Realizing that this testing was quickly becoming an effort in futility, I grabbed my scope and hooked my current probe around the red wire coming off the fuse that fed the glow plugs and connected channel two to the circuit directly to get the voltage at the glow plug terminal. I then cycled the glow plug relay using VAG scan software and got the image in Figure 2.

Not knowing what the glow plug current should be, I grabbed one of the feed wires to the glow plugs - assuming that it carried half the total current and connected the other lead to the same circuit just past the relay. The image I received is shown in Figure 3.

Realizing now that the current was the same and that the voltage was actually a little higher, I knew I had found the reason why the code was setting. There had to be a voltage drop somewhere in the parallel circuit to the glow plugs. Next, I grabbed the other wire that fed the glow plugs and got a flat current line with 11 volts showing on channel two. It was obvious to me at this point that I had a voltage drop in the second glow plug circuit. I pulled back the split loom covering the two wires feeding the glow plugs and found where someone had sliced the circuit using solderless butt splice connectors. I also noticed that there were several holes in the wires around the connectors both before and after, indicating prior testing. I can only assume that the mechanic who tested this circuit was using a test light to ground and had himself thoroughly confused by the voltage everywhere he tested.

Figure 5.

After cutting the connectors out and soldering in splice clips, I took another current reading from the fuse and captured the image in Figure 5.

The current is now evenly divided and satisfies the controllers monitoring the circuit. I now need to write a technical description of the problem without oversimplifying so as not to throw the lot tech into the fire, so to speak. I settle with "Test glow plug monitoring system with VAG tester and lab scope. Trace voltage drop across glow plug dual feed circuit to high resistance connection and repair."

A few evidentiary pictures and everyone is happy.

Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is northstarguy@zoomtown.com

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