'99 Jetta Leads to a Troubled TreePosted 1/11/2007
By Jeff Bach
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.
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.
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.
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.
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