Chasing Problems

You may remember a few months ago I wrote an article about how the ECM and engine electrical systems work in your electronic diesel engine. This month, I’d like to talk about how we used some of that knowledge to diagnose and fix some common problems – and save a ton of money. Currently in our shop, we have a truck that has been plagued with multiple puzzling electrical problems. The truck is a 1999 Kenworth W900 that has been repowered with a DDEC III Series 60. Neither the dealer nor the local Detroit Diesel shop could figure this one out, and after plenty of parts (and money) were thrown at this truck, nothing was solved. Now, that W900 is here at our shop, and it’s our turn to take a crack at fixing this electrical mystery.

The first problem we sought to fix on this KW was an intermittent loss of power. When the throttle was put to the floor then released, the throttle would go dead for a few seconds and the truck would lose power. When you have a problem like this, the first thing you need to do is check the throttle positioning sensor. The DDEC III and DDEC IV engine controllers do not use an idle validation like the ones in a Caterpillar or Cummins engine. So, before you start going through all the diagnostic tests for testing an intermittent power loss, you should take a hard look at the throttle positioning sensor.

When first checking the throttle positioning sensor, use a voltmeter to compare the voltage of the throttle positioning sensor wire with the ECM sensor return wire. You should see about half a volt if your foot is off the throttle. When you floor the accelerator, you should see no more than 4.5 volts. Anything outside of this range will cause the engine controller to recalibrate the sensor. This holds true with both Bendix and Williams throttle positioning sensors (the ECM doesn’t care who made the throttle positioning sensor – it only cares about the voltage range). Also, if you have Detroit Diesel Diagnostic Link, you can see how the engine controller interprets those voltages.

After testing the sensor, we found that it was returning nearly 5 volts on the signal wire anytime the pedal was floored. That was all it took for this truck to have a dead pedal. DDEC III and DDEC IV engine controllers will go into a sensor recalibration mode, without showing any signs other than a sharp drop in interpreted throttle position, as the sensor recalibrates over and over again, after seeing something that is out of the expected voltage range. How did we fix this problem? We installed a five cent 94K ohm resistor from Radio Shack into the throttle positioning sensor signal wire, which cured this truck’s power problem by not allowing more than 4.5 volts to pass through it. Knowing a little about basic electronics can pay you back big-time when working with these trucks. Not all electrical problems are this easy, though. Some problems, like the next one we found, require a bit more work.

The second problem that this W900 had was that the engine fan would turn on and off for no apparent reason. The manual fan switch was functional, but the DDEC III ECM wasn’t detecting the fan override on its digital programmed input. To further complicate this problem, anytime the engine fan clutch was engaged, the Jake brakes would not activate. Somehow, these two circuits were interacting with each other and the engine computer wasn’t showing any clues as to why.

The interaction between these two circuits could only happen in one of three places. The most likely location is the primary OEM wiring harness that links the DDEC’s engine controller to the connectors on the firewall. This harness supplies the input and output signals for both the Jake and the engine fan. This is the first harness that we needed to remove from the truck and inspect. The second possibility is that the Jake and fan override input signals are interfering with one another in the dashboard wiring harnesses. Dashboard harnesses are not subjected to as much heat, corrosive substances and vibration as the harnesses on the other side of the firewall, so odds are better that the problem is in the OEM wiring harness. Thirdly, the DDEC III ECM itself could be the problem, but this is not nearly as common (to date I have only seen a handful of engine controllers cause a problem like this).

When looking for an electrical problem, there are steps to follow. First, you want to determine all of the possible places that the error could be occurring and then, looking at the most probable ones first, start going through them one-by-one and checking everything. On this particular job, we have already eliminated the ECM as a source of these problems by simply replacing it with a test unit and verifying that the problem still persists. One down, two to go. If the problem was only an intermittent Jake issue, we would be taking a hard look at the sensor and injector harnesses, but as it is directly related to the engine fan, we can eliminate those as potential problem areas at this point.

We are still in the process of going through everything on this Detroit-powered Kenworth, but we have found and fixed some issues already. It is impossible to say, at this point, if the engine fan and Jake brake issue has been resolved because we are still working on the wiring harness and it has not yet been reinstalled in the truck. We think we have fixed the problem, but we will see. In the meantime, we are dealing with a few other issues we have found, as well. Next month, when we know more, I’ll bring you up to speed on what is happening with this electrical nightmare, so stay tuned.

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