The Caterpillar powered Freightliner that would not shut off. Everyday in our engineering center we see some unusual electrical problems and this one was quite interesting. These problems CAN NOT be fixed over the telephone and must come to our shop for repair.
02-26-2013 2002 Freightliner Classic with 6NZ Caterpillar
Complaint: The truck continues to idle indefinitely after the key is removed from the ignition. The only way to shut the truck down is to stall it or remove the battery terminal.
History: Two weeks ago was the first occurrence of a complete failure to shutdown. The owner operator had taken the truck to a local Freightliner dealer who had replaced the ignition switch but failed to fix the problem. The dealer told the owner operator that they could not fix the problem and they “didn’t read any codes”. They sent him away with a bill and a truck that would not shut down.
When the truck pulled into our shop the symptom was actively occurring making it easier to diagnose. Intermittent problems are the hardest to find so I am always happy to see the complaint first hand. The first thing I attempted to do was check if the ecm was powered. The fastest way to do this was though the datalink. Too bad the datalink underneath the key switch was full of corrosion. As soon as you connect to those pins they turn to green dust. Every datalink connector that I’ve ever seen that had this problem was capped and had a water intrusion problem. Those caps are good for keeping dust and water out unless the water came from the back of the connector. Then those caps are good for keeping water in the datalink, not out. Anyone who has a water intrusion problem must keep those caps off so those pins aren’t dissolving in a pool of water. We keep lots of those pins in stock because we have to. Freightliner told this guy they didn’t read any codes because they couldn’t read any codes! Without a datalink connection you have no connection to the other computers in the truck and are completely blind as to what they are doing.
After the datalink was repaired it was clear the ecm was not shutting down. After tracing out the circuit it was evident that the ignition circuit pin on the firewall was supplying voltage even if the ignition key switch was removed. This problem was clearly on the cab side of the firewall. The ecm will not shut down until voltage from the ignition is no longer supplied or engine rpm is reduced below cranking rpm. After completely removing the ignition key switch from the dashboard the next stop was the power distribution panel and the relays. This freightliner did not have fuses but circuit breakers. This makes isolating the involved circuits more time consuming. As it turns out multiple circuits were involved and the further we dug into this problem the more corrosion we found. The truck had plastic bags for windows in the sleeper so that should have been a clue. This truck had a water intrusion problem. As a power distribution panel corrodes due to water intrusion circuits will fail and sometimes the corrosion acts as a bridge to a neighboring circuit and sends voltage somewhere it isn’t supposed to go. In this case corrosion and built a good enough bridge to send 11 volts from the trailer option circuit to the ignition circuit. After the truck ignition switch was turned on the switched power circuit would energize. The switched power circuit would energize a whole bank of other circuits of which one of those circuits had enough corrosion to build a bridge back to the ignition circuit. Now the ignition key circuit is bypassed and the ignition circuit to the ecm is in a self sustaining loop. This is why the key would no longer shut the truck down.
After nine hours of repair work, several feet of wire and the replacement of eight bad relays the immediate problem was fixed. The owner operator was advised to address the water intrusion problem as soon as possible because this problem will happen again if more water enters the dashboard.
- Fernando DeMoura
Pittsburgh Power Engineering Center
Beware of Truck Stop Programmers, There are a few frustrated mechanics hanging around truck stops advertising they can program your ECM and improve your fuel mileage and performance. What they are doing is maxing out the injectors and thus taking the injector our of calibration. Most all electronic injectors have a number printed the top housing and the tells the ECM how many cc's of fuel that particular injector injects. When the injectors are changed in the engine those injector numbers are then put into the ECM to regulate how much fuel the injector receives to balance out the power. Now here comes someone with a lap top and all they can do is go into the ECM and max out all 6 of the injectors and now the flow rates are all off. Yes the engine will gain power, however it will NOT be smooth power and usually the idle is rough. This is NOT the proper way to improve the performance of the ECM. So please think about what could happen to your engine by allowing a parking lot programmer who is living out of his van, car or hotel room mess with the most critical computer on the engine.
Written by Bruce Mallinson and Fernando DeMoura
Pittsburgh Power Inc.
3600 S. Noah Dr. Saxonburg, Pa. 16056
Many of the articles compiled here where written by Bruce Mallinson. Attribution to other contributors is given in the specific articles.