8 MPG Pete & More

Carl Kellner loves power, and he knows how to use it! I recently wrote about Carl, who is from Buffalo, New York, and his 379 Peterbilt powered by a 6NZ Cat that gets 7.5 mpg. How does he get such great mileage? By driving correctly and slowly (he travels at an average speed of 56 mph), and with the help of many of our parts.

Since that last article, Carl has added more of our products to his rig, including our ported and coated exhaust manifold, a performance Cat turbo, heat wrap for the exhaust pipe and a turbo blanket. He also purchased a Pittsburgh Powerbox, but has not yet installed it. Shortly afterwards, I got a phone call from Carl during his first test drive. While bobtailing, he stood on the throttle and said the drive tires were leaving marks in the asphalt and the response was unbelievable! It’s always fun to hear excitement in the voice of a 53-year-old gear-head!

Any time you try to improve the fuel mileage on a truck that is already producing 7.5 mpg, you have to realize that the gains get smaller and harder to find. Despite Carl’s already skillful driving and his truck’s remarkable fuel efficiency, I’m happy to report that Carl gained an additional four to sixth tenths within the first week with his new upgrades. Eight mpg out of a 379 pulling a company trailer – and he still hasn’t installed the Powerbox or the 2:64 rear gears he plans to add. Carl has a very consistent run with minimal variables – he runs the same route every week, fuels up at the same pumps and parks at the same locations – so we expect him to produce some very accurate fuel mileage reports for us, based on his predictable and consistent route. We can’t wait to see what happens with this truck next!

Last month I wrote about a W900 with multiple wiring problems that even Detroit Diesel and Kenworth had given up on. As you may recall, I wrote about this truck having a throttle positioning sensor problem (and how we cured it) and how there was a problem with the engine fan (it would turn on and off for no apparent reason). Also, anytime the engine fan clutch engaged, the Jake would not activate. While digging into these issues, we found two problems. I will discuss one of these problems here, and the other in a future article.

Last month, we left off at a point where we had isolated the problem to, most likely, be in the OEM wiring harness. The laptop was telling me that another circuit related to the engine fan known as the auxiliary fan input control (connected to the air conditioning pressure switch) was receiving a constant ground regardless of the condition of the air conditioning. In order to test this circuit I needed to have the OEM harness back in the truck. Once everything was back together, I was still getting the ground signal on the auxiliary fan control input. Now it was time to start honing in on the problem, like I discussed last month, by going through and testing every connection one-by-one.

After I unplugged the auxiliary fan control input I was still getting a ground on the laptop. I then removed all the OEM harness ground cables and unplugged all connections except the ECM connector and ECM power. I also ran jumper wires for the firewall ignition and communication connectors, but the laptop still showed a ground. I checked resistance between the auxiliary fan control input and the frame and got a resistance of infinity. This wasn’t possible. I thought for a moment that my meter was faulty, but after testing it, I found that it wasn’t. When things like this happen, it’s easy to get frustrated and give up. Getting frustrated can kill a technician’s productivity and spirit. Sometimes, a tech has to take a step back, look again at all the facts, and then keep asking questions.

I then switched my meter to measure voltage and got 12 volts in a circuit that should have given me 5. I’ve never seen an ECM supply 12 volts for a sensor application. That’s when I knew the 12 volts I was measuring from the ECM auxiliary fan control wasn’t coming from the ECM. To verify this, I disconnected the OEM connector of the ECM and still got 12 volts. There had to be another 12-volt circuit shorting to the auxiliary fan control. At this point, there was nothing left to do but remove the OEM harness from the truck and gut it like a fish.

Once the OEM harness was off the truck, I traced out the damaged circuits and found what was causing some of these problems. The auxiliary fan control wire had rubbed though its insulated coating and welded itself to the 12-volt ignition wire. The DDEC III ECM was interpreting this voltage difference between its own 5-volt supply and the 12-volt ignition wire voltage as a ground. I had been chasing a ghost. I was looking for a ground that wasn’t there. The auxiliary fan control didn’t have a ground, it had a short, but the ECM couldn’t tell the difference. Since the owner bought this truck, the ECM had to be replaced twice – now we knew why (cramming 12 volts down the 5-volt throat of this DDEC III probably had something to do with that). After the OEM harness was repaired and reinstalled on the truck, I moved on to address some other related ground issues in the dash. I will get into those problems next month as this soap opera continues (but I will tell you that it does have a happy ending).

If you have any performance-related comment or questions, feel free to contact me through Pittsburgh Power Inc. in Saxonburg, PA at (724) 360-4080 or via e-mail at bruce@pittsburghpower.com.