2004 Chevy Trailblazer

This 2004 Chevy Trailblazer came in with multiple concerns. The main issue was intermittent low power. When this happened, the customer said that the Electronic Throttle Control, ABS, Traction Control, and sometimes the 4WD lights would come on. When the lights were on, he also stated that the cruise did not work and the HVAC and blower motor would quit working. First thing I did was scanned the truck’s multiple modules. These are the codes I found -

PCM – U1041 lost communication with ABS Module

Ignition Switch Wire Diagram

TCCM – B0790 transfer case neutral indicator circuit, C0379 front axle system, u1000 class 2 communication malfunction, u1041 lost communication with ABS Module

HVAC – B0263 HVAC actuator circuit, B1375 device ignition 3 circuit

IPC - U1041 lost communication with ABS Module

SDM - u1000 class 2 communication malfunction, u1041 lost communication with ABS Module

From the codes only I am leaning towards a bad ABS module that is bringing down the communication circuit causing the loss of throttle control. Could be a power or ground issue also.  I didn’t start testing in that direction because the customer stated he would lose is blower motor control every time this would happen. That one statement changes everything because a communication problem has a low chance of bringing down a manual HVAC systems blower motor control. So I went for a wire diagram of the blower motor circuit and also the power feeds for the

Chevy Trailblazer Ignition Switch

HVAC module. Referencing HVAC code B1375, I looked for the ignition 3

Chevy Trailblazer Defective Ignition Switch

feed circuit. Fuse HVAC 1 feed that circuit. With this being an intermittent issue testing is going to be tough. So my next step is to see what feed HVAC 1 fuse and try to link that power feed to my other concerns. IGN A fuse feed to the ignition switch. From the ignition switch, power feed from the run position on circuit 300 which feeds the HVAC, 4WD, HVAC 1, Brake, and TBC Run fuses. Do you see the connection? With the ignition switch feed fuse IGN A good, the biggest common failure point would be the ignition switch. A new ignition switch fixed the customers intermittent electrical concern.

2005 Diesel Dodge Ram 2500

Customer complaint was severe lack of power and performance on this 05 Dodge Ram 2500 Diesel 5.9 Cummins. Other concern was poor shifting. I took the truck for a test drive as I graphed the data with my Snap-on Modis. There were no noticeable power or performance issues under light load. As I stepped into it about 3/4 throttle or more the issue that the customer felt would show it’s face. If you jumped on the throttle the truck would get up and go and then suddenly just fall on it’s face. The low power seemed to lag a little bit after revving up. If you got on the throttle hard and held it there the power would be there, suddenly drop off, but the rpm would stay up even though all the power was gone. It really felt like loss of fuel or an excessive exhaust back pressure issue.

2005 Dodge Cummins Scan Data | Turbo Boost Pressure Loss

I continued my test drive while I analysed the data I was graphing. Another thing I noticed was if I backed off on the throttle after I would lose power for a few seconds, I could get back into it and have my power again for a second before it would suddenly lose it again. Very strange feel to this one. To me, it really felt like the turbo would just quit working. I also noticed the transmission shifting issue that the customer told me about but I was pretty sure it was all related to the power issue.

2005 Dodge Cummins Scan Data | BARO Pressure Dropout

Back at the shop I went over all the graph data I captured. I also noted that there were no engine codes. The three things I wanted to check after my test drive was to see if I maintained fuel pressure, had turbo boost, and no excessive exhaust back pressure. Going over my scan tool data PID graph data I could see all of this. I didn’t even need to do any hands on testing yet. Looking at the exhaust back pressure data PID indicated that I did not have an exhaust restriction. My fuel pressure sensor also confirmed that I did not lose much fuel pressure at the time of the abrupt power loss. The turbo boost pressure sensor showed a dramatic loss of pressure at the time of power loss. Now I have a place to start.

Was the PCM cutting the turbo charger? Was the turbo just going bad? Why would the PCM cut back the turbo charger? Did the PCM even have much control over the turbo charger on this truck? A lot of questions

Dodge Diesel | Plugged Cummins Airfilter

that needed to be looked at. Looking over some more scan data I captured, I noticed that at the same time I loss power, my BARO pressure also dived. The BARO reading is normally pretty flat lined as outside air pressure doesn’t change very much very quickly. What would cause my BARO reading to drop out? Well a bad air restriction between the air intake and BARO pressure sensor would. I checked the air filter…….. Not often you see an air filter that is plugged bad enough to cause driveability issues as severe as this truck. Another one in the books.

I believe the code was a 4wd module error code, so I went there and checked all powers and grounds. Everything look good on the multimeter but I felt I needed to dig in deeper. This 4wd issue was intermittent so I decided to hook up my scope to the 4wd module ground, power in, and power out to PCM (I believe). While on the test drive I could see if I was losing a ground or one of my powers. As you can see on my scope pattern I was losing power out to the PCM. The image shows the worse case of my power issue and most of the time it would only be one spike to ground very intermittently. You might have been able to catch the problem on your multimeter when it was the worst, unless your multimeter has a record feature, but it would have been hard and inconclusive. I don’t think a test light would even flicker over this problem. The scope showed me the problem clearly and a new 4wd module took care of this problem for the customer.

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