Monkeywithawrench
Senior Member
Ok.....hoping somebody on here has experienced this before or is familar with how this system works.
I have a customer with the above truck. He was driving it last week, loaded, when the rpm gauge and speedo stopped working and the engine lost power. This truck has never been a problem before. I plugged my laptop into it using Texa Truck software to diagnose it. It showed a problem with OIL PRESSURE SENSOR. I replaced it with a new one. That code disappeared.
New code showed up showing INTAKE AIR TEMP SENSOR was no good. Back to Mack I went. Replaced that sensor, rescanned, now the speedo sensor showed up as being bad (data invalid).
So this was Friday...I gave up around 8pm. I went home and bought the VMAC III manual online and read it. It said that if the speedo sensor was bad that it could/ would derate the power. It also talked about blink codes if you hit the cruise control resume switch so many times to display the code. I am not sure where the blink code is supposed to show up. This is an old truck. So there are a bunch of things not working and held together with baling wire and duct tape.
SATURDAY
So I ohmed out the speedo sensor and it was reading 1.4 M ohm.......no good. A good sensor is supposed to read between 150-170 ohms. Off to Mack again, new speedo sensor, checked on meter and it read 164 ohms. Plugged it in, rescanned, still showing bad speedo sensor. Pulled VECU from passenger side dash panel and ohmed out J2 PLUG J2-11 and J2-12 for the speed sensor. From the pins at the VECU, I got 164 ohms. Checked power to VECU at J3 plug. Using J3-16 (12V from battery) to J3-18 (VECU ground) I got 13V (battery voltage). J3-17 (12V from Accessory Relay) to J3-18 (VECU ground) I measured 0 volts. I tried jumping from J3-16 to J3-17 in case that should have been powered as well.........it didn't change anything. I ordered a new VECU from Mack.
WEDS.
The NEW VECU showed up yesterday and I put it in today. Code for the speedo sensor disappeared. Now it said the Accelerator Position sensor was out of calibration. I am assuming ............I don't have anything confirming this.........that the throttle position sensor calibration was in the old VECU. Tried calibrating it with Texa software. It didn't work. Tried 4 times........no goes all the way. Called Mack...........new throttle sensor is $300 and is 5 days out.
So we are up to $1200 and the truck has been down for a week.
Replace one sensor and another shows up bad. Always 1 sensor at a time, which is frustrating. Right now the Engine ECM is good, no codes. The tach, oil pressure, and speedo all work as they should now. With replacing the VECU, the speedo came back to life. Still have lack of power in the truck.
Other tests of note.........I checked fuel pressure first. It reads 110 psi at idle. On test drives, fuel pressure never dropped below 100 psi. Turbo spins free, no damage, no bearing play. No black exhaust while driving from lack of boost showing unburned fuel.
I'm at a loss here. AND WHY THE HECK DOES THE LAPTOP ONLY SHOW 1 BAD SENSOR OR CODE AT A TIME?? Fix one and the next shows up.........like its daisy chained or something. Runs into 1 bad sensor and then it stops and just displays 1 code. Is that normal from this generation system??
Or maybe its the air pressure in the LR tire is low??
I have a customer with the above truck. He was driving it last week, loaded, when the rpm gauge and speedo stopped working and the engine lost power. This truck has never been a problem before. I plugged my laptop into it using Texa Truck software to diagnose it. It showed a problem with OIL PRESSURE SENSOR. I replaced it with a new one. That code disappeared.
New code showed up showing INTAKE AIR TEMP SENSOR was no good. Back to Mack I went. Replaced that sensor, rescanned, now the speedo sensor showed up as being bad (data invalid).
So this was Friday...I gave up around 8pm. I went home and bought the VMAC III manual online and read it. It said that if the speedo sensor was bad that it could/ would derate the power. It also talked about blink codes if you hit the cruise control resume switch so many times to display the code. I am not sure where the blink code is supposed to show up. This is an old truck. So there are a bunch of things not working and held together with baling wire and duct tape.
SATURDAY
So I ohmed out the speedo sensor and it was reading 1.4 M ohm.......no good. A good sensor is supposed to read between 150-170 ohms. Off to Mack again, new speedo sensor, checked on meter and it read 164 ohms. Plugged it in, rescanned, still showing bad speedo sensor. Pulled VECU from passenger side dash panel and ohmed out J2 PLUG J2-11 and J2-12 for the speed sensor. From the pins at the VECU, I got 164 ohms. Checked power to VECU at J3 plug. Using J3-16 (12V from battery) to J3-18 (VECU ground) I got 13V (battery voltage). J3-17 (12V from Accessory Relay) to J3-18 (VECU ground) I measured 0 volts. I tried jumping from J3-16 to J3-17 in case that should have been powered as well.........it didn't change anything. I ordered a new VECU from Mack.
WEDS.
The NEW VECU showed up yesterday and I put it in today. Code for the speedo sensor disappeared. Now it said the Accelerator Position sensor was out of calibration. I am assuming ............I don't have anything confirming this.........that the throttle position sensor calibration was in the old VECU. Tried calibrating it with Texa software. It didn't work. Tried 4 times........no goes all the way. Called Mack...........new throttle sensor is $300 and is 5 days out.
So we are up to $1200 and the truck has been down for a week.
Replace one sensor and another shows up bad. Always 1 sensor at a time, which is frustrating. Right now the Engine ECM is good, no codes. The tach, oil pressure, and speedo all work as they should now. With replacing the VECU, the speedo came back to life. Still have lack of power in the truck.
Other tests of note.........I checked fuel pressure first. It reads 110 psi at idle. On test drives, fuel pressure never dropped below 100 psi. Turbo spins free, no damage, no bearing play. No black exhaust while driving from lack of boost showing unburned fuel.
I'm at a loss here. AND WHY THE HECK DOES THE LAPTOP ONLY SHOW 1 BAD SENSOR OR CODE AT A TIME?? Fix one and the next shows up.........like its daisy chained or something. Runs into 1 bad sensor and then it stops and just displays 1 code. Is that normal from this generation system??
Or maybe its the air pressure in the LR tire is low??