Monkey with a wrench

So yesterday (Monday) I took the truck out and drove it. Had warning 5 minutes until derate and speed limited to 5 MPH. Truck went into derate with warning that truck would be limited to 5 MPH at next stop (that would be engine shut down and restart). Brought truck back to the shop and ran SCR regen (crystal sublimation reduction) and Exhaust Aftertreatment System, Service Regen. Total Regens between Sunday and Monday was 6. Soot level came down from 95% to 0% on Sunday. Soot level was 0% on monday. Those last regens got the truck out from derate. I did all this with Tech Tool. I have no idea what was going on or why it came out of derate on Monday. I had no codes to work from on Sunday or Monday. I'm wondering if driving the truck had anything to do with it. So, ultimately; problem solved.

Ok.........synapses clicking away on this dilemma. Does the truck have to be driven for a certain period of time before the SCR codes would pop up?? Truck is almost in derate but no codes............stumped.

So I had a EGR differential pressure sensor high range code. Replaced sensor a week ago........code came back on. One of the tubes was clogged w/ carbon. Took housing off yesterday, cleaned tubes and housing, put back on. Cleared codes w/ Tech Tool, ran the truck.........no problems. Left for the night, came back in the morning.........turns out I left the key on........batteries dead. Put the charger on it, started the truck after charging it. SCR fault DERATE came on in display. No CEL light on dash. Plugged Tech Tool in.......no DTC's. Ran DPF regens, NOX reductions........any test I could think of for the after treatment system. Nothing worked. All sensors seemed to be in range and reading. Soot level now at 0%.........started at 95%. I disconnected ALL negative terminals from batteries. Left them off for 10 minutes, cycled key a few times disconnected, put them back on, code still there. Last resort........reprogrammed engine ECM with latest download pack from Mack...........nothing. SCR Derate in 5 minutes!!! No clue what to do next.

The VECM needs to be programmed. So the truck characteristics need to be pulled off the old VECM and transferred to the new one. And it needs to be in the truck when its done.......and since the truck only goes 10-15mph at present, guess I'll be low boying it up to the Mack dealer. So I had a chance to check the throttle position sensor today. Its getting 5.2V open voltage through the sensor harness ground. Plugged it into pedal and was getting 4.85V plugged in. Voltage on variable side was 4.85-1.2V when I pressed on the pedal.....so that's working correctly. Pulled panel for VECM, unplugged J2 and connected incandescant headlight up to it, using harness ground as well. Open voltage was 12.25 w/ no load. Headlight on it was 11.87V.....so no power problem. I then called a Mack dealer in MA, who were super friendly and helpful; and talked to parts........yes, it needs to be programmed.........they then sent me to service to verify.......again, super helpful (not like the Mack dealer in NH we have an account with). They verified it needs to be programmed in the truck (actually that was the first time I heard that). I called our local Mack dealer to try and talk to service......everyone was at lunch and they'd have someone CALL ME RIGHT BACK!! 3 hours later, I called them. They verified it needed to be programmed with the truck characteristics and that I'd need the old VECM. They said they could get to it on Tuesday....maybe........today is Weds. If the other Mack dealer in MA wasn't so far away, I'd be trucking it there. Thank you for everybodies input. I appreciate. Hopefully this post can help someone else out.

After replacing the speedo sensor, and ohming out the sensor from the pins at the J2 plug, I drove the truck with the old VECU plugged in. No speedo active. Replacing the old VECU with the new one, speedo became active and worked. Had error message on Texa showing bad or out of calibration accelerator position sensor. Tried 6 times to calibrate accelerator position sensor w/ Texa. Not sure if its working. Out of frustration, I just ordered a new one.

So I have this posted over on Heavy Equipment Forums. One of the members over there just posted that I have overlooked the obvious. I NEVER PUT A LOAD TEST ON THE VECU power supply...........or for that matter, the Engine ECM. I just assumed my trusty multimeter would tell me the truth...........and it did........without a load. Even more frustrating...........I've played this game before, I have a round headlight in my truck specifically for this purpose. I never checked the battery connections.......just the relays and fuses. I'll try and get that done today and post the results. Its odd that the speedo didn't work before, but now it does. Oil pressure gauge reads fine, tach working correctly now.

Good morning!! Long time listener, first time caller.........hope somebody here can help me. I've been reading these forums for years and learned a great deal. 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. The RPM gauge came back but was bouncing a bit. The engine never shutdown. He continued to drive, dumped the load, and came back to the shop. 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. Doing a bulb check by cycling the key, I can't find a check engine light. Shining a flashlight into the bulb bezels I see 'STOP ENGINE', ABS, ETC. One bezel is missing, the bulb for that is down behind the dash and is lit constantly with key switch on. It goes out when the dash power times out after turning key off. Using the Accel/ Resume button doesn't produce a blink code and the cruise control is in the 'OFF' position (as per VMAC III manual). 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??