2016 New Flyer Hybrid Transit Bus - Electrical & Driveability Issues
I have a interesting Case Study here. Complaint is the Bus is dead and had to be jump started to make early morning pull out. The Batteries were jumped and it was chalked up to be no big deal. Next day the Bus is Dead again and they replaced 3 of the 4 batteries. Then the following day the Bus was Dead Again.
So the bus was next given to me look over. I notice that while the bus is in the stall that 3 of the Emp Fans are modulating at high speed and the two bottom ones are off. I then see that the Battery Voltage is being Drawn down by this and that the Aps is putting out 29 Volts. Aps is the Auxiliary Power System. The Bus I am working on is a hybrid. The Low Voltage Electrical System is supplied by the Aps. The Aps converts the High Voltage from the Battery Pack into a useable low Voltage. On normal operation under full load the Aps will normally only put out 28.2 Volts. So seeing this made me believe the Aps is working overtime and it is trying to compensate for the Excessive Draw. But it is because of the Fans only or is something else going on? After this I tried to hook up to the EMP Fan Control System but had no luck because we did not have the proper software, ughh. I decided to see as much data as I could see so I hooked up my Scan Tool to look at the Engine and see if I could gather anything useful.
Here is a code list of the Engine. It has multiple codes but I only want you to pay attention to the egr codes. The other codes I will explain in a bit. It has a 3382 Code for the EGR System being under normal operational range and has a 1866 Code for EGR Differential Pressure Data being erratic, intermittent, or incorrect. I do notice that the Engine Idle is hunting up and down as well. But my next question is would the EGR Valve cause enough current draw if it was sticking to cause this issue? I did not have a Mechanical System Not Responding Code which usually indicates the EGR Valve being stuck and thus building too much current and logging a Trouble Code for it. I am not convinced this is my main issue and something else is happening in addition to these EGR Codes. So I picked some pids with the scan tool to look at. I chose EGR Differential Pressure, Intake Manifold Air Temperature, Battery Voltage, and Intake Manifold Air Temperature Sensor Signal Voltage.
So as I bring up the Graphing on the Texa Unit I notice that at idle my Air Temperature at one point is almost at 200 Degrees! Plus my EGR Differential Pressure is saying it has 148.55 Psi, WOW. And as you can see with 2 other screen shots I did of the same pids my Intake Manifold Air Temperature Sensor Signal Voltage is very low, 190 Milli Volts to be exact and my Battery Voltage is fluctuating with the Intake Manifold Air Temperature Pid. Right now I am worried about the Air Temperature in the Intake Manifold being almost 200 Degrees and I want to fix this issue first and then go to the EGR Issue later. Remember when I was talking about the EMP Fan Control System and how I saw the Top 3 Fans running at a high speed? This is because the EMP Fans are getting the Intake Air Temp Signal from Cummins and Adjusting to the Temperature accordingly. So their is nothing wrong with the Fans. The Culprit has to do with something effecting the Air Intake Temp.
So after seeing the Elevated Intake Air Temp and the fluctuating Battery Voltage I thought to myself what about the Air Intake Heater Grid and it's associated Circuit? So I got out the wiring schematic and looked at where the Relay was that controls it. I then opened up the fuse box and then got out my DVOM and checked the Volt Drop across the Fuse and I had 50 millivolts drop across the Fuse. Note everything is off on the Bus and the only way their can be a Voltage Drop Across the Fuse is if Current is flowing. I then put my DVOM on each side of the Air Intake Heater Relay and I had Voltage on BOTH sides of the Relay! I found the issue! I then inspected further and noticed the Relay was melted and it burnt the Outlet Positive Cable to the Grid Heater!
Here are some videos I did to show the issue and also to show that after I replaced the Grid Heater Relay and the cable the draw went away and the EMP Fans did not turn on at all with the Engine Cold. I also did a Bidirectional Test of the Grid Heater with Texa and I hooked up my Pico Scope to check the current draw, pretty cool! I'd say a 100 amp draw from this would make the system malfunction if it was on all the time.
Ok so I fixed the main issue but what about the EGR? I hooked up the Texa Scanner Again and looked at the EGR Commanded and actual with the Differential Pressure and it was still the same. The Differential Pressure was still at 140 plus psi. I did some additional scan graphs also to show how the Cummins Ecm was trying to compensate for the erratic readings from the EGR Differential Pressure Sensor. Also if you look at the Intake Manifold Temperature the Temperature goes up when there is more command from the EGR Valve and then it goes down when the EGR Command goes to a lower percentage. So what does this tell us? This tells me two things. Number one the EGR is opening and closing but it is probably sticking from some carbon deposits and Number 2 the holes for the Differential Pressure Sensor are either plugged or the Sensor is malfunctioning or lying. I now have enough proof to take off the EGR Valve, Differential Pressure Sensor, and I have to Take off the entire Intake Horn to inspect the holes and passages that go through to the Differential Pressure Sensor.
So I take everything off and when I try to take off the Differential Pressure Sensor it will not come off. I had to pry it off with a pry bar. Due to the fact that the Air Intake Heater Grid was stuck on it melted the Differential Pressure Sensor. I then inspected the Intake Horn and found that the passage that goes through the Intake Horn was plugged with Carbon.
If you look at the lower left you will see a round indentation that is plugged with Carbon. Also the EGR Valve has quite a bit of build up of Carbon as well and the Air Intake Throttle. Unfortunately I did not get to finish the repair on this vehicle because we were waiting on parts for it. However the next day the Tech that finished it told me it ran great the next day with no issues. The vehicle started the next day and the issue with the idle hunting was gone as well. One thing to point out here though is when I was checking for codes while it had the hunting issue again after I repaired the Intake Air Heater Grid Fault it was not coding for the EGR Valve or Differential Pressure, this issue was intermittent. The reason being because of the Sensor for Differential Pressure being melted and damaged and lying to the ECM. So if you don't look at Graph Data and don't take the time to understand how the system works, you are going to have a hard time finding and repairing this issue. I agree that using your eyes, ears, nose helps as well but in situations like this you have to understand the Data you are looking at. Just a friendly reminder that using as much info as you can from the Scan Tool will help you expedite a more efficient diagnosis. Hope everyone enjoyed this, more are on the way.
Have a good Sunday everyone.
Good one Michael. And a thorough follow up to fix the EGR as well.
Thanks Denis. It still amazes me to this day how the software on these newer vehicles can still not code all the time. I can't wait to get into the 2018 models we have and see if their is any improvement. I understand manufacturers have to put in thresholds within reason but when you bring the Bus in and you hear the idle going up and down that audibly you think it would be able to code all the time.
Michael great job with documentation...this is a very good case study. Hope your doing well...I assume I will see you in March...regards
Hi G thanks, I plan to be at Vision. I am also trying to make it out your way for the big event. I'll let you know.
Awesome case study. Thanks for sharing. Very thorough write up. Looking forward to future posts.
I’m not. We service light to heavy duty vehicles/equipment. I’m currently a light/medium duty tech. But definitely an Interesting case study.
Great post busjockey .. always with the best info . Sure looks interesting diagnosing In the hevay duty Field .I think There is much fewer heavy duty diagnsotic techs