The following was written by Michael Eilbracht on Diagnostic Network 

I have an interesting case study here. The 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. The 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 given to me to 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 usable 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. Is it 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. 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.

The Code List

Here is a code list of the engine. It has multiple codes, but I only want you to pay attention to the EGR codes. I will explain all of the other codes later. It has a 3382 Code for the EGR System being under normal operational range and has an 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. 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.

Graphing the TEXA Unit

So as I’m bringing 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. As you can see with 2 other screenshots I did of the same PIDs, my intake manifold air temperature sensor signal voltage is very low, 190 millivolts 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 there is nothing wrong with the fans. The culprit has to do with something affecting the air intake temp. 

Wiring Diagrams

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 there can be a voltage drop across the fuse is if the 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!

Videos

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.

The EGR Fix

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. First, the EGR is opening and closing, but it is probably sticking from some carbon deposits. Second, 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 is 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.

About the Author

Michael Eilbracht is a full-time transit bus tech, with 21 years of experience in electrical and driveability diagnostics. He also runs a training and mobile diagnostics business MJE diagnostics. He also specializes in oscilloscope training for heavy-duty applications, and he is the distributor for the Midwest for Autonerdz.com.  Micheal and Autonerdz sell pico scopes for the automotive, HD, and marine industries. He is also an ASE certified master transit bus tech with an L2 advanced level certification.  His website is mjediagnostics.com.