Lack of power loss after coming to a stop. What sensors are used in "Calculated Load"?

moocowman · May 13, 2020

No check engine light. No faults in any modules. After coming to a stop, engine is weak accelerating. Problem is random, but have become frequent enough that it happens about 50% of the time. When the lack of power occurs, it will randomly and suddenly go back to normal while trying to accelerate and give full power. As long I don't stop, the problem doesn't repeat. Once I come to a stop, about half the time the problem occurs again.

When it happens and I've got the pedal to the floor, target and actual rail pressure won't go past 16K PSI (normal is approx 25K PSI). Calculated load won't go past 30% (normal max is 100%). Looks to me like something isn't telling the ECM I'm asking for 100% load. APP sensors read ok (no interruption, one sensor is 50% of the other) and "Accelerator Opening" reads ok at 100% as it should when I have the pedal pressed down so far. What other sensors are involved in the Calculated Load equation? What can I monitor to help narrow down the problem area?

Hino123 · May 13, 2020

Common problem. Adjust the brake switch at the pedal.

moocowman · May 14, 2020

Hmm. I will take a look at that. They had mentioned that they thought it had something to do with the brake switch and replaced it. When I monitored withe scanner it worked ok. When it was experiencing power loss, it said "off". I'll update once I check. If you are correct, I am sending you money!

Hino123 · May 14, 2020

Have someone look at your brake lights while you try to make the brake lights stay on. If you can make the lights stay on by slowly taking your foot off the pedal then you’re switch is out of adjustment. There is also an updated spring that helps the pedal return fully. If you need more info email me at [email protected].

moocowman · May 15, 2020

No go. The brake switch is adjusted properly. I also did some more road testing last night. I can see the brake lights on the ground looking in the mirror. They're not coming on unexpectedly. Also, I can hear the relay if the brake lights come. But here's a related question. When I monitor Brake Switch in the VCS, it reads as it should. Pedal down On, let off the pedal, Off. When I monitor Brake Switch in the ECM, it's always On. It never switches to Off. Is the ECM maybe looking a pressure switch on an air brake valve? Either way. I don't think this related to any brake switch. The problem is really random. Tonight I noticed I don't have to come to a complete stop to initiate the problem. It can happen at very slow speed, like under 10 MPH. It can stay this way for 5 minutes or 1 minute. It can go away while I'm moving or after another stop. The only pattern is if I'm going over, say, 10 MPH it never happens as long as I don't slow down under 10 MPH.

I started monitoring everything possible to see if I can catch any number looking out of place when this happens. Haven't seen anything yet, but I haven't monitored everything. Something is telling the engine not to go full load.

Hino123 · May 15, 2020

Is this an air brake truck? You need to monitor the stop lamp switch and the brake switch. They should mirror each other when monitoring them in the ECU not the VCS. If this is an air brake truck the switch is on the relay valve. If that is not you issue I would be looking at the turbo. 90% change it’s a brake switch issue. This has been documented extensively and I have personally seen this exact issue over 25 times at least.

moocowman · May 15, 2020

Hi. Yes, it's air brakes. I'll have to look again to see if there are separate monitors for brake switch and stop lamp switch. I'll test the switch on the relay valve. Brake pedal switch is definitely ok. Customer replaced it. I re-adjusted it and tested last night.

As for turbo, I don't think it's a problem because a stuck VNT wouldn't kill the calculated engine load. I repaired a Hino just yesterday where the vanes weren't moving; actuator arm was stuck. Calculated load would still go to 100% as requested. I'll still check the turbo anyway.

Thanks again.

moocowman · May 15, 2020

When you say 90% brake switch issue, are you talking about the one on the pedal or the relay valve?

Hino123 · May 16, 2020

The old pic got cut off so I attached a new one 90% it’s a correlation issue between the two the stop lamp switch (pedal) and the brake switch (relay valve)

moocowman · May 18, 2020

Okay, I'm also convinced it has something to do with the brake switch on the relay valve. Stop Lamp Switch status goes Off and On with the brake pedal as it should. Brake Switch status is always On.

There are 2 switches on one valve and 1 switch on another valve. I tested the wires to the switches and each switch is getting +12V to one wire. So, we know fuses and relays are ok. The customer has replaced 2 pressure switches. (see picture) Both of these switches are NC (normally closed). I tested and they go Open when I step on the brake pedal. There is a 3rd switch that looks rusty and is faulty. It's Closed circuit whether there's pressure or not. I believe it's for the parking brake because that fitting is pressurized when the parking brake is OFF. I unplugged this switch to simulate the switch going Open, but the Brake Switch status in the ECM still showed "On". You'd think if this switch was stuck On, unplugging the switch would be the same as the switch going Off. So, I doubtful replacing this bad switch will correct the problem, but I'll replace it anyway.

Questions:
The valve on the right is for the service brakes because the hoses go directly to the service brake sides of the cans. I believe the valve on the left is for emergency/parking brake because it has lines going a valve/module on each side, then going to the emergency brake side of the cans. But why does the switch on that valve also turn on/off with the brake pedal?

Does the Brake Switch reading in the ECM look at all 3 of these switches?

Are all the switches are supposed to be NC? I read somewhere that some switches are NC and some are NO.

Thanks,

moocowman · May 19, 2020

Answered my own questions:
Valve on the right is the modulating valve. This hoses are going to the emergency brake sections of the cans, not the service brakes.
Valve on the left is the relay valve for the service brakes.
ECM does not read all 3 switches when deciding to reduce power because of brake switch correlation problems, only the brake switch that's on the relay valve (and the stop light switch on the brake pedal).
The brake switch is on the relay valve. It is NC. The lower switch on the modulating valve. It is also NC. It is also controlled by air pressure to the service brakes. The upper switch on the modulating valve gets pressure when the parking brake is off. I didn't test if this is a NC or NO switch.

I was just going to replace the upper switch on the modulating valve because it was always on, with or without pressure. But when I found out each of the 3 pressure switches has a different part number. I figured they must all work differently. But they all have the same connector. So, someone could have installed them in the wrong place. So, I replaced all 3 to make sure they're correct. Brake Switch still shows as always on. I found the pin-out diagram for the ECM. Tonight, going to test the brake switch wire for continuity from the switch connector to ECM. There's +12V to one of the brake switch wires. The switch is Normally Closed. So,when the brake is off, the ECM should see +12V. When the brake is on, it should see 0V. If the wire is broken, it would see 0V. Making it think the brake is on.

Hino123 · May 20, 2020

It is the switch on the relay valve. (the front valve in your picture) Look real close at the wires right where they go into the connector. Use the 2 finger pull test to see if they are good and not corroded there. You should have 12v on one wire. I think they are red and red with a black tracer not 100% sure as I don't know the year of your unit. Anyway that 12v wire should hold a load with test headlamp connected to a good ground. If it does not, trace it back to the fuse relay center in the dash. If it does have 12v, then chase the ground side. There are 3 wire to wire connections (connectors) for that circuit on the chassis side which gives you a few areas for testing. I would jump the switch and see if it responds in DX2. Sounds like you have a wiring issue. If you give me the year of the unit I can get you the schematic you need. I also sent you a PM with some PDF's that should be helpful.

moocowman · May 21, 2020

Update for anyone who will read this in the future. Thanks to Hino123, I have repaired the problem....

Broken wire for brake switch (BSW2 circuit) on the chassis rail just behind the cab. It looks like there was a hole in the insulation. Water got in and corroded the wire until it turned to green powder. The split loom is completely sealed in electrical tape. I think bad idea on Hino's part, or maybe the body builder did it. When water gets in, it stays in. As I cut open the loom, lots of water came out.

The brake switch gets +12V and sends it to the ECM. It is a Normally Closed switch. This means when the brake is off, the ECM sees +12V from the switch. If it sees no voltage from the switch, it thinks the brake is ON. So, if there's no power going to the switch or the wire from the switch to the ECM is broken, it will think the brake is ON.

I wasted hours studying the the shop manuals. I must have the wrong ones. This is a 2012. The 2012 manual showed a completely different ECM. It had 2 large connectors, located - I have no idea where bc the location diagram looked like the outline of an abstract blob. The one in this truck had 5 small connectors, located under the fuse panel at passenger dash.. The 2010 manual looked like the correct ECM, but wire colors weren't the same. I ended up doing it the old fashioned way with a toner and ohmmeter.