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Very bizzare "dead pedal" experience


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Well I'm only hopping that what I just experienced isnt what I think it is but nonetheless.....this is what happened.

This last weekend I took the trailer up in the mountains, as I've done many times before. As usual for the north state valley area, it was stinking hot outside and I was pulling hills in 100* weather. No fun either as everything just gets too darned hot when its like that, but surprisingly this truck does amazingly well in staying within the tolerable running gear temps.

The situation I'm posting about occurred when I crested the top of one longer grade. I was at about 5500 ft in altitude, running in 3rd gear at around 2300 RPM, and the engine was on the hotter side peaking out at around 215*-220*. Thats the HOTTEST it ever gets too and only for a few minutes at longest. Having reached the top of the grade I shifted into 4th gear to let everything cool back down but strangely the throttle was non-reactive to any further increase in pedal. Weird, so I shifted back to 3rd and all throttle was normal. Then I quickly tried 4th again but still the same thing happened where it seemed the throttle didn't react to any increase in pedal. But back to 3rd again would result in complete normal throttle. So after holding 3rd gear for a couple minutes more at most I tried 4th gear a third time and this time the throttle resumed normal action. I have no idea what happened and it didn't do it again throughout the towing trip. When I got home I checked for codes but nothing. I cant tell you how much I go through to assure the VP is happy so why out of the blue I'd start to have issues, I dont know. Its 6 years old, I only shut it off when I NEED to, and I'd be surprised if I have 50,000 miles on it. I dug around the internet to see what others have experienced with dead pedal and found the same thing over and over again but of what everyone had experienced with dead pedal, the only thing I can say that would be similar was that it was hot and the engine was hot. I've driven in higher altitude enough to know that sometimes the throttle is less reactive in the thinner air but I dont remember it being like this. I almost thought the ECM was accounting somehow for the hot engine but that sounds like I'm reaching for something.

In saying all this, I do remember feeling the dead pedal running in the higher 6k-7k ft altitude about 8 years ago with the original VP on the truck but if I remember right it was a complete but momentary loss of throttle control no matter gear or anything. What I felt the other day didn't seem to be like that but then again.....maybe I'm in denial.

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. I almost thought the ECM was accounting somehow for the hot engine but that sounds like I'm reaching for something.

That was my first thought, but the ECM does not have the capability of saving the engine.......or does it??

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There is something to note here dead pedal is when the pedal is dead no throttle just idle. But now limited pedal is a different story which I've found to be a ECM issue typically. I would start with check all grounds to the batteries and the AC noise voltage of the alternator and go from there. Is there any error codes? How about fuel pressure too?

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I didnt think of an ECM issue.  And yes, the way you described it Moparman as limited pedal is more what it was like.  I would have thought if it was dead pedal then it would have done the some thing in 3rd as it was doing in 4th.

 

My fuel pressure is always in the upper teen's and I run so much 2-stroke its almost stupid.  Like I said, I go through great lengths to take care of the VP.  I guess I'll have to make sure the batteries are in shape and see what the alternator is putting out as well.

I cant remember.....can I test the alternator while the engine is running or do I have to bench test it?

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In regards to electrical disturbances, I think its important to include that I was towing a 5th wheel whereby the truck is also technically plugged directly into the house batteries of the trailer so could the lower charged batteries of the trailer cause enough interfering draw from the alternator, and possibly truck batteries, that it threw a phantom issue with the VP?  I know I'm reaching here but it sounds logical.....  Any input?

And with that said, I'm going to test my batteries and alternator just to be sure they're on the up and up.

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At this point, I'm not sure how to test for AC noise since my local rebuilder who is a quality but small shop told me that he has no way to test for a specific level of AC noise coming from the alternator.  How else would I test this or with the trailer plugged in?

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Simple. Put a DVM set to measure AC voltage on the 12+ terminal (large one) and the other lead to the negative on the battery. Anything over .1 volt is enough to give you grief.

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Just so I dont go screwing something up.....  The only DVM I have is this one.  I never really know how to do much with it but the basics.  Can anyone explain how I would need to configure it to read off the alternator?

 

IMG_0971_zpsc02f40af.jpg

 

IMG_0970_zps9ce00774.jpg

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Plug the red lead into the "V" port and the black into the "COM" port. Turn the knod counter clockwise to the "20" designation. Then put the black probe to the battery negative and put the red lead on the alternator stud.

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OK guys, this is what I found out.....
Voltage at the batteries while the engine was running was 13.6.
Voltage at the alternator output was 13.6.
Turned on the A/C and fan on high and the voltage was 13.7.

At the alternator output, AC voltage was .020-.021. It would fluctuate a lot if I moved the multimeter sensor at all while touching the alternator output but maybe thats completely normal as I was probably reading the connection jump around.

After reading those figures, I would have thought that the batteries and alternator output should be closer to 14 volts, and more like 14.5 volts but maybe I'm wrong. The dash gauge was reading a tick over 14 but I'm not sure that means as much as the multimeter I was using.

That said, if the ambient temps were around 100* and the engine was well over 200* and the RPM's were higher.....wouldnt you expect the overall voltage in the system to drop and potentially cause the amperage to increase overall?

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Battery temperature sensor under the drivers battery will sense the battery temperature and adjust the charging rate accordingly. So if the batteries are warm to hot the charge voltage is reduced to prevent gassing of the batteries. Cold start is the best time to test with grid heaters cycling in. Then the alternator is charging cold batteries and voltage is higher and the load is heavier with 95 to 190 Amp loads. Do your check at idle.

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So you're saying it was too hot outside to check?  When I checked the first time, it was around 90* outside and I just started the truck.  I went for a drive and with the engine completely warmed up I checked again.  Both times the voltages were as I noted.  By what you're saying, if it was colder outside then I'd see a higher voltage output?

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Yes. There is a noticeable difference on the in cab voltage meter depending on the ambient temperature. The colder temps make the altenator go up to around 14.5V while when it is warm I will show just under 14V. Like MoparMan said, if you can get your grid heaters to kick on (Using a resistor in the IAT sensor or ED's fooler) and measure it while it is under a load you could experience different results.

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So you're saying it was too hot outside to check?  When I checked the first time, it was around 90* outside and I just started the truck.  I went for a drive and with the engine completely warmed up I checked again.  Both times the voltages were as I noted.  By what you're saying, if it was colder outside then I'd see a higher voltage output?

 

Colder the better. The colder the temperature is the more the field is excited on the alternator. Blue wire should be positive 12V with the key on and the green wire is a variable ground from the PCM which is controlled by the battery temperature sensor. So the colder the battery and temperatures the more the alternator is working. On hot days it typical to see charging voltage as low as 13.2 to 13.5 Volts depending again on battery temperatures. I've seen charging voltages as high as 14.9 Volts in the dead of winter with -25*F starting temps.

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