2nd Gen Non-Engine/Transmission RPM dropping out?

[Jay Schaefer]

I have a 1999 24V Cummins TD in a 3500 Quad Cab with 178,000 miles. When I get the truck up between about 60 to 70 mph and try to keep it there the rpm's will drop off about 200-300 rpm and then pick back up as if you let off the pedal yourself. This has been going on for about a month. I had the fuel pump replaced a year ago and the fuel filter replaced a month or two ago. I took it to the Dealer who put it on the diagnostic computer and found nothing. A week ago I took it to a Cummins factory shop. Same results nothing. They downloaded to the computer and calibrated it they said but it is still doing the same thing. Transmission is serviced twice a year and is in excellent shape. Has anyone incountered this same thing or could offer an opinion? Any help is appreciated.

 

[JLandry]

Jay,



Is the engine rpm really changing when this happens, or is this just the tachometer display changing? Can you hear the engine rpm drop?



If the engine rpm is really changing, do you feel a change in power?



I'm wondering if what you're experiencing isn't possibly delayed torque converter lock up?



John L.

 

[Jay Schaefer]

John

You can actually feel it drop back. It feels as if the fuel is shut off for a split second or power is shut off then kicks right back.

 

[HBarlow]

Does this occur only using accelerator pedal to control speed or with cruise control activated?

Does this occur on flat terrain or hills with cruise control on?

The Dodge-Cummins trucks I've owned will occasionally surge when using cruise control on a downgrade. The cruise control is sometimes unable to smoothly transition between fueling and defueling when attempting to maintain set speed on a downgrade.

It could possibly be the beginning of an APPS (accelerator pedal positioning sensor) failure.

 

[Cummins724]

I would say that its your APPS sensor. TDR magazine had a great article on cleaning the contacts inside of the APPS by drilling a hole & using contact cleaner in issue 54. I've attached a picture of the article for you.



View attachment 72024



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[budnate]

hey Cummins have you seen a pic of a 12V switch opend up in a old TDR Issue??



Thnx B.

 

[Cummins724]

hey Cummins have you seen a pic of a 12V switch opend up in a old TDR Issue??



Thnx B.

Can't say I have. I read every issue & don't recall that article.

 

[budnate]

ok thnx, I am on a mission to crack one open and see what the sweep area looks like.

 

[JLandry]

You can actually feel it drop back. It feels as if the fuel is shut off for a split second or power is shut off then kicks right back.

Jay,



It sounds like possible electrical noise on the APPS sensor wire or a loose connection. Either of these possibilities wouldn't necessarily show up on a scan tool with the truck just sitting in the shop.



Did the dealer try to duplicate the problem using a DRBIII on a test drive under similar conditions? I ask because the DRBIII has the ability to record the outputs of sensors for review later. It also has an oscilloscope module which could be used to monitor and record the actual electrical signal on the APPS wires under driving conditions. Another test is to wiggle wires and connectors while monitoring and recording the sensor outputs on the DRBIII.



Best regards,



John L.

 

[Jay Schaefer]

I haven't tried it with the cruise control on only with the pedal.

 

[Jay Schaefer]

Thanks guys, I'm going to take all of this to the Dealer tommorrow and see if this doesn't solve the problem. The loose wire idea may be very sound since our cat sleeps in the engine compartment for warmth, she may have loosened something.

 

[Cummins724]

Thanks guys, I'm going to take all of this to the Dealer tommorrow and see if this doesn't solve the problem. The loose wire idea may be very sound since our cat sleeps in the engine compartment for warmth, she may have loosened something.

Ya better watch leaving the cat sleep in the engine compartment. Don't ask how I know.