Dodge Cummins Diesel Forum banner

1 - 20 of 39 Posts

·
Registered
Joined
·
23 Posts
Discussion Starter #1
Hey guys! I recently had and issue come up with my rpms bouncing. Between 40-50mph the rmps will suddenly, intermittently bounce up and down about 100-200rmps for a short second. Sometimes continuously, other times, just a couple times. I never really know when it's going to happen. It seems almost as if the tranny is slipping. (214,000 miles on truck) The truck is not under any load and even does it sometime cruiseing at 60mph. I've heard that there could be a sensor that's going out somewhere?
I wanted to get some advice first before I start throwing money out the door. Let me know if you have questions to fill you in with more information. Thanks in advance.
 

·
Registered
Joined
·
23 Posts
Discussion Starter #3
Search function for AC NOISE...
I guess I'm confused on how the AC is related to the rpms?!?! On a side note, my AC/heater only has 1 speed- high, it doesn't change speeds turning the switch, but that is completely unrelated to the rpms.
 

·
Registered
Joined
·
391 Posts
Search function for AC NOISE...
Ahh yes, the typical guy who only wants to tell someone to "search" rather than type for 15 seconds longer to not leave the guy even more confused...

To the OP (Caleb): He's referring to having excessive AC voltage coming off your alternator. If there is too much, it can cause issues with your torque convertor locking/unlocking quickly, causing the symptoms that you explained. These trucks are very picking about grounding, excessive voltages, etc wreaking havoc and tossing little gremlins into our electrical systems.

This is a pretty good write-up by a very knowledgeable gentleman on the issue you're likely experiencing:


Some data points on good or bad (AC voltages):

10 to 30 mV (0.01 to 0.03) is a normally good alternator.

50 mV (0.05) is getting marginal bench testing is advised.

100 mV (0.1) is failing. Bench testing is still strongly advised to verify the alternator.
 
  • Like
Reactions: ColoradoColt

·
Registered
Joined
·
23 Posts
Discussion Starter #5
Ahh yes, the typical guy who only wants to tell someone to "search" rather than type for 15 seconds longer to not leave the guy even more confused...

To the OP (Caleb): He's referring to having excessive AC voltage coming off your alternator. If there is too much, it can cause issues with your torque convertor locking/unlocking quickly, causing the symptoms that you explained. These trucks are very picking about grounding, excessive voltages, etc wreaking havoc and tossing little gremlins into our electrical systems.

This is a pretty good write-up by a very knowledgeable gentleman on the issue you're likely experiencing:


Some data points on good or bad (AC voltages):

10 to 30 mV (0.01 to 0.03) is a normally good alternator.

50 mV (0.05) is getting marginal bench testing is advised.

100 mV (0.1) is failing. Bench testing is still strongly advised to verify the alternator.
Wow! Thank you for the clarification! Very detailed article. So, I guess I need to do a test first by pulling the alternator fuse and see what happens. I will try that and get back with the results tomorrow.
 

·
Registered
Joined
·
19,663 Posts
Welcome. It is long established issue you have. Yes it is AC voltage from the alternator bad rectifier. See "Lockup issues" in the stickies above. Caused by a bad hi AC producing alt but made worse by bad or dirty battery terminals and cables sometimes. New alternators need to be tested for AC above .03v ac, if so reject it.
An easy test is to unplug the alternator little plug on back when it is acting up and let drive it off the batteries. If AC is the issue, and is 95% of the time, it will stop when unplugged.
 

·
Registered
Joined
·
23 Posts
Discussion Starter #7
Welcome. It is long established issue you have. Yes it is AC voltage from the alternator bad rectifier. See "Lockup issues" in the stickies above. Caused by a bad hi AC producing alt but made worse by bad or dirty battery terminals and cables sometimes. New alternators need to be tested for AC above .03v ac, if so reject it.
An easy test is to unplug the alternator little plug on back when it is acting up and let drive it off the batteries. If AC is the issue, and is 95% of the time, it will stop when unplugged.
Thanks for confirming! If alternator DOES end up being the culprit, should I replace the whole thing or just the diodes?
 

·
Registered
Joined
·
391 Posts
Thanks for confirming! If alternator DOES end up being the culprit, should I replace the whole thing or just the diodes?
If it's the original alternator, replace the whole thing would be my vote...but as mentioned above, double check the "new" one you get for excessive AC voltage as well. Always bad apples out there
 

·
Registered
Joined
·
19,663 Posts
Either one wiht quality parts, no Napa alts. or take it to a good rebuilder for a lot cheaper or get diodes from one. Other rparts can do it too.
 

·
Registered
Joined
·
945 Posts
I guess I'm confused on how the AC is related to the rpms?!?! On a side note, my AC/heater only has 1 speed- high, it doesn't change speeds turning the switch, but that is completely unrelated to the rpms.
A/c = alternating current noise from the alternator dumbass.
 

·
Registered
Joined
·
23 Posts
Discussion Starter #12
So, I took the truck out for about a 30min drive including driving up a small mountain pass and I couldn't get it to act up. Everything was fine no bouncing rmp's!!! I don't know what's going on?!?!
 

·
Registered
Joined
·
391 Posts
Time is money. Same question comes every week. Gets old.
If time was truly money, you wouldn't even be on this forum...

So, I took the truck out for about a 30min drive including driving up a small mountain pass and I couldn't get it to act up. Everything was fine no bouncing rmp's!!! I don't know what's going on?!?!
You took it for a drive with the alternator fuse REMOVED and everything was fine? Or you took it for a drive with all parts still in place (as they were when you had the issue) and everything was fine?
 

·
Registered
Joined
·
23 Posts
Discussion Starter #14
If time was truly money, you wouldn't even be on this forum...



You took it for a drive with the alternator fuse REMOVED and everything was fine? Or you took it for a drive with all parts still in place (as they were when you had the issue) and everything was fine?
To clarify, All parts were still in place, I took it for the drive to see if it would act up again. Originally, if it was going to act up, I would have removed the fuse and taken it for another drive. But there's no sense in removing fuse if I can't get it to act up.
 

·
Registered
Joined
·
391 Posts
To clarify, All parts were still in place, I took it for the drive to see if it would act up again. Originally, if it was going to act up, I would have removed the fuse and taken it for another drive. But there's no sense in removing fuse if I can't get it to act up.
Roger that. Keep us posted. If it acts up, remove that alternator fuse, see what she does and report back
 

·
Registered
Joined
·
962 Posts

·
Registered
Joined
·
23 Posts
Discussion Starter #18
Update:
Ok guys, it finally acted up again, so i pulled over and removed alternator fuse. Unfortunately, after removing fuse, the rpms continued to bounce between 45-50mph. So, I guess I'm back at ground zero. I'm really hoping it's not a transmission issue! Any new advice?
 

·
On call 24/7
Joined
·
12,570 Posts
What year is the truck? Add a lockup switch and I bet it wont fluctuate. Before i swapped to a manual transmission, I couldnt drive without the lockup switch
 

·
Registered
Joined
·
19,663 Posts
What voltage was showing in dash when you plulled the fuse and drove it? Have you thoroughly cleaned the cables/terminals yet? Next time try pulling the plug on the alt instead of the fuse.
Forget lockup switches. If it is a trani problem it is minor and electrical, not major regbuild. Do you have a good voltmeter with a 2v ac scale? Has it had any other trani issues before? What if any?
Any power, trani or electrical mods on it at all?
 
1 - 20 of 39 Posts
Top