Tach bounces when idling
Thread Starter
Registered User
Joined: Oct 2002
Posts: 44
Likes: 0
From: North Myrtle Beach, SC
Tach bounces when idling
When I have transmission in gear tach bounces with either OEM or DD II injectors. When transmission is in neutral or park tach is steady. Where can I go to find problem? Any help would be appriciated.<br><br>Thanks<br>Denny
Top's Younger Twin
Joined: Nov 2001
Posts: 3,743
Likes: 21
From: Thanks Don M!
Re:Tach bounces when idling
All of my trucks have done that and I never noticed a bounce in the idle while it did that. Does you idle fluctuate too?<br>Several others have had the same thing. One day I noticed my tach to be rock steady, next time...back to what looks to be about a 25 rpm sway on the tach but the engine was smooth. I think its in the electronics of the signal.<br><br>One thing I always wondered is why DC chose a 125 rpm increment on the tach as opposed to maybe 100 or even 150. 
Guest
Posts: n/a
Re:Tach bounces when idling
It's my understanding the that the computer in these trucks does not continuously sample many of the various operating parameters - rather, it sorta samples oil PSI. moves on the temp, then to alternator, etc - thus, the sample period for the tachometer has to "take its turn" - and if the RPM drops significantly due to reduced RPM, the longer period between refreshing of the signal might cause it to fall further or more noticeably than at higher RPM... As to the tach graduations, wonder it that is to allow the readings to be properly linear between the guage faces used in metric/USA instrumentation?
Registered User
Joined: Aug 2002
Posts: 138
Likes: 0
From: El Dorado Hills, CA
Re:Tach bounces when idling
The tach is fed from messages on the CAN bus, so if your engine is idling near a step value, it will bounce back and forth between the two values. For instance, if the RPMs are measured in 25 RPM increments, and you're idling at 749 RPM, it is conceivable that the tach would bounce between 725 and 750 RPM. So a change of 1 RPM looks like a 25 RPM change because of the resolution of the tach. Correct me if I'm wrong, but the tach dial tick marks are in 125 RPM increments.
Registered User
Joined: Nov 2001
Posts: 389
Likes: 0
From: Heber, Utah
Re:Tach bounces when idling
Hell, my truck rocks side to side 2 to 3 inches at idel, and I'm more concerned about brain damage than the "tach". Maybe that's whyyyyyyyyyyy, I can't remmember what I was going to say? 
Steve
Steve
Top's Younger Twin
Joined: Nov 2001
Posts: 3,743
Likes: 21
From: Thanks Don M!
Re:Tach bounces when idling
[quote author=HeberRam link=board=4;threadid=15278;start=0#144171 date=1054387974]<br>Hell, my truck rocks side to side 2 to 3 inches at idel, and I'm more concerned about brain damage than the "tach". Maybe that's whyyyyyyyyyyy, I can't remmember what I was going to say? <br><br>Steve<br>[/quote]<br><br>I sent you a five point harness to use while sitting in traffic. It looks a bit odd watching a grown man vibrate and smile like you do at idle.
<br><br>Steve<br>[/quote]<br><br>I sent you a five point harness to use while sitting in traffic. It looks a bit odd watching a grown man vibrate and smile like you do at idle.
Thread
Thread Starter
Forum
Replies
Last Post
aroque137
2nd Gen. Dodge Ram - No Drivetrain
5
May 19, 2009 07:53 AM