Help!

[Swampdonky]

Signature 600Throws code 121 only at 1500 rpms. Lower rpms the code will go inactive. Idle or running down the road it does the same thing. They have replaced crank sensor 3 times. Timing has been checked 3 times. It has been good. They have swapped plugs 2 times for crank sensor. Any ideas?

[Chamberpains]

Has anyone checked the entire wiring to the sensor? Made sure the sensor is going into the correct depth? Has the vibration damper ever been changed? If so, how long ago? With aftermarket or oem? The damper could be way out of whack and putting a very bad vibration through the engine causing the sensor to lose signal in the higher rpms's.

[Waterloo]

Dampers need to be replaced every 500,000 miles, it is a regular service item. There is a viscous fluid sandwiched in the damper, over time it hardens and throws the balance off. There is no way to check or diagnose that I know of, replacement is your only choice. Only use a Genuine Cummins Damper, around $650. Paint it before installing to keep the warranty intact. Use the expensive Cummins red paint, one can is plenty, around $30.

[Rawze]

What model/year engine is it?
some history?


CM570/Sig. Series ISX.
Fault Code: 121 -- No engine speed signal detected from the crankshaft engine position sensor.


If it is not the crank position sensor, or the wiring, crank shaft damper is good, ecm is good, it could be that the tone wheel is slightly loose on the crank. - I have seen this before. Check to see if there is any free-play in the tone wheel to the crank.

It can also be from too much play in the crank bearings. If it has had a recent inframe or those bearings were replaced, someone needs to plastigauge the bearings and ensure they did not install the wrong ones. read through this post for some related issues known about the ISX and bottom end bearings that a lot of people simply do not know any more about them: http://rawze.com/forums/showthread.php?t...5#pid32065

lastly, if someone has installed the wrong program in it, or has made a custom program for it resulting in too high of a final crank angle (the point where peak cylinder pressure happens is too low of an angle) then it will cause the crank to vibrate internally. A person usually cannot feel this going down the roads, but over time, it beats the bearings up in the bottom end and also causes liner fretting prematurely. -> if it is bad enough, it can weaken the crank bearings and eventually start to cause crank position alarms as the bearings slowly get hammered out, the problem getting worse over time, eventually destroying the engine.

just some things that have been seen in the past on them, thoughts. nothing more.

[Swampdonky]

It is a 99 model motor I believe. It spun 2 can bearings and when it did that it almost shut the motor down. He was bobtail running around 1300 rpms. Put another head on it and new injector cam. Truck made it 7 miles cam split at an oil journal longways. Cam and head came from Cummins. Balancer has around 800,000 on it so it will be getting a new one this week. He said it was inframed about 120,000 ago. Is it possible when it almost shut the truck down something could of happened to the tone ring? It did not give the 121 code until it spun the cam bearing. Thanks for the help!

[Rawze]

getting in a hurry when installing cams into head, and bumping into the cam bearings while sliding it in will often make it spin one. Seen that plenty.

Also seen plenty of people try to re-machine a used head and end up with spun bearings or other problems.

Check to ensure your head is a certified factory new or re-man + its date...
http://rawze.com/forums/showthread.php?t...6#pid14106

[Rawze]

Since you say you have had severe problems with the head and cams, it may be the ecm throwing the wrong code, thinking it is the crank sensor out of whack, when it may simply be the cam is out of alignment or there is too much play or perhaps a vibration / damaged gear in the gear train?

just a guess.

[Unilevers]

i believe this is the one i got the phone call on.

obviously the wedge timing has been checked and the static timing measured.

-reflash it stock program
-Crank twisted/bent slightly from loading up when it died (i have seen this is happens)
-Tone ring came loose and spun slightly and is vibrating
-metal is stuck on the tone ring


if you have verified the wedging and the lower 4 things - time to measure the crank