ISB tach bouncing

[NBM]

We have a 2001 DP on a Freightliner chassis with the ISB. When the motor is cold the tach randomly will drop to near 0 and then come back up. Sometimes when the tach drops the motor dies and is hard to restart. After the motor gets partly warmed up it runs fine. The check engine light is on while the tach is acting up but goes off when warmed up. Anyone have an idea what's wrong? I have a couple ideas but not sure.

[Hooligan]

NBM: Your gauges should be the KYSOR MedallionII gauges, here is a troubleshooting guide that may help.
http://www.tayvaughan.com/appleton/f...ine2/53089.pdf

Also the VDC (Vehicle Data Computer) can cause problems due to poor connections, removing the plugs and cleaning (electronic spray) seems to help. The Plug clamps and pins are fragile.....
With check eng. Light, did it throw a code that may pinpoint the problem?
Also low voltage or weak battery may cause problems until battery charges enough to operate the system. (about the time the engine warms up) Watch your voltmeter drop as the Intake heater cycles and see if that coincides with the tach dropping out.

Hope this helps

[sc3283]

If the check engine light is on, you should have a stored trouble code. Have the code read and perform repairs for that trouble code. Odds are that trouble code repair will fix your tach and cold dieing problem

[NBM]

I had the alternator tested and it was bad so we replaced it and thought the issue was better until we took it for our first trip this weekend. The problem is still there. We also pulled every plug apart we could and cleaned with electrical contact cleaner then applied electrical contact grease.
We found a switch in the dash labeled engine warning light, it is a momentary switch. With the key on we turned then released the switch, the check engine light flashed once. I expected it to flash several times to give a code. The check engine light is not on now but was most of the way home on a 170 mile drive. Sometimes the engine goes into limp mode with no power, primarily when cold but 2x after being warm.
Any thoughts before taking it to a dealer for a code scan? Other recomendations to get a code scan?

[sc3283]

Retrieve the code....That is why the ECU has the ability to store a trouble code. It helps put a tech in the vicinity of the problem. Each code will not always give an exact failed part....but sure assists in diagnosis

[NBM]

Should have posted the results of the issue it was the crankshaft position sensor causing the issues. Fairly easy to replace and not too expensive. Another member was having the same issue and wanted to know what happened is the reason to bump an old topic.