Peterbilt 389 / Cummins ISX, Paccar MX-13 etc. FAQs

Inaccurate or Erratic DEF Level Gauge Readings or DEF Consumption Issues on a Peterbilt 389, 579 etc. with a Cummins ISX Engine FAQs

Here are some common causes of inaccurate or erratic DEF level gauge readings on your Peterbilt semi truck with a Cummins ISX engine: Actually Low DEF fluid level: This may seem obvious, but if you’re near empty on your tank it can do weird things. This is the most common cause of inaccurate or erratic DEF level gauge readings. If the DEF fluid level is low, the gauge or dipstick may not accurately reflect the actual DEF level. Faulty DEF level sensor aka DEF sending unit: If the DEF level sensor or wiring is not functioning properly, it may not accurately detect the DEF fluid level and the vehicle's computer system may not display the correct DEF level on the dashboard gauge or dipstick. This can cause the DEF level to fluctuate or appear to be erratic. The DEF level sensor is part of a multi-purpose component that also includes the DEF quality sensor, DEF tank temperature sensor, and DEF tank heater. Please note if you are seeing DEF  fault codes for any of these components you may have a malfunctioning DEF sending unit aka header, which is mounted on top of the DEF tank and looks like a cap with a clamp, but has tubes and wiring that goes down into the tank.

How often should a Peterbilt Regen?

Generally if you're a long haul truck driver, roughly about every 4-7 days is a sign of a healthy exhaust aftertreatment system. Local truck drivers should generally need a regen about every 7 days or more possibly. Again this also depends on how often your truck or if it does at all do highway regens.

How to adjust clutch on Peterbilt 389?

Remove the access cover for the clutch, if applicable Bar the engine over with a barring tool, or by the alternator (not recommended), until you find a bolt head placed in the middle of a tab (this is the adjustment bolt) Place the correct socket onto the bolt with a ratchet, afterward, press on the ratchet with a pry bar in order to press the bolt head in and begin turning clockwise Begin with 1 full turn and check your adjustment Repeat steps 3 and 4 as necessary