CTIS ECU differences.

When it comes to replacing the electronic control unit (ECU) for the CTIS on an M939A2-series 5-tonner, remember there are two different ECUs available.

You may have already noticed some differences between the older ECU and the newer ECU, and you may think that the newer ECU isn't operating properly. The newer ECUs do work, though. You just have to be schooled on how they differ from the older version.

* The ECU should only be connected when the battery and ignition switch are in the "off" position.

* When a new ECU is initially installed and the battery switch is first turned on, you'll notice the ECU lights blinking. The ECU is determining what vehicle system it's installed on, like cargo versus wrecker. Then the light defaults to the "Highway" position and the system adjusts tire pressure as needed.

Note that the battery switch must be on for the panel lights to work. And, of course, the batteries must be installed first.

* The annunciator panel lights will not operate unless the vehicle's light switch is turned to the "Stop Light" or "Service Drive" position. "Blackout" mode and "OFF" will turn the panel lights off, but even with the ECU panel lights OFF, the CTIS will still function normally.

* The overspeed light does not momentarily blink when the battery switch is turned on.

* The overspeed light will flash when tire pressures reach the emergency range of 12 psi. After a higher pressure setting is selected and the truck is driven for about two minutes at 40-45 mph, the overspeed light will turn off.

* "Highway" tire pressures are increased from 60 to 70 psi on cargo tractor, dump, and expansible van models, and from 70 to 80 psi on wrecker models.

* When the ECU detects a tire pressure imbalance, it will adjust then to whatever pressure is selected on the ECU.