Cummins CAPS Assembly Repair

Mechanics Hub - June 23, 2014

We had a 1999 Cummins ISC diesel engine quit on us while trying to leave town. It had to be towed back after having an initial look at it. The first thing was to scan for codes and at first we had no engine position and engine speed signals. So that was where we started by checking the voltage from each sensor.

They are located behind the gear cover on the left side of the engine. We hooked up the test harness to check for voltage while rotating the engine and they both read 4.8 volts. This was after uplugging both ends of the harness and checking it for opens, shorts and grounds. We eventually replaced the sensors and the codes went away.

Once that was repaired we still had a no run condition with no engine codes. So then it was time to remove the CAPS (cummins accumulator pressure system) assembly and find out what was up with the no injection scenario. The accumulator was putting out 5600 psi and we had no injection out of the distributor module. Once the assembly was removed it was obvious where the problem was.

The drive couplings on both sides of the gear pump had failed so we thought about how this happened. The bus we’re working on is 15 years old which may be a factor and the CAPS had never been replaced. Wear and tear along with age could have caused this but maybe a timing issue? I can’t say why so the only thing to do is to repair the problem and do a re-scan for codes once it gets back to running condition.

This bus is going for sale so we lucked out with an old CAPS core and used parts out of that assy. for our dead bus. This was extremely lucky since the gear pump if we purchased from Cummins would have cost at least $1500. So everybody is happy to see that all we spent on this job was labor but gained valuable experience troubleshooting these diesel engine models. They were very reliable buses with the Cummins ISC. Maybe expensive at times if anything happened to the CAPS assembly but over a 15 year period they paid us back in ‘up’ time.