Hello everyone,
Our company recently acquired a 1981-vintage PG5361P gas turbine package manufactured by John Brown. We are struggling to understand the true principle of the 20DA-1 (Intermediate) and 20DA-2 (Maximum) solenoid valves in the diesel starting means control circuit, and would greatly appreciate your insights.
The diesel engine starts via 88DS and stabilizes at ~600 rpm idle under its own mechanical governor, with both 20DA-1 and 20DA-2 de-energized. This appears to be the natural idle speed of the diesel.
Our P&ID shows that 20DA-1 and 20DA-2 share a common control oil circuit feeding the same fuel rack actuator (governor actuator). The control logic indicates they are interlocked — only one can be energized at a time.
Our field tests show the following:

• When 20DA-2 (Maximum) is energized, the actuator drives the fuel rack to full fuel, the diesel accelerates rapidly, and the gas turbine reaches ~1300 rpm. This behavior is clear and repeatable.
• When 20DA-1 (Intermediate) is energized, the actuator appears to see equal pressure on both sides based on our reading of the schematic. The fuel rack does not move, and the diesel speed remains at ~600 rpm idle — exactly the same as when 20DA-1 is de-energized.

We can hear the 20DA-1 coil energizing and de-energizing (audible click), so the solenoid armature is moving. However, we cannot confirm if the internal spool is actually shifting, or if the valve is simply ported in a way that balances actuator pressure without producing rack movement.
If the diesel naturally idles at 600 rpm with neither solenoid energized, and energizing 20DA-1 does not increase the speed above 600 rpm, then what is the actual purpose of 20DA-1?
We have two competing hypotheses:

1. "Speed-Hold" Hypothesis: 20DA-1 is not meant to raise speed from idle, but rather to mechanically lock the fuel rack at the intermediate position when 20DA-2 is de-energized — preventing the rack from falling all the way back to idle (or zero) during the transition from maximum speed down to cranking/locking speed. In other words, 20DA-1 only "works" when the rack is already at the intermediate position, not when it is at idle.
2. "Speed-Raise" Hypothesis: 20DA-1 should raise the diesel from 600 rpm idle to a higher intermediate speed (e.g., 900–1000 rpm for cranking/locking). If so, our unit has a fault — most likely a stuck internal spool, a misadjusted or detached mechanical stop, or a blocked oil passage inside the 20DA-1 valve.

Our Questions:

1. For a 1981 John Brown Frame 5 package, does 20DA-1 actually command a diesel speed higher than the natural 600 rpm idle, or is its role purely to hold the rack at intermediate position during deceleration from 20DA-2?
2. If 20DA-1 is indeed supposed to raise speed from idle, what is the typical diesel speed setpoint for the intermediate position on these vintage units?
3. Given that 20DA-2 functions perfectly but 20DA-1 produces zero speed change from the 600 rpm baseline, what is the most probable root cause — spool sticking, mechanical stop issue, or misunderstanding of the intended logic?

Attached is the diesel starting means schematic that matches our unit.
Looking forward for your reply!
Thanks!

 

Do you have the valve schematic?