It is a pretty funky throttle circuit but fairly easy to troubleshoot.
JLG used two relays in conjunction with the limit switches in the Addco Throttle control.
They were called the Mid and High RPM relays.
They are activated simultaneously to provide a polarity reversal to the Addco throttle.
The High RPM toggle switch on the control panel will provide a 12 volts positive signal to the 86 terminals on both relays.
IF, and an important IF, a limit switch is activated when the boom is in the stowed position, the two relays will activate when the High RPM toggle switch is turned ON.
The limit switch is at the pivot end of the boom.
following my diagram:
as the relays sit now without being activated by positive voltage at their 86 terminals and/or a ground at their 85 terminals, Positive coming from B+ is routed to pin 87A on relay 1 and also connected to pin 87 on relay 2. The Negative coming from B- is routed to pins 87 on relay 1 and 87A on relay 2.
This will cause a Positive signal to be present at SW1's NC post and a Negative will be present at SW2's NC post.
Without being activated, 87A of this type relay is common with terminal 30.
When the relay is activated, 87 will be common with 30.
If a Positive signal is present at the NC terminal of SW1 and SW1 is not pressed, the Positive signal will be present at SW1's COMmon post.
This will result with Post 1 of the DC motor having a Positive signal.
If the Negative potential is present at the NC post of SW2 and SW2 is not pressed, the Negative signal will be present at SW2's COMmon post.
In this condition, the motor will start turning and cause the Threaded Block to travel towards SW1.
When SW1 is pressed by the RPM Adjustment Peg, SW1 NC post will open and its NO post will close.
This will cause the Negative that is at SW1's NO post travel through its COMmon post and out to the motors number 1 post.
The motor at this point will now have Negative at both its terminals so it will stop.
The throttle cable is connected to the Threaded Block. This will be LOW engine speed.
When the Boom is lowered to the stowed position, the limit switch will be activated and a ground (Negative) will be applied to pin 85 on both of the relays.
If the HIGH Engine Switch is also turned on, both relays will activate.
This will cause the polarities of the COMmon terminals of the relays to reverse.
In this state, terminal 1 of the motor will be Negative because of the NO post of SW1 is in the circuit and the NC post of SW1 has also turned Negative. However, terminal 2 of the motor has now turned Positive because of the relays reversing polarity. This causes the motor to rotate in the opposite direction. The threaded block will now travel towards SW2. As the Adjustment Peg drops off of SW1, the negative from SW1's NO post is replaced by the Negative from its NC post. When the other RPM Adjustment Peg activates SW2, a ground will be applied to the motor's post 2. The motor will now have Negative on both its posts and will stop turning.
This will be HIGH Engine Speed.
The adjustment of the pegs determines the High and Low engine speeds.
Looking at a portion of the schematic:
With a voltmeter, check to see if there is a 12 volt Positive voltage at terminal 86 on relays R12 and R13.
Also check to see if there is a Negative (ground) at terminal 85 on both R12 and R13.
Judging by what you described, you have voltage present to have the motor turn to achieve LOW engine speed but the relays are not activating to reverse the polarity.
I think you will find the relays are not getting either Positive signals at their 86 posts or perhaps not getting Negative at their 85 posts.
It may be a bad Horizontal Limit Switch not giving it a ground.
High Drive and High Pump Volume are also manipulated by the Horizontal Limit Switch.
