Drops from 12.5 to 8 volts when turning on ignition. Won't engage the solenoid. New starter and solenoid are already installed. Any ideas to where its pulling voltage towards?
Check all of your grounds.
All connections
If not connections then bad battery.
Sometimes the battery cables will corrode inside the crimp where it is difficult to see. Seen that may times.
As others have said, check and clean connections and test your battery. After that, start pulling fuses and see if the draw is in any of those circuits, then get a hold of a meter and follow the drop.
If occured BEFORE starter and solenoid replacement: Remove wires from ignition switch and try connecting direct. Switch itself could be shorting internally. If after... Reexamine how you have the wires to new solenoid connected. Many forklift wirings connect the solenoids in many configurations. You may have the wires connected to the wrong posts.
This appears to be a problem between the key starter switch and the solenoid on the starter. We have cleaned and checked all connections. The voltage drop is when the key is turned to the start position. The solenoid on the starter will click and if you continue to turn key from on to start it will sometimes start. If you jump 12V to starter solenoid it will crank every time.
That circuit should go through some type of neutral start switch. IDE be looking there next.
As in above BEFORE: With ignition switch visible, remove wire from it going to solenoid... Jump 12 V from battery to it. If kicks in and starts, it is your ign switch shorting out/failing to make contact in the start position. If this does not function, find your neutral start switch, remove the wires from it and connect together direct. Then re feed 12V from battery as above. These should tell you it is the ign switch or the neutral start. Still dont work? Feed 12 V to the portion of the wire running from neutral start to solenoid. Don't work? Put one of the wires back on the neutral start and feed 12V to the other post. Works? Trouble is upstream in wiring going up to the key switch. Wiring, fuse, some accessory connection, is robbing our V's.
Edit above Change "Don't work? in last para to "Work's".
Is there a chance you have a "half-start" position which energizes a pre-heat relay. If you have compromised cables or a shorted grid heater, that could result in a severe voltage drop.