400ex will only start with booster pack
 

Won’t show a nuetral light unless booster pack is connected will turn over fine and start with booster I’ve changed solenoid and voltage regulator battery is good too

The battery may have a short internally, I have seen many batteries that show a full charge when tested but drop serious voltage when a load is put on them. This is normally a sign of an internal short. Try another known to be good battery and see what happens.

You can load test it with the battery hooked up. Just make sure connections are ok,battery charged up over night. Fully charged good battery should show between 12.5-12.8 volts. Have the meter leads on the battery while someone cranks it over. See what voltage drop to. If it drops past 9.5 volts,the battery is junk. Look at this good section on batteries.
https://www.batterystuff.com/blog/ho...ry-is-bad.html

Changed battery works now thanks

Glad it was something simple.