Another starter problem

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Sometimes it starts fine and most of the time it grinds. When it started to do this i replaced the starter. Same problem.
First test: I took a lead wire from batt positve directly to starter solonoid. Cranked over fine repeted test 10 times for 5 seconds and every time no grinding.
Test 2: tested starter relay = good.
Test 3: removed M.P.I. relay and started with key and cranked over fine with no grinding.
Put everything back and started it normally and grindage.
It seems like the starter is slipping off the ring gear while starting
It also seems to me something is interupting the power signal to the starter solonoid.
Please help before I destroy all the teeth of this bloomin thing.

 

[Spyder FX]

Whats the condition of the flywheel ring gear? Also this problem can happen if the dowel pins that mate the block the trans are not installed. It allows the plate the starter is mounted to to move under the torque of the starter when it's spinning.

 

[bump]

The ring gear is showing signs of rough engagement. Although I am positive this problem caused the condition of the ring gear.
Dowel pins installed.
I wish I knew why it cranks over just fine with the test's I did.
Im going to test the mpi relay and maybe try to see how it starts with a push button.

 

[bump]

I installed a push button and it starts perfect.
No more grinding
I still need to test the mpi relay.