Trim unit dies when engine runs

[neil21]

Hi all.
I just replaced the brushes in my trim motor and everything seemed to be going great. Motor goes up and down exactly as it should when it's on the trailer. Today I put it in the water and discovered that once the engine is running, the trim unit stops working. It makes the click when the relays activate and even starts to work a little, but it sounds like it's running on a flat battery, which is impossible as the battery is good and the engine is running. If I turn off the engine the trim unit works perfectly again.
Any ideas where to start looking? 2 stroke Merc 90 circa 1990.

Thanks
Neil

 

[Bondo]

Ayuh,.... Welcome Aboard,.... Start by cleannin' to shiny metal Clean, the battery terminals, 'n connectors at the battery,...

 

[jimmbo]

Rectifier may have failed. Disconnect it and see if running engine will now trim

 

[merc850]

If the rectifier hasn't failed it will if you run engine without the red wire connected.

 

[Chris1956]

I would jumper from the battery Positive post to the trim motor, and see if that fixes it. Then try ground to trim motor. I am betting ground is bad.

 

[neil21]

Thanks guys. I'll work through those suggestions.