518 shifts into 4th by itself?

Put a non lockup 518 in my 74 road runner last summer. Wired up the OD solenoid with a toggle switch. Once I got driving the car, I found that the transmission will shift into 4th without the switch, and then slips if I try to get it to downshift by accelerating. I completely unhooked the power and the ground, so that there is no power going to the solenoid. The transmission will still shift into 4th by itself. Is this the OD solenoid, or a problem with something in the transmission? I thought if its just a valve body fix, I might try to fix it and drive it this summer. If I have to pull the transmission, I will probably just get it rebuilt. Engine is a 400/451, hughes 3k converter, jvx adapter, new 3.73 gears. Transmission is unknown condition; although I put a transgo kit in it before the install. Thanks for any help!

i had a similar problem with a 518 trans,turns out there was a spring missing in the valve body,only way we found it was by comparing another valve body to the one with a problem,your results may differ!!

BTT....so it sounds like this might be one of the valves in the valve body? I followed the TranGo instructions...no leftover parts haha. Would a bad solenoid cause this?

How/when does it shift into 4th? Does it shift 1/2 then 2/4 or does it shift to 2/3 and then immediately into 4th or is there a delay?

Does it shift in to third, then waits a bit then shifts in to 4th or does it go from 2nd to 4th.

Regarding the slip - take it for a ride in manual low - get the engine up to about 3000 then get off the throttle.
Does the engine slow the car or does it just want to coast and free wheel?

I have put my share of Transgo kits in 904s and 727s without any real problems. But when I tried to install the Transgo kit in my 518, I had nothing but problems. None of them were the same as yours, but several that I was not able to resolve. I finally had A&A build me a valve body and all is good.

If you can get an unmolested valve body somewhere, I would start with trying that and see what you have.

I had nothing but problems with the TransGo kit in my 48RE. Ended up taking most all of it back out.

what should one look for in a "molested" valvebody ? i have never had one in my hands, but i have a full manual trans-go kit i will be installing at some point.