Well, I'm back to trying to help my nephew again with his '66 289 Ranchero. This time it's a problem with his C4 transmission, and being a 4-speed man, it's not something I'm all that familiar with. Here's the story.
When he got the Ranchero, the transmission had no high gear in it. On inspection, there was no lining left on the direct drum clutches, and the others weren't much better. It needed a complete rebuild. They knew the transmission wasn't original to the Ranchero, as it was a 4-speed car from the factory. The tag on the transmission said PEE 289 6M8 B2. That date code '6M8' (Dec. 8th, 1966) suggested it was likely a '67 transmission, but several of the parts inside were dated earlier.
A friend of his, who had rebuilt a few GM transmissions, did the C4 for him. He gathered advice from several Mustang and other SBF forums, from a couple racers, from the shop manual and other books, and from YouTube. He got a good rebuild kit from Oregon Performance Transmission, which included Alto clutches and plates, Kevlar bands, the Alto reverse/high 'Power Pack' six clutch disc kit, and a TransGo 47-2 shift kit.
When it came to installing that Transgo kit, they found a couple odd things in the valve body. The first was that the boost valve and bushing had been installed backwards, according to the TransGo instruction sheets. Similarly, there was a flat-sided disc or 'puck' there that wasn't in the instructions, and a spring that should have been there had been replaced with a check ball. Lastly, TransGo had provided a .110" drill bit and wanted a specific hole in the valve body separator plate drilled out. What they found was that the hole had already been drilled out...but more like .250"!
When the rebuilt 289 engine was fired to break-in the cam/lifters, after the obligatory 20+ minutes at 2,000-2,500 RPM, they dropped the engine to idle and topped-off the transmission fluid (ATI Super F synthetic). The rear axle had been lifted on jack stands, so that the tires cleared the ground. The trans was moved into Reverse, and the tires rotated backwards as they should. Then they put the transmission in low gear, and again the rear tires were rotating as they should. The revs were brought up to about 2,000, and the trans was shifted into 2nd gear, and the shift went quickly and smoothly. After about 30 seconds, it was shifted into high gear, and again it shifted quickly and smoothly. This was repeated a few times over the next couple of days, as they dealt with some issues about timing and idle mixture/speed.
At some later point, the engine was run again in gear, and the RPM's were brought up. As the speedometer indicated they were aproaching 50 MPH in High, the transmission seemed to literally 'fall out of gear'. The speed fell off, but the engine RPM remained the same. Only by letting the engine speed drop down could they get the transmission to re-engage. But again as speed got up to about 50 MPH (less than 3,000 RPM in High), the transmission disengaged again, and it has continued to do that ever since.
Asking some friends got them the suggestion that they install a pressure gauge, to see what was going on in the transmission. Fluid levels were again checked, and in fact indicated that it was slightly over-full. On running the engine, they saw 95-100 PSI at idle in Park. Shifting into Low they got the same reading, from idle up to 2,500 rpm or so. And again, in both 2nd and High, the indicated pressure was right around 100 PSI. In Reverse, pressure went up to about 165 PSI. Of course, this was with virtually no load on the engine, so the vacuum level did not drop appreciably.
At the moment, I'm leaning towards suspecting the valve body, given that the pump seems to be putting out at a good level. But as I said, I'm no expert. Any other thoughts about what could be going on? Thanks for any insight here.
