SP400 No Passing Downshift

[shoprat]

I am looking for some guidance diagnosing my SP400 in my '66 Riviera. This is one which is described as "3-2 valve used on 1st jobs only". And, yes, I rebuilt it, and someone had been into it before.

The transmission works well other than when cruising if you accelerate hard you should find the transmission downshifting from into either 2nd or low depending on speed. It does not.

If you come to a full stop, it will move away in low normally. But, if traveling at, say 30 or 40 mph, and boot it, it does not downshift. The torque converter does shift.

I have gone through the detent switches carefully and everything seem to be in order. If you energize the detent solenoid, I can hear it click.

This transmission is early and does not have a spring in the 3-2 solenoid. If you look at troubleshooting 400s not downshifting, "missing or broken spring" is one of the problems.

The modulator originally was a large one with a metered vacuum line. I replaced it with the one from the kit, it was a normal smaller one typically supplied for a 400 rebuild. I am wondering if this could be my problem. I may not be able to use the updated modulator with this early transmission.

Some guidance for troubleshooting this early transmission would be appreciated as it is different than most 400s.

Thanks

 

[TODD]

The 1st type 3-2 valve info in the '66 shop manual looks like data reused from the '65 manual. Unfortunately it doesn't indicate what the 2nd type is. Buick built both types (with and without the spring) in those early years of '64-67. Posts on other sites indicate some '67 units don't even have a 3-2 bore in the valve body. The spring was used to perform a part-throttle 3-2 downshift at speeds below 33 mph. The detent mechanism was not involved.

The '65 troubleshooting chart is clearer as it shows missing or broken spring as a cause of missing part-throttle downshifts (a symptom not shown in the '66 chart). Missing or broken spring is not shown under missing detent downshifts in the '65 chart.

Transmission nameplate codes for the 300 and 340 engines indicate the spring was used more in the small-block applications. So a 3-2 valve bore without a spring (typical for a Riviera) would not cause missing detent downshifts.

When the detent solenoid energizes, the detent oil pressure builds up and controls the valves that do the downshifts. The modulator oil supply is overridden during detent downshifts. So the modulator would not have an effect on detent downshifts.

Since the both the 3-2 and 2-1 detent downshifts are missing, it would be good to confirm 12V power is getting down to the transmission harness connector by using a test light.(Maybe you already did this.)

 

[shoprat]

Since the both the 3-2 and 2-1 detent downshifts are missing, it would be good to confirm 12V power is getting down to the transmission harness connector by using a test light.(Maybe you already did this.)

Yes, I can here the solenoid click, nevertheless, I even wired a toggle switch from the battery to the solenoid.

I am hoping when I eventually drop the pan, maybe the solenoid isn't relieving pressure, maybe somehow blocked...

Thanks,