Dear all, As already stated in my former thread I had a revison done on the automatic gearbox of my '79 400. Everything is working fine, except with normal acceleration I think the gearbox changes too late to 3th. At this moment it goes from 1 to 2th. at about 2500 RPM which is normal I believe. From second to third it only changes at 3200 RPM where I believe this should be happening at lower RPM. This is quite annoying driving at lower speeds. Gearbox stayed standard, only the technician advised me to put heavier "plates" (Sorry, I do not know the correct english word) in it to make it more longlasting and this should only have the effect of shifting a little bit "rougher" but not later .... At what RPM's are your automatics shifting ? I know want to start discussion with technician and would be nice to have some "field" expertise and feedback .... Thx. in advance David
Having had my 1980 400i Ser.1 auto tranny completly re-built a few years back, here are my experinces...I took my tranny out and took it to a shop that builds racing trannys for a living..$850 then..probably 1K or so now ..buy only a top line re-build kit B&M, TCI, ect..the cost between crap and the best kits is about $75USD..next buy a STREET shift kit at this time for $50--100USD..This will transform your car. Shift points are governed by fluid pressure, with many tiny journals, and each rebuild kit is specific for it's application, not gear ratios. Shift points are soooo simple to adjust externally on the T400, on the lower right side of the transbody on the vacuum valve with a screwdriver..cw higher..ccw lower rpm shift points (we're talking RCH's, here)(1/8th turn). The valve is there specifically for this purpose. Just remove the 'L' shaped hose and insert a small screw driver..My car also picked up <2> mpg..yes it did. But that was incidental. I have my Kevlar body armor on for those of you that always seem to have to throw rocks, so fire away. I hope that this of some help. Thank you. Jacques.
Hi, I agree with Jacques, if it shifts at 3200rpm it's too high. With a light foot on the pedal, you should get to 3rd at around 30mph. The vaccuum modulator is often where the problem is, or the hose leading to it. If it's dried up or hard, it could let outside air in, making the box think you've got a heavier right foot. Worth checking its condition. This works for other GM cars with Auto, I had a Chevy that did the same as yours. Good luck. Eric
IMO, in a performance sports car with a redline of 6,500 rpm and a torque curve that just starts to get interesting at 3,000 rpm, I don't think that a shift point of 3,200 rpm is too high at all. Once my 5 speed is warmed up, I rarely shift gears below 4,500 rpm unless I am in slow moving traffic or on low speed residential streets. Does the auto increase shift points if the throttle is pushed down farther?
It seems to, in fact, you can pretty much get it to change up just by lifting off slighty as it reaches the lower limit of the next cog. With the 3 speed, that could come as low as 30mph. In crowded places like Europe, it can take you a while to get anywhere interesting where you can open it up. It's great to get into low revs til the oil temp is up. You can cruise along at 1500RPM with barely any throttle until you're out of town then drop into second gear for fun.
Just to make sure I wasn't peddling bull, I went back to the 400i owner's manual. Page 40, it states, in all languages, that the shift ratio is dependent on throttle position as well as forward speed, but I couldn't get the ranges of rpm. The USA being the home of the GM Turbo-Hydramatic box, there is bound to be some of our learned colleagues in this community who will have details of such. After all, this box is rather similar to the one Farmer John uses to carry the corn to market in the old El Camino! My experience with frustratingly late upshifts was on a 72 Chevy Monte Carlo with the small block V8. It seemed to be always in Second round town, changing up rather late. A work colleague suggested checking the integrity of the vaccuum pipe to the box, and I found some perished rubber unions , 50 cents' worth. Changed these, and the car was transformed. Many others had suggested getting an overhauled transmission! As a side note, all automatics use throttle position to determine shift points. Some old british cars had cables , which were tricky to adjust when the throttle linkages got worn. Others use rods... Here's hoping a transmission boffin will join this thread. I might learn some good stuff!
Thanks for your reactions. I am fully with Eric. Where I am living there is alway a lot of traffic and smaller roads where you can only drive 70 Km. (40- 45 miles). In my car at the moment this means that it is always around 2700 - 2900 RPM in second gear, while warming up. This can be quite annoying and therefore I was indeed of the impression that the shifting point from 2nd to 3th should be lower. I also looked in my 79 manual an indeed the RPM's for gear change are not mentioned hence my initial question. Would still be interesting to know if somebody would have exact figures in RPM on how these boxes should gear up with normal / little throttle. Of course, as Ashman says, when it is weekend and I look for the open roads the gearchange issues is of no concern since then I can use the car for it is made ... fast cruising. My car will go back to the mechanic in two weeks, ... I will keep you informed. David
Hi! Went for a little drive today, and checked the shift points. In town with a light throttle, it goes from 2nd to third from 1600 to 2100 RPM depending on if it's flat or goes up, obviously if you push harder it will wait till higher revs. That's about 30 MPH, 50 KM/h, just legal limit in town in UK and France. It's about the same as on my Mercedes 450 SLC, and the old Pontiac Catalina and Chevy Monte Carlo I used to run. Hope that helps, Eric
Dear Eric, Thx. a lot. That was the "on the field" info I needed. I will give this info to the mechanic. I guess I owe you some fuel for the testing David
Agreed, the problem is very likely vacuum modulator related. In addition to checking for vacuum leaks be sure the modulator is seeing manifold vacuum and not connected to a source upstream of the throttle.
