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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / November 2005turbo 400 shift problems
I have a 1990 Chevy one ton HD (G30) type van. The dealer tells me that going by my VIN number I have a turbo 400 transmission without overdrive in it. Last week my son loaded about 2000 lbs into it and drove ten miles. He says it was running ok just afterwards but two days later it is now exhibiting shift problems. In drive you have to really get the RPM's up before it will shift into second and when it does it goes in pretty hard. Once I got it into second I gave it some more gas to try to get it into third, but it seemed to not want to shift into that either without further RPM's, and I didn't want to push it further. The fluid level is just a slight bit high, it is not burned and there is no slipping that I could detect. Engine idle speed does not seem to be excessivly fast as might be caused by a vacuum leak although I have not investigated the modulator or the line yet so I suppose that a leak is possible. A few weeks ago I replaced the fuel lines from the throttle body back to the tank. I noticed that the vacuum line to the modulator was a little rusty but it seemed ok. I was working near it but I don't believe that I hit it or stressed it. I just drove tested the truck today for the first time after getting it back and I didn't try to drive too far but the symptoms are the same on cold take off as they are after putting a few miles on the truck. Can someone please explain how these transmissions shift, perhaps outline a procedure for troubleshooting this problem, and possibly offer an opinion as to what might be the most common explanation for my problem? Thanks very much for any assistance. Lenny Stein. Like you said dealayed shift problems are usually caused by low vacuum to the modulator or a bad modulator, remember a poorly tuned or running engine by it's nature has low vacuum, so keep that in mind . >I have a 1990 Chevy one ton HD (G30) type van. The dealer tells me that > going by my VIN number I have a turbo 400 transmission without [quoted text clipped - 22 lines] > offer an opinion as to what might be the most common explanation for my > problem? Thanks very much for any assistance. Lenny Stein. > Like you said dealayed shift problems are usually caused by low vacuum to > the modulator or a bad modulator, remember a poorly tuned or running > engine by it's nature has low vacuum, so keep that in mind . > <captainvideo462002@yahoo.com> wrote in message >snip< Here's a second on Shep's diagnosis. I've had extensive experience with 400's, and vacuum leak to modulator can cause exact symptoms you described. BTW, off-topic, but I've always highly respected those trannies. When the THM's 1st came out in GM's, the 400 was much superior to 350's in terms of service/reliability/durability and performance(can't think of other complimentary descriptive nouns just now!). HTH, s I'm going to try to get this thing up on ramps this weekend, (if it ever stops raining here in New Hampshire that is). From what part of the engine is the vacuum line taken? About how many pounds, (inches?) should this vacuum be? I don't have a guage so to assess the amount of vacuum if I put my finger on the line should it really suck it in or feel like a typical manifold vacuum? If the vacuum seems adaquate, is there a way to test the modulator without buying a new one? Lenny >I'm going to try to get this thing up on ramps this weekend, (if it >ever stops raining here in New Hampshire that is). From what part of [quoted text clipped - 3 lines] >feel like a typical manifold vacuum? If the vacuum seems adaquate, is >there a way to test the modulator without buying a new one? Lenny Yea, pull the line off and see if there is any tranny fluid in it. If there is, then the modulator's diaphragm is bad. I'd just change it anyway, modulators aren't expensive. -LMB > I'm going to try to get this thing up on ramps this weekend, (if it > ever stops raining here in New Hampshire that is). From what part of [quoted text clipped - 3 lines] > feel like a typical manifold vacuum? If the vacuum seems adaquate, is > there a way to test the modulator without buying a new one? Lenny While it's up, disconnect the vacuum hose going to the modulator; it should have typical manifold vacuum which is definitely detectible with the finger. Also, if it is strong, then that indicates possible modulator trouble. Honestly, I never have replaced many mod's--usually a vac. leak due to rubber hose pulled off, metal hose w/a rusty hole, or a rubber hose with a burned (from exhaust) hole in it. I'm betting you'll find no, or next to no, vacuum when you pull the line off the mod. s It was the modulator. The metal line into it just rotted and basically sprung a leak. It shifts just fine now. Thanks to everyone for all the advice. Lenny >> Like you said dealayed shift problems are usually caused by low vacuum to >> the modulator or a bad modulator, remember a poorly tuned or running [quoted text clipped - 8 lines] >service/reliability/durability and performance(can't think of other >complimentary descriptive nouns just now!). HTH, s I just built my first 400 for my BIL's 64 PU. It ties a rather stout 400 SBC to a 4.11 gear set. He seems to think it shifts a bit too smooth under hard acceleration. I drove it and found the tires never quite smoking under 45 MPH at full throttle. The tack and the speedo indicate it is getting on with the program. Normal acceleration, it is simply tight and slightly firm as it should be. I think his complaint is the he could feel it shift more before it roasted the forward CP because it was so damn lazy between shifts allowing the engine to flare. I have yet to declare my build successful. I usually reserve that declaration for the first 25k miles. Lugnut |
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