iceman Posted May 28, 2001 Share Posted May 28, 2001 I have a TCI TH400 behind my BBC. I have had numerous problems with blowing out the moduluator valve on the tranny. I have the valve connected directly to the intake manifold port behind the carb so the valve sees full vacuum all the time. I just broke in the new cam yesterday and as soon as the motor fired the valve failed. This is the second valve in less than 2500 miles. Any ideas on what may be causing the failure? Quote Link to comment Share on other sites More sharing options...
alex's72fgmc Posted May 29, 2001 Share Posted May 29, 2001 hey iceman, your problem is most likely the full vacuum location of your vacuum line. since it see's full vacuum all the time and you have a "possibly" a big cam in it when it warms up or when you are revving it for the break in time the modulator fails due to too much vacuum on the valve. move it to a timed vacuum port on the carb to see if it clears up. or see if it is ajustable and adjust the valve accordingly i beleive they are adjustable. hope this helps ------------------ hmmmm the sound of wrenches turning and a air impact gun going... slowly out from the garage emerges a luxury muscle car that is anything but... it is.. the monte carlo with a new edelbrock performer rpm power package and new 350 block stroked. Quote Link to comment Share on other sites More sharing options...
Guest Posted May 29, 2001 Share Posted May 29, 2001 Hey Iceman, Not sure why your modulators are failing... Alex, I believe your modulator should be hooked to a vacuum port directly off of the manifold, and the modulator should get constant vacuum, and the vacuum will be regulated be your engine speed, this is how the modulator affects when to tell the tranny to shift. If you have a big cam and since big cams create less vaccuum this will affect when the tranny shifts, and then if you have a stall converter then it will really mess up when the tranny shifts. With the high stall and poor vacuum from a large camshaft your RPM's will be high, yet your road speed will be slow. The tranny will shift based on the vacuum created by the engine speed. High enginge speed, low road speed, tranny shifts tranny shifts to early for the road speed. Forgot to mention that you must constant vacuum to the modulator.....if you don't you will balloon your convertor. [This message has been edited by David72 (edited 05-29-2001).] Quote Link to comment Share on other sites More sharing options...
alex's72fgmc Posted May 30, 2001 Share Posted May 30, 2001 thanks david72, I stand corected then but i beleive though that the modulator is adjustable i will have to look more into this then time to crack open a chiltons. let you know what i find out. sorry about the misguided info iceman Quote Link to comment Share on other sites More sharing options...
Andreas Posted May 30, 2001 Share Posted May 30, 2001 B&M sells and adjustable vacuum modulator. However, you can vary the shift points within 5 mph only. All you need is a small screwdriver... ------------------ 71 Monte 383 "The Full Monty" 86 Monte LS ZZ4 "The Gray Mouse" Andreas' Monte Carlo Site Member NMCOA, FGMCC (Event Coordinator) --------------------------------------- "It ain't a car if it's not a Monte" Quote Link to comment Share on other sites More sharing options...
Guest Posted May 30, 2001 Share Posted May 30, 2001 Yes, the modulator is adjustable, you can adjust them to shift either early or late. There is an adjusting screw inside the vacuum port inlet on the modulator. Quote Link to comment Share on other sites More sharing options...
iceman Posted May 30, 2001 Author Share Posted May 30, 2001 Thanks for the info. I am not running a really big cam. The intake profile is 226° at .050 and 0.540 lift. The exhaust is 234° at .050 and 0.564 lift. The lobe are on a 107° center line. The cam is installed 3° advanced. Nothing radical, just a little more than stock. I rebuilt the engine this winter and changed the cam but the only difference is a slight increase in exhaust lift and duration. With the old cam I had 14 in. of vacuum at idle and about 24 in. at cruise speed. I did talk to TCI and they believe that the diaphragm in the valve could have dry rotted from sitting over the winter. I have never heard of anyone replacing these valves as a preventative maintence measure. Thanks for the tip on the adjustable valve. I'll have to look into this. Quote Link to comment Share on other sites More sharing options...
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