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Discussion Starter · #1 ·
Hello I have a 78 c3 with a 350 crate motor, recently i have been having some issues with the car changing gears until it reaches high rpms, My tech doesnt work in the car right now but i can just feel the car not shifting when it should. When i am driving it shifts into 2nd fine but shifting into third i have to give it a little more gas just for it to shift up into 3rd and same with fourth. This only happens on normal driving through the city when i give it full throttle on the high way it will shift just fine. I have checked the vacume line from intake to tranny and i just changed the vacuum modulator about 7 months ago. Also my lights have stopped going up which made me think vacuum leak for sure since the vacuum lights and the trannys vacumm line share a port so i caped off the vacumm line to the lights and kept just to the tranny and still same results, I was told that some trick shift fluid may help the problem but not solve it. I dont know what eles it could be any ideas out their would very much be appreciated
 

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Discussion Starter · #3 ·
I believe its the original transmission not exactly sure the type that it has i believe its the THM 200 but i could be wrong, and it only happens when switching from 2nd gear to 3rd gear and from 3rd gear to 4th gear, it will stay at about 35 mph then have to give it gas just for it to switch to the next gear? same with 4th it will stay anywhere between 45-50 then apply a little gas and it will switch to 4th gear have no idea what it could be?!?!?!
 

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I believe its the original transmission not exactly sure the type that it has i believe its the THM 200 but i could be wrong, and it only happens when switching from 2nd gear to 3rd gear and from 3rd gear to 4th gear, it will stay at about 35 mph then have to give it gas just for it to switch to the next gear? same with 4th it will stay anywhere between 45-50 then apply a little gas and it will switch to 4th gear have no idea what it could be?!?!?!

Google "Identifying G.M. Automatic Transmissions" and you'll find a website that shows what their oil pans look like. Then once you positively identify what kind of transmission you have we'll go from there.
 

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OK,did you just get the car? because your car came with a TH350, 3 SPEED Auto/ with vac mod...if you now have a TH2004r 4 speed auto , its been transplanted..but, the 2004r is throttle cable control shift, not vacuum modulator control .. if it is T.V. cable control, you need to loosen the cable adjustment slightly. it will lower your light throttle shift points
 

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Discussion Starter · #6 ·
I believe its a th350 but I though there was 4 gears not 3 I may be mistaken I have to look and compare because i do have the vacuum modulator and I do have the tv cable thats next to the carb I will check for sure later tonight and see does anyone know if there is a serial number any where on the tranny that I can look for ?
 

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look at the trans oil pan.......If the pan is shallow and mostly square but has one of the rear corners missing, creating a diagonal instead of a corner, then the transmission is a THM350 model..and the cable is only a kickdown, the modulator controls part throttle shifts

If the pan is deeper and elongated with a piece of the pan towards the rear of the vehicle shallower than the rest it is a THM200-R4, then the cable is a T.V. cable and controls part throttle shifts
 

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Discussion Starter · #8 ·
Just checked its.the thm350 do you think its the vacuum modulator ?? Last time it was damaged I had tranny oil in the motor but I check and theres no Tranny oil in the motor so I don't think it would be the modulator but then again I could be wrong
 

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probably a low vacuum issue.. make sure you have a dedicated direct manifold vacuum source for the trans.. you may have a vac leak and its affecting the modulator..pull the vac hose at trans while idling, see if it has good vacuum, do you have a vac gauge? if your motor is stock, you should have around 16-17inches ...how long has it been since the filter has been changed.. a clogged filter could affect it too...
 

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Modulator

Just checked its.the thm350 do you think its the vacuum modulator ?? Last time it was damaged I had tranny oil in the motor but I check and theres no Tranny oil in the motor so I don't think it would be the modulator but then again I could be wrong

If you happen to have one of those handy little hand-held vacuum pumps connect it to the modulator and see if it'll hold vacuum. If it doesn't then the diaphragm in your modulator is bad and is most likely the cause of your delayed shifts. And like Fish said, you should have a "dedicated' 1/4" vacuum hose connecting it to your intake manifold. I always use 1/4" fuel line hose because it isn't affected by oil and has the internal braid for added strength. And for added insurance against vacuum leaks put hose clamps on it.
 

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Discussion Starter · #11 ·
The modulator was just changed in Feb. Could it have gone bad already ? I don't have a pressure reader but I guess its time to invest in one considering the c3's run on a lot of vacuum i checked the line on the intake and it seems to be fine but I guess i wont know till i get that pressure reader I will see what I can do and update tommrrow
 

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The modulator was just changed in Feb. Could it have gone bad already ? I don't have a pressure reader but I guess its time to invest in one considering the c3's run on a lot of vacuum i checked the line on the intake and it seems to be fine but I guess i wont know till i get that pressure reader I will see what I can do and update tommrrow

Just hook a 1/4" hose to it and suck on it. It should hold vacuum.
 
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