Guest adigitize Posted May 25, 2015 Share Posted May 25, 2015 Hey guys just had a couple questions and would greatly appreciate some feedback, I have a 83 el camino conquista. It has a rebuilt 305 and currently has 50k miles. Ive been driving it for about a year and a half now. about 2 days ago i noticed it wasn't shifting into 3rd gear and would only if i let off the gas. If i tried to accelerate it would shift back into 2nd revving up at 3000rpms. I checked the transmission fluid and its full and red, doesn't smell burned and was changed about 6 months ago along with the filter. i went into ABC auto and was told I might have to get a new transmission but seeing as it has low milage and a rebuilt engine i wasnt sure if that was the problem. I was looking on these forums and saw that it could be a vacuum hose or the modulator/governor that could be bad? if so, could any of you guys direct me to the instructions on how to replace/fix this part or where to look to fix it? that would be helpful and i would be thankful. anyways, as i left ABC auto, i go to head home and now it is shifting with no problems. I would like to find the problem before it gets any worse or starts back up again. please any feedback would be greatly appreciated..thanks again Link to comment Share on other sites More sharing options...
Paul Dobbin Posted May 26, 2015 Share Posted May 26, 2015 Newbie, Welcome to the AACA Forum, You'll find us opinionated even if we're not always great mechanics. The modulator is an inexpensive part, less than $20.00. Easy to change, as it is on the outside of your transmission. It could be you problem, but it's not necessarily what's causing your problem. Find a reliable transmission service and tell them your problem and that you've heard it sounds like the modulator has gone bad. I've never had that problem with my 81 Conquista, but I did have it with another old GM car with the 307 engine and the bad modulator would hold it in first gear. No substitute for a good transmission guy. Goof Luck. Link to comment Share on other sites More sharing options...
JamesBulldogMiller55Buick Posted May 26, 2015 Share Posted May 26, 2015 Hey guys just had a couple questions and would greatly appreciate some feedback, I have a 83 el camino conquista. It has a rebuilt 305 and currently has 50k miles. Ive been driving it for about a year and a half now. about 2 days ago i noticed it wasn't shifting into 3rd gear and would only if i let off the gas. If i tried to accelerate it would shift back into 2nd revving up at 3000rpms. I checked the transmission fluid and its full and red, doesn't smell burned and was changed about 6 months ago along with the filter. i went into ABC auto and was told I might have to get a new transmission but seeing as it has low milage and a rebuilt engine i wasnt sure if that was the problem. I was looking on these forums and saw that it could be a vacuum hose or the modulator/governor that could be bad? if so, could any of you guys direct me to the instructions on how to replace/fix this part or where to look to fix it? that would be helpful and i would be thankful. anyways, as i left ABC auto, i go to head home and now it is shifting with no problems. I would like to find the problem before it gets any worse or starts back up again. please any feedback would be greatly appreciated..thanks again that's the proximity defect. The closer you get to the shop, the better it behaves. Seriously. as I read your symptoms, I thought "modulator". I would bet on it Link to comment Share on other sites More sharing options...
Guest adigitize Posted May 26, 2015 Share Posted May 26, 2015 thanks guys i will go and get a modulator tomorrow and start working on it. Link to comment Share on other sites More sharing options...
joe_padavano Posted May 26, 2015 Share Posted May 26, 2015 The first question is, which transmission do you have? TH350, TH250, or TH200? The TH200, which is the weakest of the three, does not use a modulator. It uses a throttle valve cable (which is NOT to be confused with the kickdown cable used on the TH350 and TH250). Improper adjustment of the TV cable on the TH200 can cause poor shifting and can eventually burn up the trans. The correct diagnosis depends on which trans you have. Link to comment Share on other sites More sharing options...
JamesBulldogMiller55Buick Posted May 26, 2015 Share Posted May 26, 2015 Well,There's someone with more knowledge on the subject than me.I don't have any expierience with the 200 Link to comment Share on other sites More sharing options...
JACK M Posted May 26, 2015 Share Posted May 26, 2015 I had the same symtoms that turned out to be a very small crack in the vacuum line that ran up from the modulator. I would take a good look at the vacuum line first.Good luck. Link to comment Share on other sites More sharing options...
joe_padavano Posted May 26, 2015 Share Posted May 26, 2015 I had the same symtoms that turned out to be a very small crack in the vacuum line that ran up from the modulator. I would take a good look at the vacuum line first.Good luck.And again, if the OP has a TH200, there is no vacuum line to inspect. One needs to know what trans we are talking about before diagnosis. Link to comment Share on other sites More sharing options...
Guest adigitize Posted May 27, 2015 Share Posted May 27, 2015 The first question is, which transmission do you have? TH350, TH250, or TH200? The TH200, which is the weakest of the three, does not use a modulator. It uses a throttle valve cable (which is NOT to be confused with the kickdown cable used on the TH350 and TH250). Improper adjustment of the TV cable on the TH200 can cause poor shifting and can eventually burn up the trans. The correct diagnosis depends on which trans you have. I am guessing it is a TH200. i do not know the difference between the 3. could you possibly tell me how to know the difference? it started back up today not shifting into 3rd gear so its going in and out of working and not working. Link to comment Share on other sites More sharing options...
joe_padavano Posted May 27, 2015 Share Posted May 27, 2015 (edited) I am guessing it is a TH200. i do not know the difference between the 3. could you possibly tell me how to know the difference? it started back up today not shifting into 3rd gear so its going in and out of working and not working. The easiest way to identify the trans is by looking at the pan. Also, the TH200 (three speed) and 200-4R (four speed) transmissions both use a on-piece case. All other GM automatics of this vintage use a two-piece case with a bolt-on tailhousing. This is because those transmissions were also used in 4WD trucks and needed the option to bolt the transfer case to the trans. The TH200 and 200-4R were never intended for truck use. Edited May 27, 2015 by joe_padavano (see edit history) Link to comment Share on other sites More sharing options...
rocketraider Posted May 27, 2015 Share Posted May 27, 2015 Odds are good that truck has a lockup torque converter transmission as well as a computer that runs everything. Either of those could also cause weird shifting. Try disconnecting the negative battery cable for a minute or so and then reconnect it. Doing that should return the vehicle's computer to factory default settings. Then drive it and see how it behaves. That's as simple and easy a fix as I know and might solve the problem. The computer will relearn your driving style and "self-tune" itself. Link to comment Share on other sites More sharing options...
Guest adigitize Posted May 27, 2015 Share Posted May 27, 2015 ok so i checked and it has 13 bolts and from the shape i can see its the TH350 i checked the vacuum line and there was no cracks in it.. there were a couple plugs that were loose from the modulator so i reconnected those but last night when my girlfriend drove it home, it was not going into gear when she had it in drive. when she would stop at a stoplight, it would just rev up and she would have to turn the truck completely off and back on in order to get it going again. could this be the bad modulator? or does it sound like i might need a whole new transmission? Link to comment Share on other sites More sharing options...
joe_padavano Posted May 28, 2015 Share Posted May 28, 2015 The lockup converter has no effect on upshifting. The modulator can definitely affect upshift, however. The modulator is essentially a throttle position sensor. High manifold vacuum is normally associated with small throttle openings, so the modulator causes the trans to upshift early. Heavy throttle openings cause low vacuum, so the modulator delays upshift. If there is either a vacuum leak in the line or the diaphragm in the modulator is bad, the trans acts as if you are accelerating at wide open throttle and thus delays upshift until the governor in the trans reaches a preset RPM. You can test this by accelerating at W.O.T. and seeing if the trans does eventually upshift (it will delay upshift until about 4500-5000 RPM, so be ready for this). Did you check BOTH ends of the vacuum line, at the trans and at the intake manifold? Before replacing anything, put a vacuum gauge on the hose where it connects to the modulator. With the engine running, you should read manifold vacuum. If not, fix the leak or mis-connected line. If you do get vacuum, replace the modulator. Link to comment Share on other sites More sharing options...
padgett Posted May 28, 2015 Share Posted May 28, 2015 I'd try a bottle of Seafoam Transtune. You may have a shuttle in the valve body that is sticking (had that revving in drive coming off a stop that "healed" before). Modulator may keep in wrong gear but not neutral (never say never). I assume you checked the trans fluid level. ps one sign of a bad modulator is ATF in the vaccuum line. Link to comment Share on other sites More sharing options...
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