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High RPMs while driving

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blazerman78

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Hi guys. Please excuse my lack of member-ness, I'll probably pay up next week when I get my paycheck. Money is tight, but this site is worth it! /forums/images/graemlins/thumb.gif
Anyway, here's my problem. Recently my motor has been turning high rpms while driving. The transmission shifts through all gears, but looking at my tach I can tell that the motor is between 500 and 1000 rpms higher than it should be. It wasn't always like this, and sometimes it works normally, although not often.
Does anyone know what coudl be wrong here? Any help is appreciated. Thanks.
 
What kind of transmission is in your truck? If your RPMs are higher than normal and it's a manual then your clutch is probably slipping. It might need to be adjusted or replaced.
 
check the fluid in the tranny and see if it is burnt or not if it is change it and try it then if not the the clutches are slipping and its time for a rebuild /forums/images/graemlins/eek.gif
 
It's a TH350. The fluid is completely clean and does not smell burnt at all. Would the modulator have something to do with it?
If not, about how much would a tranny rebuild cost? /forums/images/graemlins/frown.gif
 
It shifts 1st to 2nd to 3rd, but the shift points are alot higher than they should be, and the motor is spinning alot higher than it should be. For example, say it should be at 2000rpm at 50mph. Now it is at 2500-3000 at 50mph. If I really step on it from a stop, it shifts out of 1st at close to 5500rpm!
And no, it's not a defective tach. You can hear the motor spinning really high.
 
Sorry for the basic questions, but I don't know how much you know. /forums/images/graemlins/deal.gif
When you check the tranny fluid level, is the tranny at normal operating temperature and the truck is on a flat surface and running in park?
When's the last time the tranny filter was changed? Could be plugged.
 
Yes, the tranny is hot and the truck is level when I check it. The filter and all the fluid only have about 3000 miles on them. I've been lookin garound online and found some info that may be pointing to a bad modulator. Could that be what is wrong?
 
Check your vacuum lines. Should be one coming off the back of your carb (sometimes the front) that ties in to the tranny. If you shook it loose, you'll run WAY higher that usual. The vacuum line is what controls the modulator, if I remember right w/ my TH350. 10 to 1 says that's your problem /forums/images/graemlins/thumb.gif
 
I know that the modulator line is connected to manifold vacuum, and I'm pretty sure it doesn't have any holes in it.I checked it at the modulator when the motor was running and it had vacuum. I'm going to try and replace the modulator tomorrow and see if that fixes it.
Anyone ever done this before? It looks pretty cramped in there, like maybe the t-case shift linkage would get in the way. Should I unbolt the linkage and move it out of the way? Or is there an easier way?
Thanks for your help, guys.
 
If its shifting higher and the tranny fluid is clean and there is enough of it I would suspect a vacuum leak, mine used to do the same thing until I tracked them all down, there were a few.

if your turning much higher RPM while driving I would suspect a converter issue... which if it is your converter, if it goes out, it will take the rest of your tranny with it (at least that has always been my experience) So, I would get it to an aamco or at least a reliable tranny shop and have them check it all. If its shifting fine, then I would even more strongly suspect a converter. cheap and easy to fix when compared to the cost and hassle of a full tranny rebuild and a new converter on top of it.

just my .02

-Ryan
 
if u are going to change modulator ur going to have to pull the pan and maybe the valve body..important check balls in there... all of these people have good answers..converter...fluid level...modulator..ur linkage could be misadjusted from wheelin.. first check vacumm leaks and try adjusting modulator..try lifting the vehicle off the ground and manually shift through the gears..good way to check converter with no load..then do a pressurse check in all the gears if those are bad then u need a rebuild... get back to me if u need more help
 
on a th350 you do not have to pull the pan or anything to change the vacuum modulator it is just one 7/16th bolt on the back of the tranny holding it in kinda like the distributor
 
Well, I replaced the modulator today, and it didn't do anything /forums/images/graemlins/frown.gif On the other hand, it only took a couple of hours, and the old one was bad anyway (fluid in the line), so it wasn't completely wasted time.
I'm almost positive I have a vacuum leak somewhere, I just have to find it. Would a vacuum leak really mess up the transmission that badly?
It shifts almost exactly 500-600rpm higher than it should, and it runs that much higher in gear,too.
Which way do I turn the screw inside the modulator to bring the shift point down? i think it's counter-clockwise, but I'm not sure.
Thanks for your help guys. I hope I can get this fixed soon.
 
Well, by turning the shift point down won't you just be hiding the problem? If it is the converter or the cluth packs then you should get it fixed right away!
 
How would I check if there is a problem with the converter or the clutch packs? Would you reccomend I take it to a shop ASAP?
 
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