DeltaHF

I have a 75 mod with a 350 cid. TH400 transmission. and a 3.08 rear axle ration.

The problem is that I have 3700 RPM at 50 Mph. ( With sidepipes this makes a lot of noise)
The transmission gears from 1 to 2 of 20 Mph and from 2 to 3 at 33 Mph.
When I gear from 2 to 3 the RPM drops with 700 RPM.
The transmission was originally from a Chevy diesel truck / van.

The TH 400 should have a 1-1 ratio at 3rd gear.
My rear wheels are 64 cm in diameter.

If everything was working as it should I should have 2100 RPM at 50 Mph.

Could someone give me som tips?

MelWff

Does the fluid smell burnt? Do you hear a wining noise? Either the transmission is in 2nd gear or it's slipping or the torque convertor has an issue.

DeltaHF

Hi.

There is no burnt smell. The transmission oil looks very good.
There are no wining noise.
The converter is a Summit converter with stall speed at 1800 RPM(2 years old).
I think I will try too fit the original converter and see if it makes a difference.
If the third gear slipps shouldn`t I have problems with accelrating too higher speed?

7T1vette

Are you sure you are getting the transmission into 3rd (drive) gear? If the vacuum modulator is defective...or vacuum is not getting to the modulator (blocked tube, etc), then the trans will stay in 2nd gear beyond 50 mph and the rpms will be very high. Check out the vacuum modulator on the right side of the tranny [near the oil fill tube]. If you remove the vacuum line and oil comes out, the modulator is likely bad and needs to be replaced. Most auto parts stores (in the US, anyway) carry the replacement parts. The new replacements look quite different than the original modulator.

bashcraft

So you've been running this trans for 2 years? Has it always been like this or just recently? What trans was in it before this one?

larrywalk

Calibrate your speedometer AND your tachometer. If both are correct, the trans is slipping badly in 3rd, or never shifted into 3rd. In the latter case, it's often the vacuum line to the transmission vacuum modulator being loose or leaking.

DeltaHF

Hi and thanks for all the reply.
I have only used the car this season with that transmission. The original was a TH 350.
I know the car shifts from second too third gear. But I will take a look at the modulator.
The speedometer is perfect( cheked it with a GPS) The tachometer is also ok.
It looks like i have to get a rebuild kit for my tranny

I suppose the Transmission Clutches are the problem...