Hey guys, I have a TH400 behind my SBC and when I drop it into drive the idle drops about 250 RPM. I think this is from the vacuum module drawing vacuum from the the intake and changing the vacuum advance. If I block off the vacuum module and just manually shift the trans, will I do any damage to the transmission? Or is there something I can do to the module to stop it from drawing so much vacuum? Thanks in advance.
once the vaccum hits the module, it stops- it's not like a brake booster which has some loss to it. simple test- unplug it, plug the line, and drop it into drive. if it stays the same, You have a different problem. (my hunch is You have converter/ tune up issues.)
The rpm drops because of engine load, if you block of the vaccum line the only thing your going to cause is late shifts.
This is correct. The modulator works in conjunction with the governor valve to regulate shift points. If there is not vacuum to the modulator, the shifts will not happen until the governor valve overcomes the spring pressure on the shift valves. You would have a 1-2 shift at 45-50 MPH (even at light throttle) and 2-3 at 65+ MPH.
The modulator valve does not "draw" vacuum. Its just a device that monitors throttle position by sensing manifold vacuum. If you have your doubts about it, just replace it. They don't cost much. Like said above, the combination of the modulator and governor determine shift points. This is assuming that your pump pressure is within spec. My guess on your description is that you have your vacuum advance hooked to manifold vacuum. When you drop it in gear, idle speed decreases and thus the vacuum signal decreases causing your timing to retard. As an experiment hook your distributor vacuum advance to 'ported' vacuum and set the idle to your desired rpm. Now when you drop it in gear, the idle speed will be closer to the same as when its in neutral since your ignition timing didn't change.
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