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Old 04-27-2005, 09:58 PM
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Vacuum Modulator/84 300 turbo Diesel

I am wanting bypass the switchover valves on top of the valve cover and have a single line vacuum source from the brake booster line to the vacuum modulator and then tweak the system, does anybody no if the side of the modulator is where the vacuum source would go and the top would go to the trans. Thanks
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Old 04-27-2005, 10:22 PM
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The supply goes to the top of the vacuum control valve, the side is the vent.
http://www.ultrahot.com/vacuum/egr_82_300d.jpg
Reference this diagram, all you need to do is disconnect the white line from the three-way connector to the switchover valves (71), then plug the unused port on the connector. Or you could replace the three-way connector with a single piece of vacuum hose. There is no need to do anything with the vent line (Black) that runs from the switchover valve to the side of the vacuum control valve.

Last edited by tangofox007; 04-27-2005 at 10:35 PM.
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Old 04-27-2005, 10:33 PM
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How does the modulator meter the vacuum if the supply and the line that goes to the trans. are on top of the modulator.
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Old 04-27-2005, 10:38 PM
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The modulator is on the transmission. It is operated by vacuum, it does not meter vacuum. The vacuum control valve controls the vacuum level by leaking ambient air into the vacuum supply through the port on the side of the VCV. The more that throttle is applied, the more is leaks.
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Old 04-27-2005, 11:21 PM
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Thanks for the info now I understand, One more question my trans is shifting
just about right at low throttle but when I punch it its to soft with some flaring I disconnected the line from the modulator on injection pump that goes to trans modulator and it shifts way to hard does this mean that the modulator on the inj. pump is not venting enough vacuum at higher throttle positions. Thanks
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Old 04-28-2005, 09:16 AM
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Quote:
Originally Posted by lakerat
Thanks for the info now I understand, One more question my trans is shifting
just about right at low throttle but when I punch it its to soft with some flaring I disconnected the line from the modulator on injection pump that goes to trans modulator and it shifts way to hard does this mean that the modulator on the inj. pump is not venting enough vacuum at higher throttle positions. Thanks
I am working through similar issues It is very helpful to have a vacuum gauge or a mityvac to read your vacuum at various points in your system.

It sounds like you may have too much vacuum or your modulator on your trans may need adjusting. Put a gauge on the line that feeds the modulator - it is black and heads down towards the transmission from your switchover valves on top of the valvecover. You should read in the 15in range and then it should drop to or near 0 when throttle is applied. If it does not reach at or near 0 you are supplying too much vacuum to the modulator which can result in early shifts and major flaring.

I see you already adjusted you modulator, but it is best to solve any issues upline prior to adjusting your modulator. IOW, verify proper vacuum levels to the modulator prior to adjusting the modulator. Someone said in another post that if you get your modulator adjusted too far it can fry the tranny.
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