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Old 07-29-2010, 10:04 AM   #1 (permalink)
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C6 transmission modulator valve

How do you adjust the transmission modulator valve?
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Old 07-29-2010, 11:13 AM   #2 (permalink)
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Originally Posted by Az602623 View Post
How do you adjust the transmission modulator valve?
@the transmission vacuum line, you pull hose off and inside modulator there is a flat adjustment screw. In keeps in gear longer out get out of gear sooner. Every turn should make about 2 mph differance
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Old 07-29-2010, 11:21 AM   #3 (permalink)
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Thank you sir that helps alot. I couldnt find shit online about it
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Old 07-30-2010, 06:12 AM   #4 (permalink)
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VRV and Modulator Adjustment

I just went through this process on a used 89 F250 7.3 IDI C6 I just bought, and what a difference! The VRV (vacuum regulator valve) was stuck at 0", so somebody had jacked in the modulator screw just to get it to shift at all. When I bought it, it would shift just off idle. By the time it got up to 20 MPH it was in 3rd. The early shifts were really soft, I thought it was going to trash the tranny if I kept it that way. So here's what I did:

1) Took the VRV off the IP. Tried moving it with vacuum applied, it was really stiff and stuck at 0" vacuum. Removed the side cover of the VRV and found the rod had slipped off its the cam and was jammed. Took the cam and spring assembly out of the housing, cleaned out all the dirt with a rag, wiped in some plastic-friendly silicone lube, put the cam/spring back in, applied a bit more lube and engaged the rod, put the cover back on. Tried turning it again, with vacuum applied. Action was much smoother, and vacuum was variable from 0-14".

2) Mounted the VRV on the IP and adjusted it per http://www.intellidog.com/dieselmann/idi2.htm, second paragraph up from the bottom of the page.

3) Took the truck for a ride, with a small screwdriver and creeper. Shift points were way up near the redline, even with light throttle, and stiff for a stock tranny. Stopped every now and then, crawled under the truck and took a half-turn CCW on the screw inside the vacuum modulator. Shift points gradually dropped, I quit when they got to about 1500 RPM with light throttle and near 3000 with my foot in it, which is where it just "felt right". Shifts at low throttle are smooth, but the more I'm in it, the firmer they become.

What I learned... The VRV adjustment is not for adjusting shift points, it should be left at the factory spec so it approximates the manifold vacuum of a gasoline engine. The vacuum modulator is the proper place to adjust the shift point on a C6.
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