C6 rebuild... or not

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'73 3514V convertible, C6, roughly 137k miles. I took her in for a transmission rebuild because of a major slip between 2 and 3 and reverse slips to engage. When I picked it up he told me this transmission takes a dual modulator but his supplier only had a single modulator in stock so that was installed until the correct dual comes in. That was in December 2020. The parts involved: rebuild kit, rebuilt torque converter, 1 L/R spring, new 2nd gear servo, 10 qts ATF.

The trans was actually worse after the rebuild, and I figured I'd give a report when I brought it back in for modulator replacement. On very light acceleration shifting is unnoticeable; smooth and quiet. No slippage when shifting into reverse, On medium acceleration there's major slip from 1 to 2 and again from 2 to 3. On heavy acceleration, there is no automatic upshift. Seriously. I have to completely let off the accelerator then when rpms drop to about 1.5/2k, it will do a long slip-shift into the next gear. And, the backup lights no longer work.

I stopped by today to inquire about the modulator and report on the rebuild of December. He was understandably concerned and said he'd look at it when he installed the dual modulator, which he'd forgotten about, so also, he will call them asap and let me know.

Not sure I'm expecting anything from the forum; there's nothing for me to do until the dual modulator arrives and he gets on the tranny again. I guess just expressing my frustration.
 

Q1SVT

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I might have shared this on 71-73 before, May want to watch this video... listen to his comments on servo [you might not have the correct/big enough one] or 'spring'... The black stripe modulator tooo .... Actually all of his comments.

I like C6's built correctly and with performances parts they can kick butt while reducing the so call parasitic drag... Mine even has the first & second gearing from the E40D.

Call these guys about your needed part https://www.sonnax.com/units/119-c6

 

Q1SVT

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Here is a second one with a little more info again on servo, modulator, etc...

Richard is a good old boy, who knows his chit... love that dirty bird 'firebird'

Hope it help you get the stang back running good, your shop should be looking at how a 1971 429CJ/SCJ C6 was built... 73's 351C's were a non performance in any way. So when we rebuild them to 70-71 level other things need more toooooo. ;^)

edited: link to video 2-2 valve body, is within he 1-2 video description.

 
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My all time favorite vehicle is our 1969 Shelby GT500
In 1973 several ford models got the dual port Transmission Vacuum Modulator. This was due to the EGR (Exhaust Gas Recirculation) system used to reduce NOx emissions. The EGR was actually introduced in 1972 for some vehicles, but I do not have enough detail in my files for 1972 emission control systems documentation to see what was going on with 1972 EGR equipped vehicles to say for sure how they were set up. But, for 1973 I can tell you the center, larger port on the dual port Vacuum Modulator for automatic transmissions (C4, FMX, and C6) went to intake manifold vacuum. The smaller port on the front, side of the Vacuum Modulator went to a Vacuum Tee that was fed by ported vacuum. The Tee then send ported vacuum to the EGR diaphragm, and to the side port outlet of the dual port Transmission Vacuum Modulator. Attached is a schematic (one of many for 1973) that is fairly typical where the carburetor had a port for Spark Advance vacuum, and a port for EGR vacuum. If you need more complete info for the vacuum schematic for your particular engine you can get those schematics from Volume 6 of the 1973 Mustang Shop Manual PDF file, available from Forel Publications at the link below. The PDF file for their Shop Manual is very inexpensive. Please note, the 1973 Colorized Wiring manual also contains vacuum routings, but not for the emission ontrol systems. That is only in the Shop Manual, Volume 6.

 

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Hemikiller

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The dual port modulator was used to raise the shift points when the EGR was active. If your EGR has been deleted or disable, then the dual port unit isn't really needed.

IMO, your transmission rebuilder screwed something up, and he needs to own up and fix it. A C-6 is not exactly what anyone who's rebuilt an automatic would consider "complicated".
 
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The dual port modulator was used to raise the shift points when the EGR was active. If your EGR has been deleted or disable, then the dual port unit isn't really needed.

IMO, your transmission rebuilder screwed something up, and he needs to own up and fix it. A C-6 is not exactly what anyone who's rebuilt an automatic would consider "complicated".
See... that's what I'm thinking. Stay tuned to see how this turns out. Given my EGR has been disabled, the dual modulator isn't the issue.
 
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