C4 Slipping When Accelerating - Ford Mustang Forum
 
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post #1 of 7 (permalink) Old 08-04-2019 Thread Starter
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Transmission Slipping When Accelerating

I haven't been able to determine the source of my C4 transmission slipping when I'm accelerating. It used to slip at gradual acceleration around 30 mph but after I rebuild the transmission then that went away. It now slips around the same speed when I floor it. I played around with adjusting the front band but that didn't have any effect. I also adjusted the vacuum modulator one turn counter clockwise and that didn't help either. Not sure what the next steps should be. Continue adjusting the vacuum modulator or look elsewhere. I was wondering if the torque convertor could be the problem and need to know how I would evaluate that.


Last edited by JoeKlo; 08-08-2019 at 10:01 AM. Reason: Title change
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post #2 of 7 (permalink) Old 08-04-2019
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Have you tried changing the fluid?
I hope you tried to adjust the bands using a shop manual.


15/5/1964 260 Manual Coupe.
23/3/1965 289 Manual Fastback.
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post #3 of 7 (permalink) Old 08-04-2019 Thread Starter
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Fluid was changed during the rebuild. The bands are for reverse, low and second so it shouldn't have an effect in drive where it's slipping above 30 mph and only when accelerating. Truthfully the reverse, first and second gear work fine.

Last edited by JoeKlo; 08-05-2019 at 12:56 AM.
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post #4 of 7 (permalink) Old 08-04-2019
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Sounds like it needs a proper rebuild.
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post #5 of 7 (permalink) Old 08-06-2019 Thread Starter
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Here's the things that were done during the rebuild:
-Disassembled entire valve body, clean and lubed all parts and replaced check valves.
-Replaced bands
-Replaced clutches
-Replaced front and rear shaft bushings
-Disassembled and cleaned governor
-Replaced seals and orings where needed
-Replaced gaskets where needed
-Performed bench test on clutches and bands
-Replaced filter and vacuum modulator
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post #6 of 7 (permalink) Old 08-06-2019 Thread Starter
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I read up on the torque convertor test. The procedure says to put the parking brake on, put foot on the brake, place into drive, floor it for 2-3 seconds but not more than 5 seconds or it could damage the transmission, record the rpms off your tact. The highest rpm during the 2-3 seconds is your stall speed for your torque convertor. The factory specs for a C4 stall speed are between 1600-1800 rpms. Lower than spec means the torque convertor is failing. Higher than spec means the transmission is slipping. I'll perform the test this weekend and report the results.
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post #7 of 7 (permalink) Old 4 Weeks Ago Thread Starter
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TC test passed with max RPM just shy of 2000 using an analog gauge. The vacuum modulator adjustment seemed to help as I set it to engage an little earlier. Will make a few more runs to dial in and rule out any more slipping. The timing appears to need some adjustment. I bought a spring kit for the mechanical advance but haven't installed it yet.
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