Topic: 1966 C-4 Automatic Issue

[jlouis707]

Hello All -

I have a 1966 Mustang with a C-4 Automatic Transmission and 289 2V engine  My Mustang has 67K miles on it. The transmission had a recent fluid change this past spring.  A recent issue has come up and hoping someone on the forum may provide some guidance.

When my Mustang is idling from cold stat up, the engine idles smoothly and no noises or rough idle.  As soon as I shift the automatic transmission to reverse a loud whining/whistling noise develops.  If I step on the brake pedal the noise stops.  When I disengage the brakes, the noise returns and even continues if I when I shift to drive and proceed driving.  If I step on the brake, it stops making noise.  Eventually when the engine reaches normal operating temperature, the whining/whistling noise stops completely.

I first thought it has to do with the brake hydraulics.  I apologize I am not really that skilled mechanically so I did my best to describe the issue. 

I would appreciate any assistance from the Mustang community.

Thanks,
Jeff

[mgmradio]

Check the vacuum line at the modulator valve on the back of the trans. Vacuum line may be off or the valve may be bad.

[jlouis707]

Thank you!!!  It appears the Modulator Valve is the issue.  When I inspected under the chassis,  I noticed fresh transmission fluid leaking around the modulator valve and down the transmission pan.  I am assuming the Valve is bad in addition to the line not fitting properly.

Thanks again for the quick answer and guidance.