Chester_Kaup

I have a 1966 mustang with a C4 automatic with a transmission leak that I cannot figure out.I let the car warm up, check the fluid level and correct if necessary. I then check under the car while idling in park and drive for leaks. All is dry. I will then take the car for a drive and bring it back into the garage. I then check for leaks and everything is dry. The next morning there is a puddle of transmission fluid under the car.I have tried about every pan gasket and gasket sealer known to modern man. My procedure is to clean the mating surfaces and dry them. I also make sure the pan flange is flat. I then install the pan and tighten the bolts to the recommended torque.When I inspect the outside of the transmission after it leaks it is dry around the case fill dipstick, transmission modulator, shifter shaft and overflow tube.Is there somewhere other than the pan it might be leaking from?

Gun Jam

what about power steering?

imp

Cracked transmission case, worst scenario.

Urambo Tauro

Something has to be wet where the leak is originating from. Front pump seal? Extension housing?

I don't know what your driveway looks like, but if you're parking it inside the garage, you should have a reasonably flat surface for leaking fluid to drop straight down.You should have been able to narrow down the general area by now, just by looking at where it's dripping on the ground. Slide some cardboard underneath there if you need a fresh surface to watch where puddles to start.

groho

Common problem. The 66 c4 has a vent tube on the drivers side just above the linkage and neutral safety switch. Basically, your torque converter is draining back into the tranny through the pump and over filling the case as it sits in the garage. The problem is not when you're driving. Not a big issue, easy fix. You can add vacuum hose to the short 5/16 vent tube and route it upward so any excess fluid drains back into to tranny. The vent tube faces down, you might consider shorten it, or turning it forward before add a hose extension, otherwise fluid will just sit in the lower loop of the hose and never drain back.

Chester_Kaup

It appears that the leak is around the bolts that hold on the pan at the rear of the transmission. Thanks for the suggestions and information.

347mav

Shifter shaft seal. Where the shaft goes through the case. Runs down on the pan.
​​

Chester_Kaup

The leak has been fixed.
1. Removed the pan and cleaned the transmission and pan matting surfaces
2. Made sure the pan was straight and flat
3. Applied a bead of permatex #2 on the pan rail and around each bolt hole
4. Pressed the pan gasket gently to the pan and wiped off any excess permatex
5. Let set for 4 hours. The recommended time before placing in service
6. Applied a bead of permatex #2 to the pan gasket and around each bolt hole
7. Bolted up the pan putting permatex on the bolt threads of the bolts in the area that seemed to leak
8. Let set for 24 hours; the recommended time for a full cure.
9. Refilled the transmission
10. So far no leaks after 2 test drives a week apart

I know usually the gasket is put on dry but sometimes some old fashioned gasket sealer gets the job done.

ksrancher

I also have a 66 Mustang with a C4, only leaks above 65 MPH, Had the car out this weekend and pushed it up to 90 MPH for quite a distance and started noticing smoke rolling out the rear, dropped back to 60 and quit, after driving 2 more miles without any smoke, i pushed it back up to 90 or so and smoke started rolling out the back again, when i got home i could see transmission fluid all over the rear of the car and all over the transmission. Just put this transmission in about a month ago after the car set for 10+ years in my garage, I replaced all the seals, front pump seal, rear driveshaft seal, shifter O'Rings, modulator O'Ring and pan gasket. not sure where the oil is coming from, filled it back up with oil and let it set and it leaked out on the floor around the pan area. I power washed the transmission before installing it so the oil just went everywhere, the smoke is coming from the oil getting on the exhaust. it dont seem to happen if i stay under 60 ish. Also noticed when i use "D" it starts out in second and shifts to 3rd, if i start out in "2" it shifts through all 3 gears starting out, if i start in "1" and shift to "2" it goes from 1st to 3rd right away and skips 2nd all together. is there a ball i can change in the valve body to fix this issue?

Derf00

Most dry gaskets can benefit from a thin smear of RTV on both sides of it to fill in any imperfections of the surfaces they will be sealing. The exceptions are rubber or EPDM gaskets.