Mustang II 4 cyl gear hunting and possibly messed up vacuum system

[oesterreicher]

Hello all!

Through a few lucky coincidents, I became the owner of a Mustang II, 4 cyl, 2.3L from 1974.
During driving I noticed that the transmission was gear hunting. Essentially, the transition from first to second gear always works smooth.
However, in the range of 25mph to 45mph oscillation between 2nd and 3rd gear occurs.
I checked the kickdown rod and confirmed that it starts to move only when the gas pedal is almost fully pressed (last half inch).
I then went to check the vacuum modulator on the C3 transmission and realized that it is a dual diaphragm modulator.
Both diaphragms seemed to be intact and held 16 inHg vacuum without leaks.
Afterwards I checked the rest of the vacuum system and found no leaks. The diaphragm of the EGR valve and distributor were intact as well.
The vacuum on the intake manifold was 15-16 inHg with the engine running in idle.
Idle speed in neutral is approx. 800-900rpm (warm engine).

When I inspected the vacuum system, three facts confused me. Maybe somebody can explain this to me:
-) My vacuum reservoir is directly connected to the EGR valve and not to the vacuum amplifier or the vacuum line coming from the intake manifold.
-) The manifold vacuum is connected to the actuator under the distributor as well as to the carburator (Motorcraft 5200).
-) The second diaphragm of the transmission vacuum modulator is directly connected to another port of the carburator (same line that also goes to the vacuum amplifier)
Please see the attached diagram of my vacuum system - my description above migth be unclear. Maybe a previous owner messed these connections up??
Since I am not an expert on this matter, I would be very thankful if someone could give me advice, since I could not find diagrams that match my current vacuum system in either the 1978 Car Shop Manuals Vol 1-5 from Ford or the Mustang II Owners Workshop Manual from Haynes. Unfortunately I don't have the Shop Manuals from 74 available at the moment...
Additionally, does anyone have ideas on what I could check next, to hopefully fix the gear hunting issue?

Thanks in advance for sharing your expertise - any guidance to help me untangle this is hugely appreciated!

 

[LILCBRA]

Welcome to !

First, my disclaimer: I am in no way an expert in automatic transmissions, so all I can do is offer some ideas of what I may look at.

How are the band adjustments? I don't know if it would search for a gear if it were out of adjustment per se, but it definitely wouldn't hurt to make sure it's properly adjusted to rule it out.

How is the fluid? Maybe a fluid change wouldn't be a bad idea if it's been in there a while. Then, while it's drained, I'd inspect the valve body for anything that stood out and give it a good cleaning to make sure that that isn't the culprit.

And I'm going to list checking for leaks although I'm sure you've already done it. These transmissions work on pressure, so any kind of pressure loss will inevitably cause some kind of issue.

Beyond that, I'll admit that it's out of my league. Maybe someone else can offer something more - good luck!

 

[extra_stout]

Hi there!

I'm located in Germany. Nice to see another guy from Europe interested in this cars.

I'm also not an expert in automatic transmissions. I have just one idea for you: if the vacuum lines are original, than I would replace them. It could be, that the vacuum leak you are hunting, is coming an going with vibration?

Hope you solve that issue soon.

 

[oesterreicher]

Welcome to !

First, my disclaimer: I am in no way an expert in automatic transmissions, so all I can do is offer some ideas of what I may look at.

How are the band adjustments? I don't know if it would search for a gear if it were out of adjustment per se, but it definitely wouldn't hurt to make sure it's properly adjusted to rule it out.

How is the fluid? Maybe a fluid change wouldn't be a bad idea if it's been in there a while. Then, while it's drained, I'd inspect the valve body for anything that stood out and give it a good cleaning to make sure that that isn't the culprit.

And I'm going to list checking for leaks although I'm sure you've already done it. These transmissions work on pressure, so any kind of pressure loss will inevitably cause some kind of issue.

Beyond that, I'll admit that it's out of my league. Maybe someone else can offer something more - good luck!

Thank you!
I have been told that the transmission oil was changed a few months before I got the car.
I'll definitely check the adjustment of the bands - or rather of the one band since the C3 transmission only lets you adjust the intermediate band as far as I've read until now. And yes there are no obvious leaks.

 

[oesterreicher]

Hi there!

I'm located in Germany. Nice to see another guy from Europe interested in this cars.

I'm also not an expert in automatic transmissions. I have just one idea for you: if the vacuum lines are original, than I would replace them. It could be, that the vacuum leak you are hunting, is coming an going with vibration?

Hope you solve that issue soon.

Nice! Which part of Germany are you based in? I am from the west part of Lower Austria, near the Czech border.

I mean visually the vacuum lines look fine and seem to hold the vacuum. But you might be right. Its worth a try and would be a simple fix. I am, however, unsure if all the vacuum connections are correct at the moment. As mentioned in my original post, some of the connections don't make sense to me (e.g. EGR valve and reservoir connected together...).

Do you, or does anybody by any chance have the Ford car shop manuals from 1974? I would really like to check if the vacuum system is hooked up correctly, or if a previous owner has changed stuff. Unfortunately my 78 shop manual isn't of great help since they've changed emissions related stuff in 75 and 76, I believe.

 

[2Blue2]

Welcome newest Member Oesterreicher

Unng, I think Were gonna need a better picture to diagnose this ,

it is a dual diaphragm modulator.

You said every thing is holding 16 at idle.
Could it be as simple as the diaphragm while holding vacuum is to Old and Stiff-?- so working poorly at normal vac levels?
Idk

 

[Mcmahst]

I’m also no expert, but I think the next thing I would test is if the vacuum modulator actually functions with vacuum. I would remove it and verify when you apply vacuum to each port, that the plunger moves.

Something else to consider. This is part of the torque sensing function of the engine/trans. I know from my own experience with a 74 Pinto, that when the motor had very low compression, due to long-term maintenance negligence, that any amount of throttle pedal between 2/3, would cause abnormal and abrupt shifts (my brothers car didn’t have an oil change fir >30k miles, hence the neglect). Typically an up shift when it didn’t seem necessary. I finally determined the problem when I ran out of ideas and did a compression test. That pointed to an extremely weak engine, etc.

Good luck in solving your problem.

 

[extra_stout]

I checked the Manual i have and Made the Photos in the Attachment
It is for all years.
Perhaps it helps you to understand part of the Vacuum system.

 

[oesterreicher]

Since I was quite busy at work, I didn't have too much time to work on the car recently. The fact that the friend I share a car lift with occupied it a lot recently did not help either.

In hindsight, this was a lucky coincidence, since I looked at the kickdown rod again and embarrassingly found out that the return spring was missing. After fitting a new one, the gear hunting stopped.

Unfortunately, the shifting behavior still is not perfect, since the transmission holds second gear for a long time even when I only slightly press the gas pedal (switches to third gear at approx. 3500 rpm...). At 30mph, when I release the gas pedal, the car always shifts into 3rd (which I believe is correct). This leads me to believe that 2Blue2's theory with the diaphragm being stiff might be correct. I'll check that as soon as the car lift is free again. While I'm at it, I'll also check compression, as recommended by Mcmahst.

extra_stout - thank you for the photos of the manual.

2Blue2 - see attached a few better photos of my Mustang. XD

 

[extra_stout]

Nice!

 

[85GTStangGuy]

Do you, or does anybody by any chance have the Ford car shop manuals from 1974?

Would one of these help?

https://www.ebay.com/itm/165945078490?mkcid=1&mkrid=711-53200-19255-0&siteid=0&campid=5335821607&customid=&toolid=10001&mkevt=1

https://www.ebay.com/itm/306949209760?mkcid=1&mkrid=711-53200-19255-0&siteid=0&campid=5335821607&customid=&toolid=10001&mkevt=1

 

[manicmechanic007]

That kickdown link should be spring loaded (2 ways) and start moving at about 1/2 throttle
It should be all in by WOT (full throttle)
Gear selection in those old ones is only controlled foot feed and vehicle speed
If you keep it floored, it should not hit 3rd gear until about 45-50 mph
You foot around and it'll shift all over the place normal