So, I'm new here, but I've been reading these threads for a couple days to figure out what could be wrong.

I have an 84 bronco, 302, with a c6 transmission that will not shift. I've checked the vacuum line, which has vacuum. I've replaced the modulator as well. What could possibly be keeping it from shifting?

And this just suddenly happened one day. I was coming home, and was very low on gas, so I put it neutral going down a hill, and when I put it back in drive it refused to shift.

What's wrong with my transmission?

 

Can you be more specific on what "will not shift" means in your situation?
Like, in Drive it will engage first gear and move OK, but will never shift up to second gear, or, putting it into Drive does nothing, no engagement?
And what about Reverse?

Need specifics.

 

If the transmission is put in D, it will not shift at all. But it will be put into first. I can manual shift, however. Everything works on the transmission except shifting in drive.

 

Okay, let me try to summarize what I think I got out of this situation, as you are not using normal automatic transmission nomenclature:

In Drive and Manual 1st, the transmission does engage first gear.

But in Drive, it will not upshift to 2nd gear (engine RPM would just keep increasing to redline as road speed increases). Not a slipping situation, it is solidly in 1st gear.

In Manual 1, it can be manually upshifted to 2nd gear by moving selector lever to Manual 2.

If that is the situation you have, a few possibilities:
The lever on the trans is not totally in-sync with the column, such that the manual valve (the input lever moves the manual valve to select all trans positions) is not squarely centered in its range. I don't know if it was still true in 1984, but earlier C6s had an internal manual valve position detent, and the column shifter had its own detent, and it was important that these two both lined up, or else the trans's manual valve was off-center in its positions, limiting control fluid flow.

Another possibility is the Governor is stuck, blocked by crud, etc. The Governor is a precision spring-loaded valving mechanism that is on the output shaft in the extension housing. Once above a minimum road speed, say 10 MPH, the Governor via centrifugal force produces an output whose pressure increases with increasing road speed. Eventually, the Governor pressure exceeds the Throttle Valve pressure (developed with the Vacuum Modulator) to make the 1-2 shift happen in Drive. Same thing happens for the 2-3 shift in Drive.
No Governor output means no automatic upshift.

IIRC, if you are driving in Manual 2, and get your speed up to say 50 MPH, back off the gas and then move the selector to Manual 1, the trans would not downshift to 1st, it would stay in 2nd (because roadspeed was too high for Manual 1), until you coasted down to some point, where it would abruptly downshift on its own to 1st gear with a clunk and sudden engine braking action, sometimes chirping the rear tires on the downshift. I don't remember for sure, but I would think it would be using the Governor to determine that road speed, so you might try doing this and see if it won't downshift by itself until speed drops, and then does drop down to 1st. Do this on a dry straight road with no one behind you, as the sudden downshift and engine braking in Manual 1st can be a bit surprising, especially to someone behind you!

There are other possibilities why no upshift, gets into valve body and understanding the whole operation, gets deep.

 

Okay, sorry about the wait, I've been super busy with school.

Sorry about the terminology. Yes, the transmission can be manually shifted into 1st or 2nd, with no slippage, it is solid on gear. I did get going 45 in second (manually) then down shifted into first (manually). The engine did downshift into first once I slowed down to about 25, but the shift was weak.

Does that help?

 

I was thinking that if the manual 2 - 1 downshift delay due to high speed worked, that that might exonerate the Governor. But I've been unable to find anything that describes that action in terms of what goes on in the trans. But I can't think of any device other than the Governor that knows what the road speed is.

I don't know what to think of that downshift being "weak". At least in the older C6s, and other old Ford transes, that downshift was brutal, with it dropping in, and with the engine braking in Manual first just about locking the rear axle for a moment.

Disconnecting the shift linkage at the trans and verifying that when in "D" at the column, that the trans is solidly in its "Drive" detent, and not pulled off to a side, is a first step in verifying that the manual valve is centered. The idea is to check simple things first.
Also like verifying that the Kickdown Rod from throttle linkage to trans, that the rod gets pushed back only near wide open throttle, and that it is indeed spring-loaded to hold it forward.
And that there indeed is proper level of vacuum at the modulator (basically idle intake manifold vacuum).
Although lack of vacuum at the modulator, also a misadjusted Kickdown Rod that comes into play way too early, although these will delay upshifts to higher road speed, if you keep at it, it will shift up with enough road speed, where a Governor circuit failure, or 1-2 Shift Valve failure, etc. will not ever shift up automatically, engine would reach red-line in first gear and still no upshift.

Other than the easy checks I outlined above, there may not be much you can do. If you would like to learn more on diagnosing, hydraulic shift circuit diagrams for each action, etc. etc., you may want to look at FordManuals.com, where you can download a complete official Ford C6 book as an e-book for $10.95 An advantage of the e-book rather than paper is supposedly that you can zoom in on the diagrams, can't do that with paper.


Beyond that, it gets deep and you need knowledge and equipment. Like doing a Governor check up on jackstands with a hand vacuum pump and gauge to simulate engine vacuum at certain levels, and a pressure gauge to look for cutback. It still could be the Governor, or the 1-2 Shift Valve, or ?

One last thought... there were vehicles with the C6 that used plastic grommets in the shift linkage at hinge points, my all-linkage (no shift cable) 1970 cars had them, if any of those plastic grommets crack/break/fall out, then the driver's movement of the shift lever is not 100% accurately telegraphed down to the trans' input lever (manual valve may not be solidly in trans' detent positions). I don't know if there are any plastic grommets in a cable-operated shift lever setup, if that is what you have.