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Discussion Starter #1 (Edited)
Went out this morning to tackle the transmission shifting issue. My C-6 seems to only want to shift at 3k RPMs when accelerating or if I take my foot off the throttle.

I thought it was a Vacuum issue so check the vacuum line and all seemed well. Next I replaced the Vacuum Module:



Went ahead and did a trans fluid, filter and gasket change.







Drove it around and same issue. So I moved the vacuum line from here:



It was running from vacuum module to this inline tube than to the tree on the upper intake.

I moved it from here straight to the upper intake tree and no change after I drove it.

So I took the vacuum line/hoses and replaced the hoses, inspected the metal line. No leaks.



Installed it back and no change still shifting high RPMs or if I lift off the throttle.

It was pretty clean in the pan when I dropped it.

Is my Trans Vacuum line in the right spot? On the inline vacuum tube connected to the back firewall? Info: 88 XLT 351W C6
 

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Yes, the factory location is on the plastic vacuum manifold on the firewall.

Why do you have a loose MAP connected, instead of mounted where the (apparently) dead one is? Why is there a spare orange line going to the HVAC box bolt? It's probably leaking thru the threads, which isn't helping anything.
 

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Beach bumming it ........
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Discussion Starter #3 (Edited)
That's a old picture and not how its hooked up now. MAP is secure and that egr vacuum line is hooked up correctly now. After some searching everything leads to the vacuum line to the module or the trans vacuum module its self. Both have been replaced so I am stuck.

Sent from AutoGuide.com Free App
 

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yo Saltlife;

Here is my vacuum check;
http://fullsizebronco.com/forum/showthread.php?t=206824&highlight=Vacuum+Leak+Test;+idling
See my post #11

Vacuum Gauge, PM1520, 2 inch (called a gas saver...keep vac @ high lvels for better mpg)

C6 Trouble Shooting Flow Chart
Source: by jasperengines.com



Check the shift linkage. Make sure the linkage moves freely.

Shift Linkage Parts Break-Out Diagram in an 88 with nomenclature
Source: by Dustin S (Dustball, Mellow Yellow, Mr. Laser Boy)




Shift Linkage Adjustment & Removal & Installation in an 80 Bronco, F & E Series; part of column R&R 11.2 MB pdf file Source: by broncohq.com




C6 Description & Operation, Adjustments, Diagnosis & Testing (partial), Disassembly & Assembly, Parts Break-Out Diagram, Hydraulic Control System Diagram,
http://www.thedieselfiles.com/Ford/C6 Automatic tranny.pdf
http://www.autoparts2020.com/rsdev/part_detail.jsp?PART_HDR_ID=87069
 

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IIRC the modulator should have a tiny screw in the vacuum fitting to compress or decompress the diaphragm. I believe turning it counter clockwise shifts it higher in the rpm range and clockwise puts it lower, can't remember. Recently went through a similar problem with a 70 mustang that had had power brakes added to it and sucking away some vacuum...
 

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Beach bumming it ........
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Discussion Starter #10
Update:


Checked the Vacuum modulator and there is no adjustment that I can do.

I drove it this weekend to help move someone. Still having the same issue but noticed that the trans does downshift fine. If I give it gas it will downshift like normal.

Still not shifting till high RPMs (harsh shift) or until I let off the gas.


So next steps (After reading miesk's C6 Trouble Shooting Flow Chart):

1.) Should I clean/check the governor? Where is it located and what do I do?

2.) IF # 1 does not work should I Check valve body?
 

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If there is no adjustment in the modulator then there is an issue. Pop the vacuum line off and there should be an adjustment screw in there. Also, make sure you have vacuum going down there. Pop the line off with the engine running (safety first, truck in park) and put your finger over the line to make sure there is vacuum going to it.

Here is a picture
 

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The governer is only for shifting back to 1st when coming to a stop, so that won't be the issue.

Check your kickdown cable/linkage. Make sure the lever it's attached to on the trans springs back when pulled.
 

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Beach bumming it ........
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Discussion Starter #13
If there is no adjustment in the modulator then there is an issue. Pop the vacuum line off and there should be an adjustment screw in there. Also, make sure you have vacuum going down there. Pop the line off with the engine running (safety first, truck in park) and put your finger over the line to make sure there is vacuum going to it.

Here is a picture
I get it now I was thinking by the wording he meant on the side of the modulator. so do I tighten of loosen the adjustment there?

I will check vacuum going coming from the line as well (kinda thought vacuum is because it does downshift fine)

. Thanks :rockon
 

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Salt, I can't recall which direction does what. Play around with it. Screw it all the way in, drive it, then make some outward adjustments to your liking.
 

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:twak My bad on the wording
 

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yo,
Vacuum Modulator Adjustment; "...Most modulator diaphragms have an adjustment screw through the vacuum tube hole. Screw clockwise for later/firmer shifts and counterclockwise for earlier/softer shifts. However if more than two or three turns don’t get what you want, you may have other problems."..
Source: by Kenneth C at Bad Shoe Productions

http://www.badshoeproductions.com/trans.html


Vacuum Modulator, Diagnosis & Replacement in an 89; "...I started out having shift problems, my tranny wouldnt shift out of first after a dead stop until after 40mph/3K-RPM, and once it did it would shift up and down between 35mph and 45mph, from 2 to 3 and 3 to 2 and 2 to 3, etc.. It wasnt a shudder, it took a few seconds between shifts. Then it started not wanting to shift into 3rd. I would have to manually shift to 2 and back to 3 and most of the time it would kick into 3rd.

Terrified it would cost me a mint, I posted a Q in the tech section at FSB. Got a few ideas, and meanwhile I dropped down to a Cottman's transmission shop for a free checkout. The tech said he removed the vaccuum line from the modulator and oil (transmission fluid) ran out, so the modulator was bad. The modulator simply has a diaphragm that is vaccuum actuated to adjust the shift points (tell the tranny when its okay to shift). If you pull the vaccuum line and fluid runs out, the diaphragm is busted and fluid is getting by. This means the vaccuum on the line is not pulling the diag. and hence the small shaft inside is not moving.
The different color bands indicate the diamater of the rubber diaphragm inside the modulator. The two COMMON colors from auto parts suppliers are PURPLE and GREEN. These are for CARS. TRUCKS like OURS REQUIRE the BLACK banded modulator, likely from a transmission shop or dealer. This is reguarding the mid/late 80's models, maybe more... Id venture to say any C6 with a slip in modulator.
Remove the bracket bolt with a 14mm socket, pull the old modulator from the rear of the C-6 transmission. Be carefull not to loose the plunger, its about a 2 inch piece of metal with rounded ends. Put the plunger into the new modulator, slide it onto the transmission and bolt down the bracket. Remove the vaccuum line from the old modulator and plug up the new, but allow for any excess fluid to drain fron the line.

Failure to replace the modulator could result in bruning up the transmission. I noticed mine not "acting right" since I first put it on the road (2 months), but it was nothing major. Yesterday on my way to work, the tranny's shif went all to hell, nothing was right.

Below, please see a picture of the modulator on the C-6. There is also a before and after video. Notice the MPH and RPM on both the before and after. At one point before, I pushed 3K RPM at 45MPH and it never shifted out of 2ed.

With the new modulator, the tranny shifts like a war hawk, just like its supposed to! And, this may also, most likely be the source of my tranny fluid leak (As verified by the tech at Cottman's)..."
Source: by MyFullSize (Andrew) at FSB http://fullsizebronco.com/forum/showthread.php?t=17161

READ MORE esp about the discussion on green vs black Vs MyFullSize & Matthew

Because Ford Lists;
1988 Ford Bronco
7A377 (TM-43)
Auto Trans Modulator Valve
5.8L 351 CID 8 Cylinders; Color - Green, SAD Trans
http://www.fordparts.com
 

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If that doesn't do the trick then you may have a sticky valve in the valve body. The fluid change should eventually help it. However, if the adjustment on the modulator doesn't cure it I would give it a dose of the sea foam tranny treatment. Put a can of that in there and drive it. Find a big parking lot or dirt road with no traffic and throw it up in reverse and haul ass! This will back flush it some as well. Read the directions on the can. I think it has to be drained out and the tranny fluid changed again.
 

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Beach bumming it ........
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Discussion Starter #19
Tried adjusting the Vacuum Modulator over the weekend. It didn't really seem to make a difference if I adjusted it mostly out or mostly in. If I adjusted it mostly out it would shift softer but still at 2600 RPMs or when I let off the gas a little.

Trans never did this with the old motor. This new motor may have a mild cam in it (Can't confirm but was a HO from a 85). Could that build up vacuum levels at different rpms causing this? Or is that me over thinking.
 

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That could, but I don't remember the mild cam I had causing this.

Try moving the vacuum line right to the manifold. Straight source it.
 
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