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Discussion Starter #1
Thanks for taking a look:

I have a 1986 XLT with a 5.8L and a 3spd C6 transmission. Whether I have it floored or am just cruising, the transmission will shift around 1500-2000 RPMs and will be in 3rd gear by 20-25mph. If I want to get any real acceleration out of the vehicle, I have to move it from 1 to 2 to D. Additionally, if I want to pass or need a little oomph to go uphill, the transmission will never downshift on its own.

Looking at a vacuum gauge and wideband O2, I can see that the engine has to bury itself in the powervalve to get enough power to keep accelerating after each shift.

I had the transmission rebuilt a few years ago and I'm not sure if I had issues with it shifting like that before that.

Do I need a shift kit to fix something like this or is there somewhere else I can look to get it shifting properly? I wouldn't mind being able to actually have the engine be in its powerband some time.
 

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i believe the c6's shift using a vacuum modulator at the back of the tranny. Id check that first. I had a similar issue on my aod when i swapped motors. the kickdown was never put back on and the first run around the block was 4 gear by 25-30 mph and it wouldnt downshift until it wanted to stall itself.
 

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Driving Stuff Henry Built
-90 xlt, 351w, e4od, man 1356, 3.55, sag, warn hubs, 35s. -73, 400, np435, d20j twin, 35s
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I'm no trans guy, but here's the bits & pieces that come to mind.

Have you checked the vacuum line to the trans? Sometimes they dry out & split or just quit sealing at the modulator. IIrc a c-6 won't shift properly without a good vacuum signal. Also look for a pinch in the line, especially near the turn at the modulator. Make sure it's not clogged as well.

If there's good vacuum to the trans, there is an adjustment screw in the modulator. It is in the center of the barb where the hose connects. You take the hose off & use a narrow slotted driver. It's been too long, I don't remember the adjustment procedure, or which way to turn it, but it seems like small turns of the screw made big changes. It's probably best to look that up.

Look for ATF in the vacuum line. If there is, then I think the modulator needs to be replaced.

The adjustment of the downshift rod (Or is it a cable by 86?) from the carb to the trans may be part of the issue as well. If it is the problem, it would need to be adjusted so that pressure is applied to the trans earlier than it is now as the throttle opens.

That's what I remember. I'd start with checking for vacuum line issues. I don't know if the throttle rod or vacuum modulator comes next. Bump for a real trans guy. :toothless
 

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My '83 351W with a C6 does the exact same thing. I thought maybe it was just the way the tranny shifted. Mine too is in 3rd gear by about 20-25 mph. In fact the 1-2 shift happens so low that I never even feel it shift. I am anxious to see what others say about this. :popc1::popc1::popc1:
 

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Remove the vacuum line off the vacuum regulator on the tranny and look into the vacuum nipple - should see a small straight blade screw head. I forget which way is which, but if you screw it one way it will increase the shift point, screw it the other way it will decrease the shift point. Sorry I don't remember which way does what.

When my C-6 was rebuilt it shifted really early too, took it back to the shop for the 100 mile checkout, he adjusted this, shifts right now.
 

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Discussion Starter #8
RE:

Thank you for the replies. I'll take a look at the modulator and the vacuum line. I hope that does the trick.

I tried opening up that PDF file and I get a broken link. Does it still work for anyone else?
 

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just tried it and worked for me, thanks for the post
 

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I tried the link but it says the website akpp.in.ua doesn't exist.
 

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yo,
yw

Still works

it is a pdf file so you need the free Adobe Reader; http://get.adobe.com/reader/
I save file (it is clean, and checked w/AVG before use)
and then click to Open w/Adobe reader X
 

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Okay, got it to work. I think the issue is that the server it's on is kinda slow. When I just click on it, it doesn't work. When I right click and go to "save as...", it does. But, it's taking 30 minutes to download a 5mb file. I think if you straight up click on it, it times out, giving you an error.
 

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yo, good, thanks!
I was able once to dnld that in secsonds, but now C is so slow here in da woods that I saved it to my pc instead of opening it on the www.
I have more on the:

E4OD

including the Update Handbook; "...covers engineering changes that have occured from 1989 to 1995, parts affected by the change, interchangability & part numbers. Includes: Valve body and checkball changes thru 1996; 1989-1995 valve body checkball location;
1996 valve body checkball locations; early 1989 case checkball locations; late 1989 case checkball locations; 1990-1995 case checkball locations;
1996 case checkball locations; spacer plates and identification; main valve body differences and identification; lower valve body differences and identification; case differences and identification; output shaft changes; overdrive section failure; 4th clutch snap ring change; low roller clutch inner race change; cooler line fitting changes; low roller clutch assembly change; overdrive sprag changes; ball bearing center support change; overdrive carrier and input shaft change; cast iron overdrive carrier; direct clutch housing change; solenoid pack change for 1995 models; front pump change for 1995 models; intermediate servo piston change; sun gear differences; 1989-1995 valve body; 1996 valve body (exploded view); new design pump cover and stator shaft (blocked cooler); reverse clutch lip seals for 1996; larger design low roller clutch change; new design forward ring gear hub and planetary; ew design overdrive clutch piston and housing; cooler by-pass valve added to some 1997 models; lube orifice cup plug added to 1997 models; new design front pump for 1997-1998 models; new design 6 pinion forward and rear planetary arriers; new design sun shell; ntermediate diode freewheel failure; increased overdrive clutch capacity..."

and on the AOD Description & Operation, Identification Tag, Parts Break-Out Diagram w/Nomenclature, Seals, Rings & Gaskets Locator, Throttle Valve (TV) System, Transmission Shift Patterns, Main Components & Functions, Diagnosis & Testing, In-Vehicle Service, R&R, Cleaning & Inspection, Specifications, & Special Service Tools/Equipment
 

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Discussion Starter #16
RE:

I had to have someone send it to me but I now I have the file. Again, thanks for the link.

After taking a look at it, the only 'adjustment' I can find in it is to two different mechanisms (3 for the 7.3) The two that I can work on are the Intermediate Band and the Neutral Start Switch. Basically the Intermediate Band is a removal and re-installation of its locknut.

They do cover some vacuum and pressure tests on page 37 (I think that's what some of the previous posters were referring to) on page 37. However, I don't see any form of adjustment to adjust the shift points. Just how to tell if a few of the items need replacing.

Also, other than the dipstick is there any other, more reliable way to tell if the transmission has too much fluid? Whenever I pull the dipstick, it always always seems to rub against the tube on the way out of the transmission, getting covered in fluid and thus making it impossible to read. Every time I pull it out, there's fluid 6-8" above the full mark. Or did the transmission shop really overfill it that much/I have the wrong dipstick. :banghead

Any ideas?
 

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Just to be sure, are you checking the fluid level with the engine running?
Ditto, the tranny fluid must be checked with the engine warm and the tranny in park. If you are taking a reading when the engine is off then you are not getting an actual fluid level reading.

Also you can adjust the modulator. Turning the screw on the inside in (counterclockwise) 2-3 turns should raise the shift point enough to make a difference. This is all that I have been able to find out about the C-6 and this particular issue. The screw that is on the inside of the modulator is a flat head. It takes a very small flat head screw driver to adjust it. I think they call them jewelers screw drivers or sometimes just precision screw drivers.
 

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Driving Stuff Henry Built
-90 xlt, 351w, e4od, man 1356, 3.55, sag, warn hubs, 35s. -73, 400, np435, d20j twin, 35s
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I've had the same problem since I bought my truck. I tried the adjustment and it helped very little. I did read that you weren't supposed to turn it more than a couple turns either direction.
 
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