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5.0L Fluctuations & C6 Tranny Hard Shifts when warm

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5.0L Fluctuations & C6 Tranny Hard Shifts when warm

 
  #1  
Old 07-16-2018, 05:56 PM
nelly781
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5.0L Fluctuations & C6 Tranny Hard Shifts when warm

Hello Everyone, I am in great need of some help fixing a problem I have w/ my '88 Bronco 5.0L engine surging and C6 tranny shifting hard (205k miles).

Let me provide some chronological events (please note the mileage is as best as i can remember and that the problems are building so all previous symptoms listed exist):
1. ~175k miles I noticed some hard shifting between Reverse (R) and Drive (D). I noticed one of my universal joints was worn; replaced it and it helped, but not completely fixed. I ended up ignoring it b/c it was small at that point and i just thought it was normal b/c you're switching directions.
2. ~185k miles I noticed that the shift from 1st to 2nd feels like its dropping into gear instead of transitioning, but not always consistent (i think it gets worse as the drive-line warms up) When its trying to shift from 1st to 2nd I can sometimes give it a little throttle and then back off and it will jump into 2nd, but not always.
3. ~ 195k miles I noticed that the engine would fluctuate when I come to a stop and sometime stalls. It fluctuated from ~400rpm-1000rpm, and it took about 1sec to complete one complete fluctuation (I dunno if 1sec is important, just trying to communicate how fast it is fluctuating.) But the fluctuations DONT occur EVERY time I stop. (and the symptoms occur sometimes when I put it in Neutral (N)) Again, I think this is when the engine warms up (or maybe at least the ECU sees that the engine is warm).
In addition, when driving down the road at a constant speed (i.e. 45mph) the truck feels like I temporarily loses power and then returns to normal; the rpms drop and therefore speed drops some and the truck feels sluggish; the nose even goes up and down accordingly. This fluctuation of feeling sluggish/normal while cruising is sporadic but fluctuates several times when it does.
4. Today (205k miles) the engine and tranny perform same as above at first, but after it heats up all things get worse; power is low, going from 1st to 2nd is very late and slams in there, 2nd to 3rd is delayed, and after driving for a while it will stall and wont crank back up until it cools down. When I floor it, the truck will not shift from 1st to 2nd and the rpms will fluctuate from ~3000 to 4000rpm or so and the truck will back fire.

I currently do have some Check Codes:
33 ALL - (O, M) EGR did not open/ respond during test or if memory code, did not open intermittently <-- My EGR is disconnected.

67 Park/Neutral circuit fault <-- Issue is when in Drive (D)
Transmission Manual Lever Position (MLP) sensor circuit
(M) Intermittent Park Neutral Position (PNP) sensor fault

Thanks in advance,
Nelly781
 
  #2  
Old 07-16-2018, 06:18 PM
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are you certain you have a C6 and not the AOD which was really common for that year and motor setup? have you serviced the transmission with new fluid/filter?

C6 will require a properly functioning Vacuum modulator

AOD will require the TV cable to be adjust properly
 
  #3  
Old 07-25-2018, 03:28 PM
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Hey Kemicalburns,
No, im not sure if it is a C6. I was told that along time ago and just stayed with that info, but never had trouble until today. How do I tell if it's AOD or a C6?

I havent serviced the transmission in a long time, but i hardly drive the thing, so i never thought to service it.

When you say "AOD will require the TV cable to be adjust properly" <-- what is the TV cable? and are you saying this will help w/ the
 
  #4  
Old 07-25-2018, 03:29 PM
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Also, sorry for the delay on this topic. I'm not very active on this topic.
 
  #5  
Old 07-25-2018, 09:10 PM
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Your Bronco came from the factory with an AOD. Unless someone swapped in the three speed C6, you have an AOD.

Video for how to adjust the TV cable:
 
  #6  
Old 08-02-2018, 10:59 AM
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I think I have the C6 transmission; from what i've read the C6 has only 3 gears-which is what I have.

I'll look into the vacuum modulator some more.
 
  #7  
Old 08-02-2018, 11:22 AM
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Take a look at the shape and number of bolts on the transmission pan:
 
  #8  
Old 08-02-2018, 06:30 PM
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Two things, what is the trans code on your door tag? And what does the D on your shift position indicator look like? does it have a circle around it or just a plane "D"
 
  #9  
Old 08-03-2018, 10:42 AM
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Hey,

@ rla2005: based on you diagram, my transmission has the "S" shape towards rear of transminssion, and has 17 botls...therefore a C6.
@ Aciancio: the trans code is "K". which bases upon a quick google search shows a C6 transmission (even though it says 78 - 86) Also, the "D" on the dash is just a "D", with NO circle around it, so I dont think i have overdrive.

I'm thinking I have a C6
 
  #10  
Old 08-03-2018, 10:44 AM
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This is also helpful, just in case:
Door tag codes. - Ford Truck Club Forum
 
  #11  
Old 08-03-2018, 12:50 PM
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I have never run across a factory 1988 Bronco with the 5L/C6 combination. Good to know. Looks for signs of a ruptured modulator. If you find transmission fluid in the vacuum line, you probably found the root cause. If that is good, the modulator could just be bad.
 
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Old 08-03-2018, 01:09 PM
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Now that we have your transmission identified we can move forward. The C6 uses a vacuum modulator and "kickback" linkage to downshift when you punch it. It sounds like both problems are vacuum related, have you noticed your heat/ac vents changing to defrost randomly when your having problems? I would seafoam the top end (kind of my default start to any problems if it hasn't been done in the last 30-50k miles), then check as many vacuum hoses and fittings as you can, maybe get a vacuum gauge to see what is going on. Check your transmission vacuum line, shift and kickback linkage for proper fit and tension and check your transmission fluid level, color, odor for anything strange. Also when was the last time you checked/changed your air filter?
I think I'll stop there and not send you down to many roads at once, let us know what you what you find and if it changes anything.
 
  #13  
Old 08-03-2018, 02:18 PM
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@rla2005 & @Aciancio...

Thanks so much guys. I'll take a look and get back to ya. It may be a while due to availability, but yeh for sure.
 
  #14  
Old 08-05-2018, 04:51 PM
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Hey guys,

Just got out from under the truck...sure enough, fluid came from the vacuum line connected to the vacuum modulator. Good call!

Can I get some help determining what "color" vacuum modulator i need? mine seems to have no identification on it at all.

Thanks
 
  #15  
Old 08-06-2018, 07:55 AM
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Which modulator should I use on my C6... Black, purple or green stripe?


The black stripe is a small diaphragm/low vacuum modulator and is mainly used in heavier vehicles and trucks. It has a low shift point. This modulator is also popular in performance applications due to the low shift point.

The purple stripe has a slightly larger diaphragm and is designed for use in normal to heavier passenger vehicles. It has a medium shift point.

The green stripe is similar to the purple stripe, but will provide a slightly later shift point. It is designed for normal passenger vehicle usage and is the most popular modulator used the C6.


Remember... all three types are adjustable.



Based on the above I would install a black stripe version on a truck/Bronco
 

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