C6 transmission on my 1989 Bronco 351W

L\Bronco

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Id start by checking your vac at idle, you said it was around 17", then rev it to around 2000 in neutral, the vacuum should climb around 1-2" (It will drop as you open the throttle, and then recover as Tiha said.)
(The engine is more efficient at 2000, the vac should be better)
If it goes down but is running smoothly, you likely have restricted exhaust, drop the pipes at the manifolds (I they will come off without breaking all the studs) and check again. If it comes up now, the exhaust is plugged.
If it runs bad at 2000 RPM with low vacuum, chase the engine performance first.
I would start here, (while you are looking, check the vacuum line from the intake to the modulator valve, its just behind the trans pan, the rubber sometimes swells or the steel tube gets rubbed through and a small vacuum leak in the hose or tube can cause havoc in the shifting of a C-6. Also make sure no fluid leaks out of the modulator or hose when you disconnect it. (I've seen new ones fail right out of the box.)
Cheers
 

L\Bronco

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Sorry capts, I just re read your first post. I think Tiha may have hit it near the beginning.
Did you try disconnecting the kickdown cable at the throttle?
It might be adjusted too tight, (Especially if the trans guy thought he was setting a TV cable) It should only pull on the valve (in the valve body) near the end of its travel.
As Tiha said it should shift normally with the cable off and just not downshift when you floor it.
Maybe try that first (if you haven't already)
Cheers
 
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captsb890

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I would start here, (while you are looking, check the vacuum line from the intake to the modulator valve, its just behind the trans pan, the rubber sometimes swells or the steel tube gets rubbed through and a small vacuum leak in the hose or tube can cause havoc in the shifting of a C-6. Also make sure no fluid leaks out of the modulator or hose when you disconnect it. (I've seen new ones fail right out of the box.)
Cheers
Thanks for that information---will do
 

joserios1970

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vacuum shouldn't be an issue for you.

It is created by the pistons going up and down, plus the cam profile. The way the camshaft is grouind has a big effect on vacuum, but in the end it is the butterfly in the throttle body that creates vacuum. It is the restriction that controls the air intake.
So when you step on the gas, the butterfly opens and allows more air in. Lowering vacuum until engine rpms come up and compensate for the new air flow.

Seeing the vacuum drop is normal, but it should recover.

When you let off the gas the bufferfly closes and the vacuum spikes up.
I believe u are right I had the same shifting problem on my bronco sligly mis fire .I found the distributior had a play at the shaft y put a new one and now it's terrific good ..I hope these help you...
 

johnnyreb

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Well it sure sounds like it is still the trans. But we can maybe do some more trouble shooting.

I am not sure if the Neutral safety switch has any effect on your model year, but you can start the truck crawl under it and unplug it. Then go for a drive, see if there is any change. My thinking is if maybe it is telling the ECM to open the EGR valve or something like that when in 3rd gear.

You said it cuts out, or runs rough in 3rd gear, if you pull it back into 2nd gear does it start running properly?

I am pretty sure there are no solenoids in the C6. It should not need any electrical to run. I think you can completely unhook the modulator as well and take if for a drive to check for change.

Off the top of my head sounds like there is a sticky valve in the valve body, or a servo piston that is sticking, could be from housing wear. But after it shifts that would not cause it to run poorly. Might be fighting more than one issue there.

I assume it shifted okay before the trans was rebuilt? And ran okay in 3rd gear?
Tiha, I just read where the C-6 IS NOT a electrical transmission--meaning NO ELECTRICAL PARTS IN IT.
 
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captsb890

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Tiha, I just read where the C-6 IS NOT a electrical transmission--meaning NO ELECTRICAL PARTS IN IT.
Thanks for that---Since we last wrote--I cut out the Cat's as both were plugged and didn't have money for new ones---Tad better----------also-----let me run something by you-------someone said the EGR valve is in the same vacuum line as the transmission modulator----- I'm going to block it off to day--,they said the EGR opens at 2500 rpm and that's where my shifting from 2-3 takes place--after 2500 ?? A thought---He said it could be lowering the vacuum and it hangs up on shifting?
 

johnnyreb

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The vacuum modulator controls shift points and line pressure based on engine load.
Vacuum will indicate engine load if the engine performance is good. High vacuum = low load, so low pressure and early shifts. Low vacuum = high load, so high shifts and high pressure.
It sounds like the root cause might be engine performance. restricted exhaust will **** the vacuum as the engine revs up and it will get worse with load which could explain the late\hard 2-3 shift, but it doesn't normally cause misfiring, so maybe look there too, Cracked plugs/wires, worn exhaust cam lobes (rare), ect.
Id start by checking your vac at idle, you said it was around 17", then rev it to around 2000 in neutral, the vacuum should climb around 1-2" (It will drop as you open the throttle, and then recover as Tiha said.)
(The engine is more efficient at 2000, the vac should be better)
If it goes down but is running smoothly, you likely have restricted exhaust, drop the pipes at the manifolds (I they will come off without breaking all the studs) and check again. If it comes up now, the exhaust is plugged.
If it runs bad at 2000 RPM with low vacuum, chase the engine performance first.
If you pull the vacuum hose off of the modulator on a C-6 it will not shift into 3rd until the road speed is crazy high. (Depending on gear ratio)
Hope that helps
Cheers
To make things easier. When you put the exhaust back on. I started useing BRASS _DOUBLE-- WITH LOCK WASHERS) on instead of the steel ones. Saves alot of trouble later on and easier to put back on.
 

johnnyreb

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To make things easier. When you put the exhaust back on. I started useing BRASS _DOUBLE-- WITH LOCK WASHERS) on instead of the steel ones. Saves alot of trouble later on and easier to put back on.
two brass taps with lock washers on each stud.
 
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captsb890

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To make things easier. When you put the exhaust back on. I started useing BRASS _DOUBLE-- WITH LOCK WASHERS) on instead of the steel ones. Saves alot of trouble later on and easier to put back on.
Thanks--it has a tad more breathing with the Cat's off--but I will block off the EGER-----yes, as I understand--could be the manifold as they are susceptible to cracks---
 

johnnyreb

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Thanks--it has a tad more breathing with the Cat's off--but I will block off the EGER-----yes, as I understand--could be the manifold as they are susceptible to cracks---
I started useing studs and brass taps on the ehaust manifolds. With a couple of studs in. All you havw to do is place the mnifold on and start a couple of taps and the hold the manifold on for you. I use two taps and a lock washer. To keep it tight and they don,t rust or break any bolts off.
 

lynchsg

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You should have a vacuum canister (reservoir) on the right side inner fender well in the engine bay that makes up for engine vacuum fluctuations. If you have air conditioning it will be coffee can size. Mine cracked at the seam on the top. I soldered it and got my loss of vacuum back.
 
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captsb890

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You should have a vacuum canister (reservoir) on the right side inner fender well in the engine bay that makes up for engine vacuum fluctuations. If you have air conditioning it will be coffee can size. Mine cracked at the seam on the top. I soldered it and got my loss of vacuum back.
Thanks--I'll check that area-----by the way, my 89 351w has no acceleration--poor--like it has a governor attached---what would that be--put an OBD 1 on it and showed egr/ thermactor---I thought it might be the TPS---I t has 204,000 and valves were done at 135,000---I can beat it with a pair of roller skates ! Do you think it could be the EEC computer? I took it out and no burned smell or anything--it looked ok----but maybe-you can never tell--they are expensive----
 

Bronconyc

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I still cannot find an answer to my shifting problem after the transmission was overhauled. AAMCO Had it back many times and they stated the miles on the motor 202,000 and tired etc. It has 17 + on the vacuum ----It shifts a little better when it is cold rather than hot-----Has late shifting into 3rd gear - and Gerks but here is what I tried- I can, from a standstill, put it in low gear---accelerates and runs smooth---then shift to D2 still a smooth shift -- but when I get to shift at a higher rpm into drive--it stutters, breaks down, runs rough----What is it-? linkage adjustment-? shift solenoids-? motor inside the transmission? Thsy adjusted the modulator out as far as it would go. Without shifting through the gears manually, it shifts pretty good low into 2nd but hangs up on 3rd? until I let off a little gas pedal and then it shifts with a Gerk-------------any advice would be greatly appreciated----No, I won't take it back to AAMCO as after 4 times, they can't find their ass with both hands and a map!
The fluid options we have today for the C6 are limited. The Dex 2 or 3 doesn't matter it most likely came out of the same drum and wasn't even dexron.

Overheating, or running hot, without a trans temp gauge it is just a guess.
Running differently between cold and hot would suggest fluid leaking past sealing rings or apply clutches. When aluminum warms up gaps expand allowing more fluid to leak by. Or sometimes they expand in the direction of closing a bore so valves and serovos hang up.
Also adding a pressure gauge, take reading from the test ports in every gear, cold and hot, that would also help you see where the problem is, or isn't. They are like $45 at harbor freight.

Adjusting the kick down or the vacuum modulator should not affect the way it shifts into 3rd gear. You can completely unhook them and take it for a drive and see if there is a difference.

Completely understand your frustration and I agree the trans shop should be taking better care of you and they most likely do not know what they are doing.

I don't know if it is still the case, but test drives from trans shops used to be free. Maybe call a different shop and see what they say? If they would look at it? If you could get them on your side then you have a good reason to go up a level or two in amco management and maybe get something done. At least have some good technical information instead of just saying "it isn't right" Gives them more to work with, upper level people respond to that much better.

Maybe someone else can come up with some better ideas than myself. Being at wits end at this point, myself, I would probably buy a used c6 of the internet or junkyard and stick it in just to see/prove that was the problem.
Have you checked to see the linkage under the transmission is actually still connected? On mine I had the exact same problems and the wire linkage just came off. Popped back on and problem solved
 

PrettyCesspool

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I still cannot find an answer to my shifting problem after the transmission was overhauled. AAMCO Had it back many times and they stated the miles on the motor 202,000 and tired etc. It has 17 + on the vacuum ----It shifts a little better when it is cold rather than hot-----Has late shifting into 3rd gear - and Gerks but here is what I tried- I can, from a standstill, put it in low gear---accelerates and runs smooth---then shift to D2 still a smooth shift -- but when I get to shift at a higher rpm into drive--it stutters, breaks down, runs rough----What is it-? linkage adjustment-? shift solenoids-? motor inside the transmission? Thsy adjusted the modulator out as far as it would go. Without shifting through the gears manually, it shifts pretty good low into 2nd but hangs up on 3rd? until I let off a little gas pedal and then it shifts with a Gerk-------------any advice would be greatly appreciated----No, I won't take it back to AAMCO as after 4 times, they can't find their ass with both hands and a map!
Check your motor mounts and replace them if they are broken.
 

Motech

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I agree with @L\Bronco that your downshift cable is too tight. Hold your throttle wide open, push the adjustment button on the cable retainer and let it snap to it's spring-loaded position and road test again.
 

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