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TimmyG

Auto trans upshift problem

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84 King Cab with z24/ 3 speed auto.

I was actually surprised at how much I liked the auto. I had an 88 hardbody z24/auto and it was a real dog. I hated it. This 720 auto has a kick down switch that I don't remember having in the hardbody, and the HB was just a real turd for some reason. 

 

This 720 is peppy, responds to throttle inputs well, and shifts great with the auto MOST of the time... except one very specific situation. 

 

Truck warmed up and in D. From a stop, floor it and hold WOT. The truck revs out and upshifts from 1 to 2 normally, but when it reaches the top of 2 it won't upshift into 3. If you continue to hold WOT, it stutters and struggles at what I assume is redline (no tach). As soon as you let off the gas at all, it upshifts into 3. You can then immediately return to WOT with no problems. 

 

This no upshift into 3 problem ONLY occurs in this specific situation: flooring it from a stop and holding WOT all the way to highway speed using gears 1, 2, 3 sequentially as if drag racing. 

 

Every other driving scenario it shifts great. Fluid level looks fine, although my experience with auto trans is limited. The fluid color looks normal. I will say it looks more red on the end of the stick vs thinner in the good range on the stick...but there is definitely oil on the stick if you check using your finger to swipe in the good range. 

 

 

 

 

 

 

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I'm going to make a big guess here.

 

Somewhere on the intake, probably where the large vacuum hose for the brake booster is, is a tiny vacuum line, probably going to a metal line than goes down behind the engine to the left (driver's) side of the transmission. There it changes to a short rubber elbow hose and connects to the vacuum diaphragm. This tells the transmission how hard the engine is working. A very low manifold vacuum means don't up shift too soon. If the hose is loose or cracked the transmission will be fooled into delaying the up shift.

 

Just curious. With the engine off, step down on the gas slowly and just as you hit the floor, you should hear a switch click. Does it?

 

 

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Yes sir, the kick down switch clicks with engine off..  and works great with engine on 😎. I will take a look at what you described. Thanks!

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Also, I just wanted to say after adding a tach today... holy rpm! uA2sJsr.jpg

Edited by TimmyG
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The red line on the Z24 is 6,300 so don't worry.

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8 minutes ago, datzenmike said:

The red line on the Z24 is 6,300 so don't worry.

It's just funny because I've only had 5 spd 720s previously.  And why did they mark the red zone as such on the tach if the redline is 6300? Seems a little silly.

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If it ever stops raining I will go check this... I can't currently pull my running 720 into the garage. First world problems. 

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On 9/15/2019 at 4:49 PM, datzenmike said:

I'm going to make a big guess here.

 

Somewhere on the intake, probably where the large vacuum hose for the brake booster is, is a tiny vacuum line, probably going to a metal line than goes down behind the engine to the left (driver's) side of the transmission. There it changes to a short rubber elbow hose and connects to the vacuum diaphragm. This tells the transmission how hard the engine is working. A very low manifold vacuum means don't up shift too soon. If the hose is loose or cracked the transmission will be fooled into delaying the up shift.

 

Just curious. With the engine off, step down on the gas slowly and just as you hit the floor, you should hear a switch click. Does it?

 

 

 

Well, unfortunately I did not find anything obviously wrong with the vaccuum hoses from the firewall to the hard line on top of trans or the vaccuum elbow to diaphragm on the side of trans. 

 

Now that I have a tach, I'm seeing when the trans is actually shifting. 

From a stop, WOT in 1st, it shifts into 2nd at around 6,000. Keep holding it WOT, and it starts struggling and sputtering around 5,000 in 2nd and will not rev over this or upshift into 3rd unless you let off the gas for a moment. If you let off the gas, it immediately upshifts into 3rd and you can get back to WOT with no more drama. 

 

Now the weird part. If you're already in 3rd and you go WOT, the trans will immediately kick down into 2nd and rev smoothly all the way to around 5500-6000 and upshift into 3rd with no bullshit. 

 

The truck revs out to 6,000 in neutral with no issue. Now I know you're thinking why rev this underwhelming z24 engine to the moon and why do I care? Because I just want it to run correctly and give me all she can at WOT. I mean, I might want to drag race another 720. 🤣 I could win! 

 

 

Edited by TimmyG

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Try a new fuel filter. I think there's one just before the electric pump ? If this works I'll explain. If not it's $5 well spent anyway.

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Simple enough. I'll put it on the list! Probably needs to be done anyway, it just wasn't a priority...until now. Auto trans issues is not my strong suite as most of the ones I've owned were newer vehicles that didn't have problems. Thanks, Mike.

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@datzenmike Well, I solved this today. Took off all the vaccuum lines and capped them off except for one... Ran one line from the carb and split to the vacuum advance at distributor and to the fuel charcoal canister. Truck runs smoother and shifts normally at WOT in all scenarios! No more problem upshifting into 3rd!

 

Also, bogging under hard throttle is gone if you roll into full throttle and its 50% less severe bogging if you smash it wide open suddenly. More on this in a moment...

 

Concerning vacuum lines, I discovered the vacuum advance from distributor wasn't hooked to the carb at all. It was probably mistakenly routed from the carb to the hard line on timing cover that runs to the charcoal canister, instead of the one that runs to the distributor. 

 

Also, concerning the bogging...when the truck is idling, if you spray carb cleaner at the base of the carb the engine will bog. There is definitely a vacuum leak at the base.

 

I discovered the mounting nuts on the studs at base of the carb/adapter plate were loose. Tightening them didn't seem to help at all. Test drive showed it still cuts out for a moment if you suddenly go WOT and still bogs if you spray carb cleaner at the base when idling. Seems worse on the side of the carb facing the head/rear of carb. 

 

I suppose I need to remove the carb and adapter plate, then replace the gaskets. Maybe I should use some type of gasket sealer and blue locktight the mounting nuts. 

 

The before vacuum lines....

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The after vacuum lines...

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Edited by TimmyG

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Canister purge and vacuum advance are connected. It goes through the TVV to only allow vacuum advance when the engine warms up but you could skip it.

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9 minutes ago, datzenmike said:

Canister purge and vacuum advance are connected. It goes through the TVV to only allow vacuum advance when the engine warms up but you could skip it.

 

Not exactly familiar with how this works. Could you explain?

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Vacuum advance is a ported vacuum source on the carburetor base. It goes to the T V V that only allows it to pass when the engine is warm (but you could skip this) it then goes down the front of the engine to the distributor and to the purge port on the canister. It's a closed system that fluctuates with the carburetor throttle position and engine speed. That's it.

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23 minutes ago, datzenmike said:

Vacuum advance is a ported vacuum source on the carburetor base. It goes to the T V V that only allows it to pass when the engine is warm (but you could skip this) it then goes down the front of the engine to the distributor and to the purge port on the canister. It's a closed system that fluctuates with the carburetor throttle position and engine speed. That's it.

 

So... now it's just pulling ported vacuum all the time, not just when it's warm? 

 

What problem could this create or disadvantage could it have?

 

I don't even know what the canister purge means. I never had anyone really explain any of this to me because most people don't know I guess. 

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The T V V (thermal vacuum valve) simply prevents the vacuum signal from getting to the distributor when the engine is cold. It's merely to reduce pollotion when the engine is cold and the choke is on. It will work perfectly fine without the T V V connected.

 

The charcoal canister is a storage device for gas fumes generated in the gas tank that can later be drawn into the engine and burned. It's all automatic and has no effect of running, but it needs a vacuum signal to tell it when to empty.

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1 minute ago, datzenmike said:

The T V V (thermal vacuum valve) simply prevents the vacuum signal from getting to the distributor when the engine is cold. It's merely to reduce pollotion when the engine is cold and the choke is on.

 

The charcoal canister is a storage device for gas fumes generated in the gas tank that can later be drawn into the engine and burned. It's all automatic and has no effect of running, but it needs a vacuum signal to tell it when to empty.

 

Ahhh, makes sense. I understand the TVV operation, just didn't fully know why to limit the ported vacuum going to distributor or canister. 

 

Is deleting the canister dangerous? I'm assuming the fuel tank has another vent for filling purposes and pressure relief with rising temps? 

 

 

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The tank is sealed at the cap but if over pressurized, the fumes are fed to the canister and stored in the activated charcoal in it. During running as the gas tank empties air from the air filter is allowed back down the same pipe to replace the gas used and prevent a tank implosion. If removed you may smell gas fumes. I would just leave it alone it has no effect on running.

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2 minutes ago, datzenmike said:

The tank is sealed at the cap but if over pressurized, the fumes are fed to the canister and stored in the activated charcoal in it. During running as the gas tank empties air from the air filter is allowed back down the same pipe to replace the gas used and prevent a tank implosion. If removed you may smell gas fumes. I would just leave it alone it has no effect on running.

 

Thanks. I don't mind the canister being there as long as I know what its function is. 

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Well, the upshift into 3rd problem is  back 👎🏼😂. At this point I'm gonna blame the vacuum leak at base of carb/adapter plate. The only things that changed are me spraying carb cleaner there to identify the leak, and the motor cooled off and warmed to operating temp again. Thinking this area is causing a variable degree of vacuum leaking. Could explain it.

 

The new vacuum line routing definitely has the motor running better, but the no shift situation with stumbling around 5,000 rpm in 2nd gear is back. Still only happens if you go WOT and attempt to run through 1st, 2nd, and 3rd sequentially. 

 

I figure one day this thread might be useful to someone else scratching their head, so I'll keep updating. 

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Did you change the fuel filter???

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4 minutes ago, datzenmike said:

Did you change the fuel filter???

 

Negative, boss. Haven't made it to that yet. I have the filter now. Will change her out today...need to address this exhaust flange leak right now. It's more annoying. 

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@datzenmike After unsuccessfully fixing my exhaust leak today (discovered cracked Y pipe, the part that bolts to bottom of cast iron exhaust manifold)... I decided to change that fuel filter. Tiny little guy cost me $4 haha. 

 

Had half a tank of fuel so used a hand pump to empty the tank. Boy that took a while. One old, broken bolt later I got the filter in and she's running. 

 

The trans upshift problem is gone. Its fixed again, at least for today!

 

Motor still falls on its face for a second if you floor it. Still bogs if you give it lots of throttle rolling into the gas.... but she shifts into 3rd real nice at WOT about 5500 rpm consistently. This is drag race style WOT through 1, 2, and 3 where she previously would stumble and hang up around 5000 without upshifting into 3rd (just to be clear). 

 

You think the fuel filter cured it? If so, why? Thanks.

Edited by TimmyG

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GTH2ZW9.jpg

 

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Safety first! 😁😁😁

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Driving along at speed, flooring it to down shift to second and letting it up shift to third uses a lot less gas than accelerating the car from a stop through first and almost all the way through second.  Accelerating from a stop, uses a lot of gas very quickly, and by the time it's gets to the top of second the carb begins to run dry and it stumbles and can't shift. The carb runs dry because the fuel filter is partly blocked with debris. In effect you are using gas faster than it can be sucked through the filter.

 

 

The bog when you floor it is common with the Weber. At about 1/2 throttle the secondary is forced to open even though the engine is not ready for it. Try 1/2 throttle and learning to step into it as it's needed. Some practice and you'll be able to feel when it's best to step down the rest of the way.

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