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5 speed transmission issues


bigleagueteague

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Hey all. '84 4x4 here.

 

So the other day I noticed what seems like a pretty sudden change in the tranny - I’m having trouble downshifting into 2nd from 3rd at higher RPMs (tach is out, so can't say how high). Tranny only downshifts into 2nd smoothly below about 15mph, maybe less. If I want to get it into 2nd at higher RPMs it takes more effort than I have been used to and the gear engages with a rough klunk that I can really feel through the shifter. It doesn't sound like grinding, but maybe that's what this issue will become???

 

Yesterday I checked out the tranny fluid and it seemed low. I bought the 720 back in July and was told it had recently had a fluid change at Valvoline. No record. I took the sellers word for it, but the oil that came out of the tranny looked like total shit. Seemed pretty nasty for 6 months and only a few 1000 miles. Maybe that's normal for gear oil? The drain plug was pretty foul with metal shavings but no major chunks. I put Sta-lube in it and took it for a drive but the issue is still there and noticed there may be a similar issue, but not as drastic, when downshifting into 3rd from 4th.

 

~205000 miles, but no other noise from the tranny. Upshifts really smoothly. Haven't changed out fluid in transfer case or rear diff, but wouldn't think those things affect tranny performance at all.

 

Checked clutch fluid level - fine. Perhaps I need to adjust the clutch?

 

It has cooled off here in Boise pretty significantly in the last couple weeks. Perhaps it's an issue exacerbated by the cold?

 

Thoughts?

 

Thanks.

 
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My first thought was clutch adjustment, but there could also be other causes.

 

Did you do anything prior to the change? Gear oil change or have the trans apart in any way? Any other things that may have been done?

 

How do you drive it? Do you like "grabbing gears"? If so, maybe you broke a roll pin. Even if you drive like a grandma, the factory roll pins are know to become loose or break in half, causing hard shifting.

 

Proper gear oil needs to be used in these transmissions. If using regular old gear oil, it's too slippery for the synchros. I use Redline MT90, but others here use other brands with success.

 

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Perhaps my clutch is out of adjustment. That's the next thing to check.

 

I just replaced gear oil w/ Sta-Lube GL4 85w90 hoping that might help the issue - no dice. Didn't do anything to the trans prior to the change in performance.

 

I think I drive the truck pretty gently. Not smashing through the gears or anything. Not sure what "grabbing gears" means.

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10 minutes ago, bigleagueteague said:

 Not sure what "grabbing gears" means.

Just being aggressive and quick with the shift....

Is the problem only with the downshift?

 

The first thing I'm thinking are possibly worn synchros in the transmission....

 

I had this issue just last year before I finally fixed it....

 

Try when you shift to let the clutch  out for a second in neutral and then clutch back in and finish the shift....

This allows the synchro to move just enough....

My problem was a flat spot on the synchro wouldnt allow the shift because it was hanging up on the edge... they should be a nice v shape to guide it to the next gear....

 

This can also be done while not driving too..... I tested mine on the bench and could feel it catch... so I'd say without the ebrake on just have the truck in a static position....

You should be able to shift the gears without a problem..  

 

 

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I wouldn't say I'm too aggressive with shifts.

 

The problem is only with the downshift. It feels great upshifting.

 

I was concerned about the possibility of worn synchros... a transmission rebuild sounds pretty expensive. I don't know if I'd be comfortable tearing into it myself.

 

I've tried double clutching, can't say I noticed a difference. I'll try again.

 

I threw the transfer case in neutral and then shifted through gears w engine running at varying RPMs - couldn't reproduce the problem. Not sure whether this is a viable test for synchro function or if it's even a wise thing to do...

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The transmissions aren't horrible to repair yourself but if you have never opened one it can be a little intimidating.....

 

If it up shifts everytime then it doesnt sound like the synchro to me.... still a possibility but doesnt sound like it.....

Mine was 5th so it was always the up shift..... 

 

Just a word of caution... after I ignoring my inability to shift to 5th correctly I ended up breaking an ear off the shift fork inside the transmission.... it wedged its self under the reverse gear..... 

So you may have something starting to fail inside ... I'm not saying it's what happen just to be careful forcing the gear till you figure it out...

 

When you changed the fluid I know you said it was old and nasty, but was it shinny at all?

Was anything on the drain magnet?

 

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3 hours ago, Stoffregen Motorsports said:

My first thought was clutch adjustment, but there could also be other causes.

 

Did you do anything prior to the change? Gear oil change or have the trans apart in any way? Any other things that may have been done?

 

How do you drive it? Do you like "grabbing gears"? If so, maybe you broke a roll pin. Even if you drive like a grandma, the factory roll pins are know to become loose or break in half, causing hard shifting.

 

Proper gear oil needs to be used in these transmissions. If using regular old gear oil, it's too slippery for the synchros. I use Redline MT90, but others here use other brands with success.

 

I use Redline MT-90 in my 85 4x4,works great.I drive like Grandma all the time,had a kid in the back seat pass me and he rolled his window down and asked me why I was going so slow.I just laughed.

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There's no clutch adjustment but there is an adjustment for pedal play. Push on the pedal rubber with finger and it should have about 1/16" to 1/8' of looseness before it becomes firm.

 

Worn synchros will grind on down shift because of the extreme miss match of counter and mainshaft speeds. You can minimize this by letting the clutch up in neutral, quickly revving the engine which spins up the counter shaft and then quickly depressing the clutch and down shifting quickly before it slows down. Worn synchros don't grind so much up shifting as the counter shaft is spinning at the same RPM as the engine and once the clutch is depressed the thick oil causes drag that slows it and when they are close to the same speed as the mainshaft the synchros easily complete the shift. Naturally if they are worn and you force a fast shift they will likely grind.

 

Mine was so bad that I skipped down shifting all together. Then I tried a synthetic oil. It's the same protection but much thinner. Up shifting was faster with the same effort and down shifting much improved. The thick 80w90 oil was always a stiff up shift for the first few minutes when starting off cold. This went away completely.

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Theres more than 1/8" of pedal play, probably more like 3/8". I don't see how adjusting that will do much to remedy my problem.

 

Drain plug was pretty gnarly, definitely some shavings but no chunks.

 

I've tried double-clutching, upping RPMs in neutral and then shifting into 2nd - still doesn't want to go in smoothly.

 

Clutch fluid is filthy, but I don't see how changing that out will make a huge difference. I'm not having any issue getting into 1st or reverse, so I don't think there's a clutch issue.

 

At this point I'm guessing it's a synchro issue. I'm just surprised that shifting would go south so suddenly. The only thing that has changed is ambient temperature!

 

U0vmnzo.jpgKCZzPPg.jpg

 

 

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Well, since brass synchros aren't magnetic, the junk on the drain plug doesn't mean you have a synchro problem. I would start by knocking down the low hanging fruit.

 

There are two adjustments at the pedal - one is the high pedal stop, which is sometimes also the brake light switch, and then there is the adjustment at the pedal to master cylinder. I like my pedals to be level with each other, but before you go there, make sure you have no more or less than 1/16" of free play where the pedal meets the master.

 

If your fluid is nasty, try filling with new fluid and bleeding it to verify that the clutch pedal/master/slave is not the problem. After you verify that, then flush and bleed again.

 

Mike noted that the slave is not adjustable on these trucks, but it is possible that someone at some point installed an adjustable slave or rod from an earlier car/truck, so check that to make sure. It will be easily identifiable by the threaded rod and/or the presence of a spring or spring hole on the arm and the slave.

 

Back to the metal on the drain plug - this is probably bearing material, and is not totally detrimental, but could be a sign of things in need of repair. It's not the smoking gun that your looking for, but should still be investigated.

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Excessive pedal play will mean you are not getting maximum clutch lever travel so this exaggerated, would be like depressing the clutch half way and expecting the transmission to shift properly with the clutch not fully disengaged. Probably not the problem but many small things can add up to one big one.

 

Have some fresh brake fluid and open the bleeder on the slave. As it drains keep the reservoir topped up till only clear fluid drips out.

 

Drain bung has has normal build up. Wipe clean so you can compare next oil change which should be in 5 year/30k miles.

 

Only the L series 510 and 521 had the adjustable push rod.

 

 

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On 11/30/2021 at 2:20 PM, datzenmike said:

There's no clutch adjustment but there is an adjustment for pedal play. Push on the pedal rubber with finger and it should have about 1/16" to 1/8' of looseness before it becomes firm.

 

Worn synchros will grind on down shift because of the extreme miss match of counter and mainshaft speeds. You can minimize this by letting the clutch up in neutral, quickly revving the engine which spins up the counter shaft and then quickly depressing the clutch and down shifting quickly before it slows down. Worn synchros don't grind so much up shifting as the counter shaft is spinning at the same RPM as the engine and once the clutch is depressed the thick oil causes drag that slows it and when they are close to the same speed as the mainshaft the synchros easily complete the shift. Naturally if they are worn and you force a fast shift they will likely grind.

 

Mine was so bad that I skipped down shifting all together. Then I tried a synthetic oil. It's the same protection but much thinner. Up shifting was faster with the same effort and down shifting much improved. The thick 80w90 oil was always a stiff up shift for the first few minutes when starting off cold. This went away completely.

What synthetic oil did you use for your transmission? I am about to change mine and I have the same problem with hard shifting when cold. 

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GM no longer has it available. It was the GM Delco Friction Modified Synchromesh Transmission Oil. But I believe there is only one maker of the base synthetic and it's also sold under Royal Purple, Redline? and a few others by simply adding a color to it. It was used in the 'New Camaro' and the Corvette. So I asked the dealer what the current Camaro and Corvettes are using and he said it was basically automatic transmission fluid (ATF) and it's $25 CDN a liter. ATF, although a hydraulic oil, also has to lubricate gears and bearings just like a standard. ATF has no standard for viscosity or weight and is somewhere between 6w and 12w. I used it in my sedan struts. It might be worth trying ATF some day. I wonder if the new Corvette and Camaro has a transmission oil cooler????

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At this point I'm thinking the recently arrived cold temps brought a synchro issue to light. 

 

I can double-clutch and match revs and make it in. Trans won't make the downshift otherwise. Seems like it's time to learn how to crack open a transmission and rebuild. I might try changing the Sta-Lube out with a synthetic GL4 in the meantime. 

 

I haven't swapped out the clutch fluid yet. I'll try to do so this coming weekend.

 

Thanks for all the pointers folks. Greatly appreciated.

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It's hard for the synchros to squeeze the cold thick oil out of the way for engagement. I think you'll find the synthetic really improves down shifting.

 

I don't down shift coming to a stop using engine compression, I just slip out of gear once slow enough and let the clutch up. This lessens the wear on the synchros and the release bearing. Brake pads and shoes are way cheaper and easier to change than clutch discs and release bearings. I start in neutral, clutch up to lessen the wear on the crankshaft thrust bearings and also the release bearing.  Of course there are times that conditions change, like a light turning green, or where you might need to accelerate and are in too high a gear and a quick down gear is necessary but overall brakes are for slowing down not down shifting.

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Transmissions are not cheap to have another one installed. I got a Jasper remanufactured one from a Jasper installer. It came with a 3 year or 100,000 mile warranty just like my Jasper engine.Another installer had one for a year warranty but it was about the same price.I also got a new yoke installed.Hard to find any one to go in your transmission and fix it cause it's hard to find parts and more easier for them to just install another one.A lot of people won't put that kind of money in a old 720.But I will in a New York minute.My wife had a Jasper transmission installed in her 2004 lincoln LS.

 

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Edited by Thomas Perkins
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  • 2 weeks later...

I have a new rear seal for my trans. Once I get that in I'll fill up with something different.

 

I have found the AC Delco Friction Modified Synchromesh fluid for $12.13/qt. Looks like the stuff @datzenmike mentioned. Cheaper than Redline MT-90, so I'm thinking it's worth a shot. It's clear to me now that the Sta-Lube I threw in there is pretty heavy stuff in low temps.

 

7561V9z.png

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  • 1 year later...

Long overdue update for y’all.

 

It took a while but a few months back I finally got around to changing out the trans rear seal, draining the Sta-lube, and replacing it with the AC Delco synchromesh trans fluid (not friction modified).

 

The change made a world of difference. I have changed my driving habits and rely less on downshifts (got used to rev matching last winter), but it goes into gear way easier, even down into the teens. Overall, much better cold weather shifting up or down.

 

All in all, I highly recommend this stuff if you’re having any synchro issues.

 

Now to swap out tach and neutral switch o rings (assuming they have em). The trans seems to be leaking the thin fluid from somewhere. Rear seal looks to be dry.

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Tu0tY8g.jpg

 

No tach switch. Probably the speedometer cable. The pinion sleeve has a large O ring but they seal very well. However there is an internal seal (lower left) on the spinning pinion. Try plumber's teflon tape on the threads.

 

Pull the sleeve straight out do not twist or you'll damage the plastic gear. I loosen the cable and use it to pull out, then remove the cable from the sleeve. Raise the rear or drain the transmission or you'll be wearing the oil.

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