Jump to content

5th gear sounding rough


bylzebub

Recommended Posts

Hi guys, been lurking here for about a year since I got my 720 (1985 720 ext cab 4x4) and finally signed up for the forum. I've got a couple topics with questions but the latest and most pressing issue right now is my 5th gear just started sounding bad. . .  like an impact or jack hammer are having a nice time in the tranny every time I shift into 5th.

 

All other gears are great, smooth, no issues. No popping out in 5th either, it just developed a good racket and vibration. The more throttle in 5th the louder it gets, if I back off so does the sound. Pop it back in 4th and its not an issue.

 

The main questions I have:

 

1. Whats most likely going on in there?

2. Am I safe to run the truck in the other 4 gears in the meantime? (I hardly need 5th and could easily go without if thats safe)

3. Are we mostly likely talking a new tranny or would a rebuild fix this?

4. What other transmissions are options here if I need a replacement?

 

Just in case it matters, so far I've replaced the carb with a weber 32/36, replaced the water pump and I'm about to throw on some pacesetter headers cuz my exhaust manifold has a descent crack. I thought maybe the knocking sound was the exhaust leak but its definitely a 5th gear issue. I do have a knock sound at idle tho since installing the weber.

 

Anyway all help is appreciated. I tried getting some answers at the Nicoclub.com but I think that 720 forum is dead. Thanks!

 

 

 

 

Link to comment
  • Replies 20
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

You can run the other 4 gears. The good thing is the 5th gear is easy to get at by removing the rear half of the transmission. Sounds like the 5th gear or the 5th counter gear or both are pooched.

 

Does reverse work ok? It's just forward of 5th, and right beside it.

 

Gear chunks will be trapped behind the adapter plate but they can get into the two adapter plate ball bearings. Tilt the vehicle up at the front and remove the speedometer pinion. Oil will gush out so be ready. This may flush out some chunks or shiny pieces. I expect the oil to be sparkly with metallic pieces. If for some reason it's clean maybe the noise is something else.

  • Like 1
Link to comment

You can run the other 4 gears. The good thing is the 5th gear is easy to get at by removing the rear half of the transmission. Sounds like the 5th gear or the 5th counter gear or both are pooched.

 

Does reverse work ok? It's just forward of 5th, and right beside it.

 

Gear chunks will be trapped behind the adapter plate but they can get into the two adapter plate ball bearings. Tilt the vehicle up at the front and remove the speedometer pinion. Oil will gush out so be ready. This may flush out some chunks or shiny pieces. I expect the oil to be sparkly with metallic pieces. If for some reason it's clean maybe the noise is something else.

 

Thats sweet! I was hoping I wouldn't have to sit the truck in the meantime.

 

Reverse works fine, its really just 5th.

 

I fully planned on draining the gear oil and seeing whats in there, forgot to mention that. Changing every fluid the truck had was one of my first projects when I got the truck so that oil isn't old. Are you saying I shouldn't drain ALL the gear oil and just remove the oil behind that last adapter plate through the speedo pinion?

 

Whats the recommended gear oil weight for these trucks? If I remember correctly I think I used synthetic 75W-90, also in my transfer case.

 

Thanks for the quick reply! its nice finally having a place to ask these questions.

Link to comment

All the oil will drain from the bottom drain bung but if there are chunks they are stuck behind the adapter plate. If you drain through the speedometer pinion sleeve they may flush out... that's all. Either way I'm expecting the oil to be full of metal. BTW the drain bung has a magnet on it that catches ferrous metal.

 

GL-4 80W90 is fine. NOT GL-5. This transmission has problems, I wouldn't bother with the added cost of synthetic.

  • Like 1
Link to comment

Given that this transmission has problems, I assume it is time to get a rebuild. Would a reputable transmission shop be able to do the job? And does the price vary wildly, depending on the nature of the problem?

 

My 4x4 has a bad gear sound in 3rd gear, so I parked it right away and I haven‘t pulled the speedo cable or the drain plug yet. But it sure didn’t sound good. As I leap frog rebuilt engine / transmissions into the trucks, I sure would want to know that the tranny is solid. I am just trying to figure out whether this is one of those deals where it is best to just take it in to a tranny shop or maybe I can do it myself.

  • Like 1
Link to comment

Well I spent about an hour trying to get the fill plug out. Luckily I was smart enough to try that one first before the drain plug. Anyway its not budging. Anyone got a tip? I'm gonna take it to a shop on Tuesday probably and get a good impact wrench work on it. My compressor and impact driver has no juice.

 

Can someone drop a pic of the speedo pinion? I think I found it but to me it seems like its more on the top of the transmission and would therefore need quite a tilt to drain out that way. Maybe I'm looking at it wrong but I decided to just drain everything when I ran into the fill plug problem.

 

On a side note I'm looking for theories on a particular sound I assume is related to my transmission. When the truck first starts up there is a slow rhythmic knocking sound, if I push the clutch pedal in it goes away. How hard is it to drop the transmission in the 4x4 truck. Seemed fairly extensive from looking at everything while trying to get that plug out. Has anyone done a transmission walk through yet?

Link to comment

Speedo pinion is in your transfer case. There isn't one in your 4x4 transmission.

 

Dropping your transmission is a nightmare. It is better to pull the engine to do the clutch than it is to pull the transmission. Even the original Factory Service Manual says to do it that way.

 

That being said, I pull my motor then pull my transmission. That's how much I hate dropping the trans out the bottom. I have pulled them together, but I wouldn't recommend trying that either.

 

To get the trans out the bottom, you need to competely remove the front diff and crossmember, the front driveshaft, the intermediate shaft to the tcase, the exhaust must be moved or removed. Getting all that back together is worse than taking it apart in my experience.

Link to comment

Sorry, did not read you have a 4x4. Best to just remove it. If replacing, a 2 or 4 wheel drive transmission will work. If you do serious off road and want to keep the wide ratio then you'll have to find another 4x4 box.

Link to comment

So.... exactly what I said.

Yes, but forgive me if I take more then one person's opinion before I jump into something like pulling my entire engine. 

 

Since everything shifts fine except into 5th what are your thoughts on leaving it as is and possibly running a heavier weight gear oil for added protection? I obviously don't know transmissions very well but its a theory I was thinking about.

 

Sounds like GL-4 is a must though and my closest auto parts store is all GL 4/5. I'll be looking around for some straight GL 4 I think. 

Link to comment

Napa has gl4 in stock. Heavier will work, but cold shifting will get harder and mileage may go down.

 

If you are not messing with the trans, then just leave it in the truck. If you look at that how-to you posted, he has the engine up extremely high in order to get the trans over the front end. He's using a gantry crane which makes that easy. If you are using an engine hoist, that process is a joke.

 

The diff has one large bolt through the cover holding it to the front crossmember. If you remove that bolt, you can slide the diff down about an inch. It may not sound like much, but that additional clearance at the oil pan makes a world of difference when fighting your engine back in to the truck.

Link to comment

I was only looking into this for the sake of doing some transmission work, I don't want to the pull the engine except to get back there and fix that 5th gear. 

 

I'm really starting to wonder if when I changed the gear oil when I first got the truck and undoubtedly put in some GL4/5 oil it didn't mess up my transmission more. . . 

Link to comment

It would only affect the brass synchros. You might have trouble shifting in and out of gear or it might stick in gear but iot wouldn't affect the gears themselves. Yes you can continue driving it but I would dump the old oil and change it to get any fine metal filings out.

Link to comment

After I purchased a 2 ft long iron pipe from home depot for a cheater bar, that fill plug twisted right off with no issues. Of course the real fun started after I took off the drain plug. . .

 

drain%20plug_zpszx0ks4rr.jpg

 

I'm hoping that those are only chunks from my 5th gear but only time will tell. The oil was very dirty and had plenty of very fine shiny metal flake in it. I tried taking a picture of it but the camera would only capture the reflection. I changed this oil approximately 10 months and 5000 miles ago when I first got the truck and there was one little piece of metal on the magnet then. The next photo is just an oil sample from what came out vs new. I have no experience with how quick gear oil gets dirty but probably not this bad in that amount of time.

 

Oh and I found GL4 85W90 at NAPA. Tried 2 other parts stores before that one, everything seems to be GL4/5 now. Fun fact, my service manual calls for GL 5 in the differentials. GL4 is only for the transmission.

 

gear%20oil_zpsutqe6tpk.jpg

 

As I was under the truck though I found what I thought might be a reason the oil is getting so filthy. Keep in mind I'm no mechanic but I don't understand why a transmission would have a giant open hole leading to the clutch. I had hopes of 4x4ing in the truck but one rock in there could really ruin a day I'm thinking.

 

clutch%201_zpszg1brhey.jpg

 

clutch%202_zpsvwvvnwdf.jpg

 

Correct me if I'm wrong but that white thing in the rectangular hole in the side of the case is my clutch. The arm going in there would have to be working the pressure plate or something like that. Wouldn't water or rocks really make a mess if they got in there? Is my transmission missing something there that it should have?

 

Anyway my plan is to run the truck in gears 1-4 for now and try and track down a transmission to replace or rebuild for the truck. This one feels like a ticking time bomb now. Anyone in the pacific northwest have a transmission for me?

Link to comment

Transmission oil does not get dirty like engine oil. It will usually pour out the same as it went in. Metal filings are normal to find on the magnet but chunks are not. I pulled one once with a couple of ball bearings on it.

 

The clutch arm should have a rubber boot on it like the shifter does. Yes dirt and water can get in but only at the clutch/pressure plate and the flywheel, it can't get into the transmission oil. That is sealed to the outside. 

 

Differentials have a different sort of gear in them than a transmission. Probably because they transmit power through a 90 degree bend they need additional anti scuff agents in the formula that GL5 provides. These same anti scuff additives are also in the transmission but at a much lower concentration. About 50% less. These sulphur additives attack copper and erode/corrode them if in too high a concentration.Transmissions transmit power through the same axis so the gears are different and (I'm guessing) rub less and need less protection.  

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.