Keeping up with a leaky transmission, newbie question. . .

[DecoDog]

Salutations kind Internet car folks!

I’ll preface this post by saying I’ve never performed an oil change before. Apologies in advance if this is a no-brainer, but I’m new to the hobby! 

 

My 1936 Airstream DeSoto Sedan has a leaky transmission—at the seals on either end it seems from my cursory inspection (This is an issue I knew about when I purchased the car). It’s hard to get a good idea how much leaks, as when it’s sitting it’s just a small pool, but I have the hunch that it leaks significantly more when the oil is warm and the car is moving—but of course it’s hard to monitor a moving car! I don’t currently have the means to have it properly serviced, so for the time being I’m flying blind.
 

I imagine the car is still perfectly drivable so long as I keep the fluids at a healthy level. 
I was planning on draining the fluids and measuring what comes out, to see how far off the capacity it is and then calculate a rough leak rate. Then, proceed to fill it up and “top it off” every so often to compensate for the loss. 
 

My questions would be:

-Can you overfill a transmission with fluid, is this easy to accidentally do? (My plan would’ve been to just keep filling the car to “capacity” every so often, but I figure there’s more space in the recess for oil than is required).
-Is there a dependable way to check fluid levels besides draining? 
-What would be your suggestion to dealing with this issue?

 

So far, I’ve clambered under the car to check where the transmission oil drain and fill plugs are, as well as where a bulk of the oil leaks to and placed a dish under to measure the amount while the car sits.

 

As with anything, I’m sure my specific car would change any usual procedure, but a general suggestion is highly appreciated! 
 

Thanks in advance for taking the time to share your knowledge with a newbie, I’ll take any tips I can get! 
 

(Attached is a photo of what I believe to be my transmission fill point, as well as some of the leaking seals) 

[Dave39MD]

You may have already purchased an owners manual and shop manual but if you haven't it would be good to have them at hand. The leaks look similar to many well used and driven cars.

 

You can overfill a transmission but it is more likely worn seals, bearings or bushings. It can also be the wrong oil which is why the manufactures information is so important along with experience from others with the same vehicle.

 

Marque car clubs are great to join for not only the like minded fellowship but the knowledge the members have on vehicles like yours.

 

Dave

 

 

[Rusty_OToole]

To fill the transmission you need to unscrew the fill plug. If oil drips out you are fine, if you can stick your finger in the hole and touch oil you are fine. Otherwise put in oil until it drips out and put in the plug. This is easier said than done unless you have a pressure fill hose like a garage.

The first thing you need to do, is clean off all the old oil and grease then drive around a bit and see where the leak is coming from. Sometimes it is obvious, sometimes you have to start with a clean dry transmission. A spray can of brake clean will make short work of the cleaning process.

It could be something as simple as a loose bolt or fitting. Or it could be a bad seal requiring the trans to come out. Be sure you have the correct grade of oil.

It is even possible the leak is from the engine and is running back onto the trans.

[DecoDog]

9 minutes ago, Rusty_OToole said:

To fill the transmission you need to unscrew the fill plug. If oil drips out you are fine, if you can stick your finger in the hole and touch oil you are fine. Otherwise put in oil until it drips out and put in the plug. This is easier said than done unless you have a pressure fill hose like a garage.

The first thing you need to do, is clean off all the old oil and grease then drive around a bit and see where the leak is coming from. Sometimes it is obvious, sometimes you have to start with a clean dry transmission. A spray can of brake clean will make short work of the cleaning process.

It could be something as simple as a loose bolt or fitting. Or it could be a bad seal requiring the trans to come out. Be sure you have the correct grade of oil.

It is even possible the leak is from the engine and is running back onto the trans.

Thanks for the pointer! I hadn’t considered unscrewing the plug, as for some reason I had it in my head that it could only be filled via a pressure system. That should give me the ability to visually check the level. Appreciate it! 

[Frank DuVal]

My "pressure" system is usually folding up the plastic gear lube quart container* while the spout is in the hole! And of course, I only stop when I see fluid floe back out, which means a lot flows out before I get the plug back in. Just some of the old car fun we all enjoy! I get less loss of fluid this way that using those cheap suction gins at the auto parts store...😉

 

One usually has to be creative in getting that plastic bottle to fold up and out of the way of everything else under there. But it still beats the cheap suction guns.

[rocketraider]

Young'un, we will turn you into one of us!👨‍🔧

 

Being willing to try will take you far.

 

Take it you're assembling a tool set? For time being a SAE set from Harbor Freight will handle your basic needs on the DeSoto. Later on you can start haunting swap meets and estate sales for quality vintage tools.

 

If you're lucky the fill plug will be either 3/8" or 1/2" square drive, and a ratchet will screw it out. If not, you'll need to get a set of what's called pipe plug wrenches.

 

Brake cleaner will clean off oil and grunge or if you can swing it, have the underside pressure washed or steamed. It will make finding source of the leaks easier.

[Grimy]

5 minutes ago, rocketraider said:

Brake cleaner will clean off oil and grunge or if you can swing it, have the underside pressure washed or steamed. It will make finding source of the leaks easier.

I recommend against pressure washing or steaming because those processes are very likely to remove the cloth-only factory insulation on wiring.  If you do use either process, watch for wiring locations FIRST and avoid them as best you can.

[jpage]

Looks like most of the leak is coming from the rear seal. Does that car have an overdrive in it, as the rear of the transmission looks different? Seals are usually easy to change, but for now just keep an eye on the level. I would recommend using at least a 140wt. gear lube which might ease some of the leakage. I don't think that there is a seal in the front of the transmission,  if it's similar to the Dodge unit, but they can leak from the front. Are your u-joints exposed or are they covered with a leather boot?

[Rusty_OToole]

The trans pictured shows a pipe plug with a square recess that will take a 3/8 socket handle or extension. Some have a pipe plug with a square protrusion you can put a wrench on. Pro tip: On these, use a socket extension turned backwards then put an 8 point or 12 point socket on the other end of the extension and turn it with a socket handle or ratchet handle.

[DecoDog]

2 hours ago, jpage said:

Looks like most of the leak is coming from the rear seal. Does that car have an overdrive in it, as the rear of the transmission looks different? Seals are usually easy to change, but for now just keep an eye on the level. I would recommend using at least a 140wt. gear lube which might ease some of the leakage. I don't think that there is a seal in the front of the transmission,  if it's similar to the Dodge unit, but they can leak from the front. Are your u-joints exposed or are they covered with a leather boot?

Yes! The car does have an overdrive, which does add a bit of complexity to the transmission for sure. Comparing diagrams there’s a whole extra one and a half feet of transmission to account for with numerous extra seals, so it’ll certainly be an undertaking down the line. I believe the original manual recommended 90 weight for a overdrive transmission, but I have been told that well used engines and transmissions require different weights than the original specs might suggest. 
 

universal joint is exposed, right at the back of the transmission. 

[DecoDog]

24 minutes ago, Rusty_OToole said:

The trans pictured shows a pipe plug with a square recess that will take a 3/8 socket handle or extension. Some have a pipe plug with a square protrusion you can put a wrench on. Pro tip: On these, use a socket extension turned backwards then put an 8 point or 12 point socket on the other end of the extension and turn it with a socket handle or ratchet handle.

Thanks for the tip! Borrowing up my stepfathers toolkit today, as he said he’s got something that’ll fit. I’ll keep this in mind as I’m fiddling with the plug, it is in a rather inconvenient spot. 

[JACK M]

6 hours ago, DecoDog said:

it is in a rather inconvenient spot. 

 

Get used to that.

[keithb7]

I have a ‘38 Chrysler with overdrive. There are 3 plugs. The tranny and OD oil are shared between both compartments. There are 2 drain plugs down low, to drain oil.  The OD and tranny have lower sumps. So both plugs get removed to drain all the oil. 

There is only 1 fill plug. Square drive as shown above.  I fill it with oil until it starts to come out of the hole. Then both the tranny and OD are full. 
 

Mine leaks pretty good too. Not quite as bad as yours. I keep an eye on it. Get a large syringe and put a piece of small hose on the nipple. About a foot long. Suck GL-1 gear oil from your jug, into the syringe. Get under the car and squirt it into the tranny. Repeat until full.  My syringe is about 50ml capacity if I recall. My last top up took about 250 ml. I figure I lost about 100ml per month.  
 

I’ll drop my tranny and reseal it eventually. Put in a new clutch (synchro) gear as well. Maybe this winter. 

[keithb7]

[DecoDog]

4 minutes ago, keithb7 said:

Thank you very much! This is extremely helpful. My model has a overdrive as well, and I’ve actually yet to find a second drain port, but I was also in a poorly lit claustrophobic underside of the car. I’ll have to double check tomorrow. For the time being got the transmission filled with SAE 90 as the owners manual specifies for overdrive equipped transmissions. 

[richwill262]

The leaks look similar to many well used and driven cars.

[m-mman]

On 8/20/2021 at 9:34 PM, Frank DuVal said:

One usually has to be creative in getting that plastic bottle to fold up and out of the way of everything else under there. But it still beats the cheap suction guns.

these are readily available at AutoZone etc. They thread into to top of the gear oil jug and quickly hand pump lube into the trans filler hole. 

 

[c49er]

Why not just just pour the gear oil into the standard size suction gun. Same capacity as a grease gun.

Then just one long push of the suction gun handle and all the gear oil is in the trans.

These suction guns have a foot long hose on them.

Buy a good one and it won't even leak like the cheap Chinese ones!

Been doing it this way for years.

[rocketraider]

D-dog, another of our young Forum members, Mr Reed, has a 50 Buick with a leaky DynaFlow. Oldtimer's advice given to him was it would probably be fine as long as it was marking its turf, but when it stopped that meant it was empty!😃

 

 

I truthfully think your leak may be easier to fix. Buicks with their torque tube drives have to have the rear axle slid back several inches to get at the transmission output shaft seal. Major job, but probably very routine in those days.

[Richard1]

I will address the leak issue, as it is always a problem when oil isn't changed frequently enough or thee is too much wear. Since the axle and pinion seals only get completely wet when the oil is thrown on them during use, the more the car sits, the more the top half of the seal dries (since you never want to fill more than just below the half-way mark on the shaft).

 

The new oil comes with a small amount of additive that maintains the seals soft until it is used up. We have no way of measuring how long that is. But I do not know of anyone in the US that sells the concentrated seal swell to the public or in quantities less than 5 gallons. 

 

I buy the 5 gallon pails ($600) and import them to Bolivia, where I mix formulas for automatic transmission conditioning and power steering sealing. For my Corvair I was able to find a rebuilt transmission that had been on a shelf for 40 years. Runs great, but leaked. I did a calculation, and added 25 ml (just under 1 ounce) of seal swell additive to my transaxle (75W-90 GL-4), and within days it stopped leaking.

 

I think that if I was in the US, I'd look at the different stop-leak products on the market and (knowing that they are probably about 5% or so seal swell diluted in some other oil), choose one that will not reduce the viscosity too much, or compensate with a little gear oil that is thicker than needed so the end result would be correct. 

 

 

[Frank DuVal]

Hi Richard! I've posted a link to your oil paper just last week on this forum.👍

[DecoDog]

On 8/28/2021 at 11:43 AM, Richard1 said:

I will address the leak issue, as it is always a problem when oil isn't changed frequently enough or thee is too much wear. Since the axle and pinion seals only get completely wet when the oil is thrown on them during use, the more the car sits, the more the top half of the seal dries (since you never want to fill more than just below the half-way mark on the shaft).

 

The new oil comes with a small amount of additive that maintains the seals soft until it is used up. We have no way of measuring how long that is. But I do not know of anyone in the US that sells the concentrated seal swell to the public or in quantities less than 5 gallons. 

 

I buy the 5 gallon pails ($600) and import them to Bolivia, where I mix formulas for automatic transmission conditioning and power steering sealing. For my Corvair I was able to find a rebuilt transmission that had been on a shelf for 40 years. Runs great, but leaked. I did a calculation, and added 25 ml (just under 1 ounce) of seal swell additive to my transaxle (75W-90 GL-4), and within days it stopped leaking.

 

I think that if I was in the US, I'd look at the different stop-leak products on the market and (knowing that they are probably about 5% or so seal swell diluted in some other oil), choose one that will not reduce the viscosity too much, or compensate with a little gear oil that is thicker than needed so the end result would be correct. 

 

 

Some great insight here. I was confused when I replaced the oil to find that the car isn’t leaking anymore, at least not in a way that leaves a puddle right away like it used to. Oil was milky and dark when I drained it from the transmission, so I’m assuming it was well past it’s point of usefulness. New oil in the transmission seems to have either softened the seals again as you suggested, or the oil weight is better suited than the previous owner had installed. 

[rocketraider]

Try running it a couple hundred miles and then change the oil again. That will help clean out all the grunge of the last no telling how many years.

 

Proud of you for learning how to service a car nearly four times your age! 👨‍🔧

[trini]

If it was mine the first thing I will do is to get a manual for it. You hear hear the  saying " when all fails consult the manualll". The easiest way to tackle the problem is with the transmission on the bench. After so many years gaskets could be deteriorated. And as one blogger said it could be the wrong oil . It could be oil seals. It could be engine oil leaking from the valve cover gaskets and flowing back.

Varsol in a garden spray can does a nice job cleaning up old oil and grease. Catch the drips and recycle. Varsol does not evaporate and when settled the gunk sits at the bottom of the container.