Advice on Transmission Fluid Change for 1980 C10

AlabamaDan · Sunday at 5:31 PM

Hey everyone,

I'm working on bringing my 1980 Chevy C10 back to life after it sat in a garage for around 15 years. The truck was my grandfather's and is all original, so I'm trying to keep things as close to factory as possible. I've got the engine running smoothly, but when I checked the transmission, I found the fluid level was low. I haven’t seen any obvious leaks, so I’m guessing it just needs a full fluid change.

Since I've mostly worked on imports before (done a few drain and refills on Nissans), I wanted to check in here about what’s needed for the C10's transmission. I don’t know exactly which automatic transmission it has—I'm guessing it could be a TH350 or TH400, but not 100% sure. Here are a few questions I’m hoping you all can help with:

How do I identify the transmission model? Anything specific to look for?
What’s involved in a full fluid change? I assume I need to drop the pan on these, right? And is there a filter I should replace while I’m in there?
What parts and supplies should I buy? I'm guessing transmission fluid, maybe a new pan gasket, and possibly a filter kit?

I appreciate any tips, advice, or things to watch out for with these old automatics. Thanks in advance!

Turbo4whl · Sunday at 6:06 PM

Look at the pan..

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Yes there is a filter screen. Use ACDelco Dexron 3 or 6. I would not use any fluid that states Dex-Mer or multi-vehicle. Others may have a different opinion on the fluid.

Get a pan gasket too.

DoubleDingo · Sunday at 6:28 PM

@Turbo4whl I can't find plain Dexron III here, so I buy DEX/MERC V Valvoline. Bought my 1991 Cherokee in 2002 with 88k on it, and did a drain and refill with DEX/MERC III at the time, then only thing I could find was V. It now has 220k on original drivetrain. I cannot knock Valvoline DEX/MERC V.

When I bought the 81 C20 in 2012, knowing V was the only thing I could get (staying with Valvoline) I have been using that in the TH400 all this time. When I have the AW4 and TH400 rebuilt, I will consider swapping brands at that point, but again, I can't knock the Valvoline DEX/MERC V.

AlabamaDan · 2024-11-04T11:36:56-0500

I was thinking to switching to synthetic. What about this one:
Valvoline DEXRON®-VI ATF Full Synthetic Automatic Transmission Fluid, 1 Quart

Robert Bare · 2024-11-04T11:47:39-0500

If it has not been rebuilt in a long time, I would not use synthetic, but old style dex or dex/mercon. Clutch material was different. My opinion. But, we do have a resident trans guy here, he will chime in,I bet.

DoubleDingo · 2024-11-04T11:49:55-0500

AlabamaDan said:
I was thinking to switching to synthetic. What about this one:
Valvoline DEXRON®-VI ATF Full Synthetic Automatic Transmission Fluid, 1 Quart

I would recommend against going synthetic. If you can find Dexron III, go with that.

Ricko1966 · 2024-11-04T11:50:18-0500

AlabamaDan said:
I was thinking to switching to synthetic. What about this one:
Valvoline DEXRON®-VI ATF Full Synthetic Automatic Transmission Fluid, 1 Quart

I wouldn't unless I was going to have a shop do a flush,or I was going to install a drain plug in the pan and change the fluid frequently. A plug and change the fluid frequently is what I would do regardless of if I went synthetic. Reason is when you drop the pan you will change out only approximately 4 qts,the whole system holds like 16 qts,if you drain and fill frequently eventually you will have mostly fresh fluid. Also when I do a fluid and filter a couple of tips for you. Check your fluid level first, use an upside down trash trash can lid with a hole in the center on top of your drain pan. Most of them already have a hole. Measure what you took out,put that much back in, then you can double check it,but it will be right.

DoubleDingo · 2024-11-04T11:56:54-0500

Ricko1966 said:
I wouldn't unless I was going to have a shop do a flush,or I was going to install a drain plug in the pan and change the fluid frequently. A plug and change the fluid frequently is what I would do regardless of if I went synthetic. Reason is when you drop the pan you will change out only approximately 4 qts,the whole system holds like 16 qts,if you drain and fill frequently eventually you will have mostly fresh fluid. Also when I do a fluid and filter a couple of tips for you. Check your fluid level first, use an upside down trash trash can lid with a hole in the center on top of your drain pan. Most of them already have a hole. Measure what you took out,put that much back in, then you can double check it,but it will be right.

I took an old milk jug and used a 4-cup measuring cup and a sharpie to mark 1 quart, 2 quart, 3 quart up to 4 quart using water to get the marks on the jug. In between 3 and 4, I went with 8 oz increments. Just have to transfer what drained from the transmission into the milk jug to see how much to pour back in the transmission.

Grit dog · 2024-11-04T12:04:09-0500

Have you even tried to run it yet?
What the fluid look like now?
No sense in changing fluid and filter on a bad transmission so verify it’s not inop first.
Do NOT take it to be power flushed. You can research why.
Once you e verified it’s a good transmission, IMO the best way to do a fluid change so you get the tq converter fluid out as well is to drain pan, new filter, refill and then disconnect cooler lines and use the trans pump to pump out the old fluid not sure specifics on old th350 but basic principle is do a pan/filter swap and then run the engine, meter fluid output. Let some out, put some in. Until you’ve reasonably gotten most all the old fluid out.

No comment on fluid. It’s a basic trans and new juice is good, as long as the trans isn’t on its way out. Anything Dex compatible imo.

82sbshortbed · 2024-11-04T12:08:11-0500

Robot or human?

I've always used this in my transmission. Never used the blue.

AlabamaDan · 2024-11-04T15:13:26-0500

Robert Bare said:
If it has not been rebuilt in a long time, I would not use synthetic, but old style dex or dex/mercon. Clutch material was different. My opinion. But, we do have a resident trans guy here, he will chime in,I bet.

It's never been rebuilt. It has always run fine. In fact, it's still driving okay right now, I just noticed it was low and thought I'd proactively do a little maintenance instead of just adding a quart and moving on. It probably has a small leak and over the last 15 years has gotten low while it sat in the garage.

DoubleDingo said:
I would recommend against going synthetic. If you can find Dexron III, go with that.

Noted.

Ricko1966 said:
I wouldn't unless I was going to have a shop do a flush,or I was going to install a drain plug in the pan and change the fluid frequently. A plug and change the fluid frequently is what I would do regardless of if I went synthetic. Reason is when you drop the pan you will change out only approximately 4 qts,the whole system holds like 16 qts,if you drain and fill frequently eventually you will have mostly fresh fluid. Also when I do a fluid and filter a couple of tips for you. Check your fluid level first, use an upside down trash trash can lid with a hole in the center on top of your drain pan. Most of them already have a hole. Measure what you took out,put that much back in, then you can double check it,but it will be right.

Good info. I've never worked on a tranny before, I was thinking the pan drop would dump all fluid - but I didn't realize it was 16 quarts! Wow!!!

I'm not home right now to crawl under it, but I don't think it has a drain plug. I don't know why they don't have one. So you're recommending adding one?

DoubleDingo said:
I took an old milk jug and used a 4-cup measuring cup and a sharpie to mark 1 quart, 2 quart, 3 quart up to 4 quart using water to get the marks on the jug. In between 3 and 4, I went with 8 oz increments. Just have to transfer what drained from the transmission into the milk jug to see how much to pour back in the transmission.

Great tip!

Grit dog said:
Have you even tried to run it yet?
What the fluid look like now?
No sense in changing fluid and filter on a bad transmission so verify it’s not inop first.
Do NOT take it to be power flushed. You can research why.
Once you e verified it’s a good transmission, IMO the best way to do a fluid change so you get the tq converter fluid out as well is to drain pan, new filter, refill and then disconnect cooler lines and use the trans pump to pump out the old fluid not sure specifics on old th350 but basic principle is do a pan/filter swap and then run the engine, meter fluid output. Let some out, put some in. Until you’ve reasonably gotten most all the old fluid out.

No comment on fluid. It’s a basic trans and new juice is good, as long as the trans isn’t on its way out. Anything Dex compatible imo.

I've driven it slowly around the farm off and on the last few years and it runs fine.

Ricko1966 · 2024-11-04T17:08:49-0500

AlabamaDan said:
It's never been rebuilt. It has always run fine. In fact, it's still driving okay right now, I just noticed it was low and thought I'd proactively do a little maintenance instead of just adding a quart and moving on. It probably has a small leak and over the last 15 years has gotten low while it sat in the garage.

Noted.

Good info. I've never worked on a tranny before, I was thinking the pan drop would dump all fluid - but I didn't realize it was 16 quarts! Wow!!!

I'm not home right now to crawl under it, but I don't think it has a drain plug. I don't know why they don't have one. So you're recommending adding one?

Great tip!

I've driven it slowly around the farm off and on the last few years and it runs fine.

It 99% does not have a drain plug,which is why I said use an upside down trash can lid over your drain pan,otherwise you are going to have a mess. Yes I'm recommending adding a drain plug,and drain and fill the transmission pan on the same day you do your oil changes until you've done at least 4,that will get most of the fluid changed out.

AuroraGirl · 2024-11-04T19:45:44-0500

DoubleDingo said:
@Turbo4whl I can't find plain Dexron III here, so I buy DEX/MERC V Valvoline.

with that attitude you wont

The only product that "IS" dexron iii is this:

But youre using synthetic dex/merc so youd want to aim for Dex VI anyway

You must be registered for see images attach

NickTransmissions · 2024-11-04T21:27:49-0500

AlabamaDan said:
It's never been rebuilt. It has always run fine. In fact, it's still driving okay right now, I just noticed it was low and thought I'd proactively do a little maintenance instead of just adding a quart and moving on. It probably has a small leak and over the last 15 years has gotten low while it sat in the garage.

Noted.

Good info. I've never worked on a tranny before, I was thinking the pan drop would dump all fluid - but I didn't realize it was 16 quarts! Wow!!!

I'm not home right now to crawl under it, but I don't think it has a drain plug. I don't know why they don't have one. So you're recommending adding one?

Great tip!

I've driven it slowly around the farm off and on the last few years and it runs fine.

The pan itself holds 4-5 quarts; the entire system holds like 8-10 (TH350/TH350C)...

Anything DexMerc will be fine. If your TH350C (Im assuming that's what you have, correct me if I'm wrong) has a cloth style filter, remove and chuck it in the trash - install a screen style filter with new filter-vb gasket.

Check the vacuum modulator hose for small cracks - replace if observed. DO NOT use a cork or rubber pan gasket - they leak and suck. Composite gaskets are fine.

DoubleDingo · 2024-11-04T22:35:09-0500

AuroraGirl said:
with that attitude you wont

The only product that "IS" dexron iii is this:

But youre using synthetic dex/merc so youd want to aim for Dex VI anyway

You must be registered for see images attach

Good to know. Thanks for that. I'll see if I can find that when have the rebuilds done

Advice on Transmission Fluid Change for 1980 C10

AlabamaDan

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Turbo4whl

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DoubleDingo

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AlabamaDan

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Robert Bare

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DoubleDingo

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Ricko1966

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DoubleDingo

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Grit dog

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82sbshortbed

Fuckemall!!

AlabamaDan

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Ricko1966

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AuroraGirl

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NickTransmissions

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DoubleDingo

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