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Discussion Starter · #1 ·
Hey guys, wondering what your thoughts are on this transmission mated to the duramax? My concern is specifically while towing before the TC locks in 4th, it will allow up to 1200 rpm of slip and rev the engine right at about 3000 rpm, which as we know makes the cabin extremely loud. Also, the trans only locks in 4th gear (have had it occasionally lock 3RD at borderline redline, but not sure what the exact parameter is for making it do this).

it seems like this transmission is much better mated to a gasser that can rev. I installed a TC lock switch to lock 3RD going up hills. Only problem with this is it throws a trans code and maximizes line pressure for shifts until I restart the engine.

Anyone ever attempt to tune the trans to lock out other gears at a certain slip point or engine rpm? We have an LBZ work truck with an Allison and it tows night and day better than my van

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I would leave it alone and drive it. I went over 500,000 miles on my original transmission with no issues.

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2006 LBZ, 2009 LML, 2022 L5P. 1993 6.5 parked in the shed.
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630,000 miles on mine, bone stock except for a deep pan (I wanted a drain plug) still in daily use.

How much are you towing and has the converter possibly been replaced with the wrong one at some point? Never seen 1200 rpm of no lock/lock difference on mine.
 

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Discussion Starter · #4 ·
630,000 miles on mine, bone stock except for a deep pan (I wanted a drain plug) still in daily use.

How much are you towing and has the converter possibly been replaced with the wrong one at some point? Never seen 1200 rpm of no lock/lock difference on mine.
Towing about 8000 lbs almost every weekend. I had the transmission/TC rebuilt about 5000 miles ago. Previous transmission was so bad I wouldn’t even call it an even comparison. But this one just doesn’t seem right, seems like it’s slipping a ton. By the time it goes from 1200 rpm slip to locked it feels like a double up shift based on how much my rpm’s drop. Is there an easy way to inspect the TC and tell what’s in there?
 

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2006 LBZ, 2009 LML, 2022 L5P. 1993 6.5 parked in the shed.
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I'd double check with whoever you got the TC from to make sure they put in the correct one, diesel applications with the 4L85 are certainly way more rare than gas, it would be an easy mistake for some place to make. Other than part numbers, not sure how you would tell. You keeping the fluid under 180F? What fluid are you using?
 

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Discussion Starter · #6 ·
I'd double check with whoever you got the TC from to make sure they put in the correct one, diesel applications with the 4L85 are certainly way more rare than gas, it would be an easy mistake for some place to make. Other than part numbers, not sure how you would tell. You keeping the fluid under 180F? What fluid are you using?
Will do, do you know if the gas torque converters are different than the diesel ones? Wouldn’t be surprised if that is my issue, although the place that I had do it also builds race diesel transmissions.
Fluid level is good and when I had the trans done they also installed an XL trans cooler, never seen the fluid get over 175 degrees, even when towing as heavy as I do. Using whatever synthetic stuff the shop put in there
What slips speeds do you guys normally see before the TC locks up?
 

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2006 LBZ, 2009 LML, 2022 L5P. 1993 6.5 parked in the shed.
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Diesel TC is way different than gas, much tighter since it doesn't need to help turn gas engine speed into torque. IIRC I see 2 or 300 ish rpm drop, but I'm not towing, just hauling (so 12,000 gross max) so not exactly apples to apples.

Do you use the tow/haul button on the dash?

There is EFI live support for the TCM the vans use, I've never tried it.
 

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Discussion Starter · #8 ·
Diesel TC is way different than gas, much tighter since it doesn't need to help turn gas engine speed into torque. IIRC I see 2 or 300 ish rpm drop, but I'm not towing, just hauling (so 12,000 gross max) so not exactly apples to apples.

Do you use the tow/haul button on the dash?

There is EFI live support for the TCM the vans use, I've never tried it.
I used to use the tow/haul button, but all it did was rev the hell out of the engine. For example it would hold 2nd gear until 40mph, and by that time I was redlining the engine. I'll have to look into the TC first and figure out what's going on there, then possibly EFI live. Otherwise the TC functions just fine, no problems locking/unlocking, just getting crazy slip speeds out of it.
 

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You need to use tow/haul when towing. You are not going to hit redline at 40 mph in 2nd gear. Have you talked to the shop that rebuilt the transmission?

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Discussion Starter · #10 ·
You need to use tow/haul when towing. You are not going to hit redline at 40 mph in 2nd gear. Have you talked to the shop that rebuilt the transmission?

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I understand I should be using tow/haul mode under ideal conditions. But when the tc is slipping 1200 and the trans input speed is 1800, that’s running the engine at almost 3000 rpm.
Even under normal operating conditions not towing, if I accelerate hard I can get up to 1200 rpm of slip. It just seems like the engine is revving a ton and not really “hooking up” if that makes sense.

Reached out to the guy that built the transmission and he said he used a factory spec torque converter (Edit: 1600-1800 stall speed)

I’ll try to get a video for you guys this weekend when I’m towing
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Has it done this since day 1 after it was rebuilt ? You need to go back to that shop ASAP and get it fixed. You have a rebuild and or TC issues that adding switches or trying to tune the TCM will not fix.
 

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Discussion Starter · #12 ·
Has it done this since day 1 after it was rebuilt ? You need to go back to that shop ASAP and get it fixed. You have a rebuild and or TC issues that adding switches or trying to tune the TCM will not fix.
It has, although I never thought anything of it until I started towing heavy recently. Unfortunately I moved across the country and am no longer anywhere near that shop. Anyone have transmission shop recommendations in East Tennessee? Transmission otherwise shifts great and never heats up, I think it’s isolated specifically to the torque converter
 

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2008 Chevrolet Express 3500, V8 6.6L LMM w/ DBL7483 oil filter mod, HydraMatic 4L85-E 4-speed w/ OD
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I know it sounds ridiculous but have you checked your plastic water drain valve on both fuel filters for leaks or damage? I had air being sucked up through one of mine because a mechanic used vise grips & overtightened it. It was causing all sorts of fuel rail pressure issues causing the gears to space out and even tripping a code for my thermostat because the engine would take too long to warm up.

Also it wouldn't hurt to check the trans fluid level if you haven't already, press the dipstick over a bleached coffee filter to see the level accurately. The 4L85-E will space out your shifts if it can't get the fluid pressure high enough due to low fluid. I know this because I have a stripped pan bolt with a DIY patch of mine that leaks sometimes when going over massive pot holes and I have to watch it.
 

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Discussion Starter · #14 ·
You reminded me that I’m actually due for a fuel filter change, so I’ll check those valves out when I’m down there. I did get a P0087 code twice towing this past weekend. Maybe I’ll pick up one of those billet filter drains from KD that was mentioned previously on the van forum

here’s some pics from towing this past weekend, no crazy acceleration, but when towing and trying to accelerate I feel like it just revs and goes nowhere. Also when pulling a boat out of the water in 1st up a ramp, I’ll see as much as 1500 rpm of slip
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It wouldn't hurt to add 1/4 of one of those Dexron VI trans fluid quarts and see if the slippage goes away. This transmission is really picky with fluid levels, even 1/4 quart too low and it'll slam in to reverse. I'm actually considering investing in a deep pan over my low profile one because of this.

But it sounds like you may have found your problem with that fuel rail pressure code, shifting and fuel rail pressure go hand in hand. The TCM needs a specific torque to engage the next gear and if it isn't reaching that torque value because of fuel rail pressure that's where you get the spaced out shifts.

If you go for the aluminum valve remember to shorten the steel handles, you'll need vice grips at that point to tighten it, and it doesn't hurt to go a little past hand tight since this is aluminum, and tighten the plug itself using the square part above the drain valve, not the 12 pointed part (unless you have the special OEM filter wrench).
 

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It wouldn't hurt to add 1/4 of one of those Dexron VI trans fluid quarts and see if the slippage goes away. This transmission is really picky with fluid levels, even 1/4 quart too low and it'll slam in to reverse. I'm actually considering investing in a deep pan over my low profile one because of this.

But it sounds like you may have found your problem with that fuel rail pressure code, shifting and fuel rail pressure go hand in hand. The TCM needs a specific torque to engage the next gear and if it isn't reaching that torque value because of fuel rail pressure that's where you get the spaced out shifts.

If you go for the aluminum valve remember to shorten the steel handles, you'll need vice grips at that point to tighten it, and it doesn't hurt to go a little past hand tight since this is aluminum, and tighten the plug itself using the square part above the drain valve, not the 12 pointed part (unless you have the special OEM filter wrench).
What ? There will be no issues with only being down a 1/4 of a quart. While a deep pan is a good investment it will not fix someone not correctly reading the dipstick.
 

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What ? There will be no issues with only being down a 1/4 of a quart. While a deep pan is a good investment it will not fix someone not correctly reading the dipstick.
Are you speaking from experience? I am.

Each time my stripped pan bolt begins to leak, I'll lose ~ 1/4 of a quart, give or take and get a hard shift from 1st to 2nd, and from drive to reverse which triggers max line pressure and limp mode.

If I don't add any fluid and start it up the next day it does the same thing. I add 1/4 of a quart after adjusting my clamps and seeing a tiny puddle below my vehicle and it is fixed.

A deeper pan would hold more fluid and prevent this from being such a common issue.
 

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Are you speaking from experience? I am.

Each time my stripped pan bolt begins to leak, I'll lose ~ 1/4 of a quart, give or take and get a hard shift from 1st to 2nd, and from drive to reverse which triggers max line pressure and limp mode.

If I don't add any fluid and start it up the next day it does the same thing. I add 1/4 of a quart after adjusting my clamps and seeing a tiny puddle below my vehicle and it is fixed.

A deeper pan would hold more fluid and prevent this from being such a common issue.
I have 1+ million miles with 4L80 transmissions. Sounds like you have several issues that need to addressed. 1/4 of a quart low from full will not cause issues. A deeper pan will not help correct the fluid level. The pickup does not get longer when you install a deep pan, the extra fluid will be below it.
 

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.... The pickup does not get longer when you install a deep pan, the extra fluid will be below it.
Any deep 4L80 pan I bought came with required parts to lower the filter to the bottom of the deep pan. Last 3 4L80/5's I did were B&M pans. Can't remember what brand is on the 93 but it had one.
 

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Any deep 4L80 pan I bought came with required parts to lower the filter to the bottom of the deep pan. Last 3 4L80/5's I did were B&M pans. Can't remember what brand is on the 93 but it had one.
A quality pan like Mag-Hytec keeps the pickup at stock height.
 
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