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Transmission issues, read 4L80e build thread, now what? #5168327 07/22/19 05:27 PM
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Roadtripp Offline OP
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I’m a almost Newb. I’m enjoying this forum. A wealth of good information. Thank you for interesting in-depth 4L80e transmission build thread. I went through all 18 pages but got confused. It’s a lot to digest. I’d like to build my first transmission and my current unit may be the one. I don’t have the budget to build like the one in the thread. I’ve done basic engines ring and rods, carburetor rebuild kits, etc, with success. Alternatively, if this 4L80e can be driven as-is for the foreseeable future that might be best as I have other demands on my time and money. But I’ll happily do a basic rebuild it if necessary. If it’s major then maybe getting a different transmission is best?. So I’ll describe as best I can what it’s symptoms are:
It’s a 1995 Chevrolet Chevy Van G30 with 117,000 miles. I’m fixing it up into a budget camping and expedition van. Needs to be reliable as possible as will be in remote locations some of the time. It sat for about 1.5 years before I got it. I don’t think the fluid has been changed but did not smell burned. Did a lot of stop and go driving as it was a Seattle short bus.
When I got it it was slightly low fluid level and slipping a little then engaging. Can that do harm to it? It seemed ok when I added fluid. After driving about 200 miles back to my location it suddenly dropped out of overdrive and coasted then engaged again but at a higher rpm. I assumed it was in 3rd but then realized it was probably 2nd. I dropped the pan and the reddish brown fluid. A little black sediment in pan. No chunks of anything. Replaced filter added Maxlife fluid and test drove hoping it was a plugged filter and dirty fluid. It did the same thing over again. Perhaps “limp” mode. After some research I learned how to count the flashing CEL. Got 68, 74, and 87 codes. I replaced the input and output speed sensors as they were inexpensive and easy. To no effect that I could tell. No joy.
I then tried cleaning and inspecting external wiring. No joy.
Then someone mentioned turning the ignition switch on and off several times. It drove very normally again for a few miles then drop out of overdrive again. After cycling ignition switch again it drove normal then dropped out again. Next I replaced the valvebody with one from Shift Rite and added more Maxlife fluid and filter. The valve body was an tricky job upside down dripping in my mouth and I think I torqued the valve body correctly. I used a small amount of Vaseline to hold the chrome balls in place. I’ve driven about 10 miles test driving. It dropped into second one time but now seems normal again. I wishfully attributed this to computer relearning? I hope there is such a thing!?
Hmmm, now I don’t have a lot of trust in this transmission. Might it be on its way out?! Today’s 5 mile test drive seems ok. Got up to 60 mph.
There is a slight whine in 1st gear, almost no whine in 2nd, and silent in high gears. I see from my reading it’s not a early, but a ‘94-‘96 version. For a basic rebuild which kit, torque converter, what specific improvements are recommended and what signs should I look for inside it??? I have not bought the service manual for the transmission yet but plan to. Or should I buy a different transmission, rebuild it, and swap it in. What about the computer??
I’d like to add a better cooler and a temperature gauge as I plan on towing but only a small cargo trailer with 1500-2000 lbs gross weight . I’m generally very easy on my rigs. I’d rather pull over and park in the shade when it’s hot. This is the first transmission issue I’ve had in 40 years.
Oh do I need the special tools like I saw in the Build thread?? That’s amazing he has a machine shop and can craft such nice specialty tools. Perhaps someone rents out the pullers and what not. Sorry for the rambling post with too many questions.

Re: Transmission issues, read 4L80e build thread, now what? [Re: Roadtripp] #5168414 07/22/19 08:12 PM
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clinebarger Offline
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Your "EPROM" PCM doesn't learn or have any adaptive ability of transmission functions. It's purely Inputs & Outputs.

This sounds electrical to me......You can't take the DTC's at their face value with old OBD 1 controllers.

DTC 74 intrigues me.....For one, DTC 68 will not run with 74 hard set. Second....Speed Sensors on this unit do not fail very often.

68,74,87 ALL center around Speed Sensor differential as means to calculate these particular faults.

You need good scan data to prove that this isn't an electrical issue.
*Does the Speedometer act-up at all?, Though your G-series uses a VSS Buffer Module & can send proper signals to the Speedo Head & false signals to the PCM.
*It's best to use 3rd gear (TCC Locked) as all 3 (Engine RPM/ISS/OSS) should read exactly the same.....Complete drop-out of ISS or OSS speed readings indicates a wiring issue in their respective circuits.

*Keep in mind that the PCM is very primitive & can only run a couple functional/sanity test at any one time......Those 3 codes could be caused by the same issue (ISS drop-out for example)

There aren't many good OBD 1 scan tools anymore, I use a GM Tech 2 or a Snap-on MT2500 "Brick".

A cold/cracked solder joint on the PCM board can cause intermittent issues like this.....Same goes for the VSS Buffer Module if drop-out is noticed on the OSS signal.


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
Re: Transmission issues, read 4L80e build thread, now what? [Re: Roadtripp] #5168531 07/22/19 10:53 PM
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LDM Offline
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A good scan tool for OBD1 was the TTS Powersystems software. It was discontinued but picked up by Tunercat and he is now developing it under the DataCat data logging program. I've used TTS for years on my 94 Z28, DataCat has some newer updates for functionality. Looks a bit differently but you can choose all the different parameters you want to look at in the program now. I've been using Tunercats OBD1 tuning since the late 90s, its one of the best GM OBD1 tuning options available. You probably don't need to do your own tuning though so you should just need data logging and ALDL interface.

www.tunercat.com is the website. He also sells the OBD1 ALDL cables you will need for your datalogging as well as the program. It can definitely help with diagnostics.

I'm not really as good with the automatics as my 94 Z28 is a 6 speed T-56 manual and my 89 IROCZ has the 700R4. No electronic controls for the transmission on those cars. The 95 does use the VSS buffer module so you will want to check that to make sure it isn't causing an issue. Speed sensors generally don't fail but it is possible. Checking all the wiring for damage/corrosion is also a good idea, although since you live in Washington state you problem don't have the issues with all the salt they put the roads like they do around where I live.


2015 GMC Sierra 1500 5.3
1994 Chevy Camaro Z28 5.7
1989 Chevy Camaro IROCZ28 5.7
Re: Transmission issues, read 4L80e build thread, now what? [Re: Roadtripp] #5168847 07/23/19 11:36 AM
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Roadtripp Offline OP
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Was not aware of the VSS buffer module or options for OB1 diagnostics. The Tuner Cat software at $69 is very affordable. All good items in my toolbox. Ive been wondering what to carry along.
A cracked PCM, might that be repairable, would I need a magnifying glass to see it?? I’ve read changing modules can lead to more issues. I remember seeing a GM technical bulletin document saying something about the EPROM but could not decipher the technojargon with my layman knowledge.
This mornings 5 mile test drive went normal. Phew!! Intermittent events are a worry. Will check for codes again and get back to the forum. I really appreciate the suggestions.

Re: Transmission issues, read 4L80e build thread, now what? [Re: Roadtripp] #5168854 07/23/19 11:46 AM
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bullwinkle Offline
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Not sure about your gas application, but a bad TPS can cause weirdness, it did on the 6.2 OBD1 4L80E in my sig. Limp mode, whiplash inducing upshifts, no TCC nor OD, and burnt up fluid all resulted.


06 Ram 3500 CTD 4X4(FG Venturi), 93 GMC C3500 6.2, 89 F-450 7.3, 98 XJ 4.0(XG8A), 05 xB(XG3600), 18 Transit 3.7, 03 Merc Grand Marquis 4.6 2V(XG2)
Re: Transmission issues, read 4L80e build thread, now what? [Re: bullwinkle] #5169009 07/23/19 03:14 PM
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Roadtripp Offline OP
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Thanks, I’ll look up the TPS. Not a difficult or expensive fix. Is it adjustable at all or plug and play?
I’ve read the brake light switch is wired into the transmission somehow. I’m still learning the technojargon and how a automatic works but I assume it unlocks whatever is locked in the transmission if the brakes are applied?
I think I’ll swap out more old fluid for Maxlife ATF. It seems to run better every time I swap some.
Cline: no I have not seen the speedo do anything unusual. Very steady. Thanks

Re: Transmission issues, read 4L80e build thread, now what? [Re: Roadtripp] #5169302 07/23/19 09:38 PM
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Thanks for your patience with my Newb thinking. Is it a rule of thumb that intermittent (erratic) electrical issues are often bad grounds? Or water getting in a sensor connection then taking a while to dry out?. I’ve been informed it was a cardinal sin to pressure wash the engine. My bad, I just couldn’t take the squirrel poop on the engine. Maybe the TPS was compromised and now dried out?
Cline, I’ve been going over what you wrote wrapping my head around it. If it boils down to one of the ISS or OSS circuits.
How do all the various sensors connected to the PCM get the power?. For example the TPS (throttle position sensor). What signal does it get? Would a digital multimeter tell me anything on a sensor?
When I changed drivers seat in this short bus I located the PCM and cleaned all the dust and crud off it. It looks ok but how on Earth does one tell if it’s functioning properly. I’m confused what “good scan data” is. I dont know anyone with a GM Tech 2 or Snap-on MT 2500. Only once when I briefly owned a VW TDI I witnessed a aircraft mechanic connect a laptop to it and find what was the issue and repair it all in one hour. It was an amazing time saver in the right hands. I forget what software he was using for VW but I hope the Tuner Cat software has that potential.
The squirrel poop episode reminds me of a story about a girl I dated. She had returned to Montana from a Summer in Europe and started up the Dodge pickup her ex gave her. She got a few miles down the road when it burst into flame. A huge rat nest was in the engine compartment. A guy happened to be loading dirt with a tractor nearby. He dumped a load of dirt on the engine compartment and put it out quickly enough that the truck was saved. But what a mess!! I’ve found several nests inside the air cleaners of vehicles so always check.

Re: Transmission issues, read 4L80e build thread, now what? [Re: Roadtripp] #5169385 07/24/19 12:39 AM
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clinebarger Offline
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The ISS & OSS sensors create their own power.....A/C Signal Generators.


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
Re: Transmission issues, read 4L80e build thread, now what? [Re: Roadtripp] #5171528 07/26/19 05:17 PM
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Roadtripp Offline OP
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Ok, thanks Cline. The intermittent transmission is driving ok so far. Maybe water evaporated from a connector. Hope it lasts.

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