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forman

1991 Chevrolet C1500 Automatic Transmission Advice Needed

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Hello Moses!  I bought a truck from a relative for my son. I also purchased your book Chevrolet & GMC Light [Duty] Truck Owner's Bible that I am having sent to him, so I won't get to read it until I visit again. I should start by saying that I delivered the truck to him and will be visiting for a few more days before returning home.

The truck has 171,000 miles and has been maintained fairly well aside from a slight leak in the radiator and a bad signal from the coolant temperature sensor, an oil pressure gauge that seems to read high, and a sticky brake light switch are going to be fun to repair with my son.

The transmission, however, has been neglected and seems ok now but I need your advice on the best way to approach "preserving" it for a time when we can either rebuild it our selves or save up and have the work done by a professional.

As I mentioned it seems ok now but occasionally seems to have a hard shift while shifting down. The trans fluid is more brown than red and I am getting ready to change it and the filter and if possible the fluid in the torque converter if you think that needs to be done.

I think I was able to read the letters MD8 on the bell housing. Could this transmission be a 4L60? Also could you give us someway to test the function of the transmission so we would know how to assess any problems?

Looking forward to hearing from you!

Jeff

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Hi, Jeff!  Best at the Holiday Season, good to get your post...The 1991 Chevrolet C1500 automatic transmission would be a 4L60, not the 4L60E electronically controlled unit phased into production in 1993.  Your code is a 4L60 (GM RPO MD8), essentially an evolved 700R4.  You will find my comments and details on the 700R4 in the book, which will be applicable to your 4L60 transmission.  In this iteration, the transmission is refined and updated for reliability. 

 

I trust the book will be a good read for your son and furnish useful insights.  A factory workshop manual would be advised for major unit work, used books are out there in both complete editions and also as "Unit" books that detail the transmission and other unit repairs.  I can help with any advice, schematics, etc.  My library is extensive on all GM automatic transmissions back to 1941!

 

I have rebuilt the 700R4 and equivalent transmissions.  They have a lot of components and must-follow assembly sequence concerns.  If you decide to tackle the 4L60 as a father/son project, I believe you will be successful as in your other recent projects. 

 

One "specialty" tool required is a universal AT clutch spring compressor available through K-D and others.  You can improvise beyond this tool...The rebuild kits are not expensive, and hard parts damage can be avoided if the rebuild takes place early enough in the wear cycle.  For those who have rebuilt an automatic transmission before, this is an "accessible" though very busy transmission.

 

As for a "health check", I like to run proscribed pressure tests to make sure circuits are intact and functioning with a full flow of pressure.  Gear applies and other direct results are helpful.  There are a variety of air pressure checks for that approach. 

 

If the 4L60 is not slipping or showing signs of a torque converter issue, the complete unit and converter flush and fill with a new filter can be restorative.  With the coloration of fluid, I would do the converter flush along with the unit fluid change plus add a new filter.  Use GM recommended fluid, do not get creative at this mileage with exotic or expensive fluid.  Make sure the transmission is fully functional before upgrading anything.

 

Check linkages and the signal switches for kickdown, as these boost pressure and can sometimes create issues like the harsh downshift you describe.  Harsh shifting can also be sticky or maladjusted pressure switch signals, line pressure too high or pressure regulator issues.

 

I can be more specific once you've done preliminary cleanup of the unit.  If that does not "cure" the maladies, share specifics about when the unit shifts poorly or malfunctions on any level.  We can troubleshoot from there.  Or, if you'd rather plunge into diagnostics before spending cash on flushing and a filter, we can address drivability and shifting symptoms now...Your call, Jeff.

 

Moses

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We will get the fluid and filter changed tomorrow. I think the transmission will respond well. I found myself thinking about how a 1991 automatic transmission is supposed to feel like and how can I relate what I'm feeling and hearing?

The tranny has some age on it and I am used to newer hi tech components that shift smoothly. Maybe you could tell me at what speed I should shift from first to second and so on. Also could you describe what a hard shift or a slipping shift is in your words so we can be on the same page when I describe what happens tomorrow.

thanks

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forman...Jeff, the 700R4 and 4L60 have an extra low first gear ratio.  These transmissions were developed to offset ridiculously tall (numerically low) axle ratios in emissions cars and light trucks.  In the truck applications with reasonably low (numerically higher) gearing and stock diameter tires, the upshift to 2nd is noticeable, feeling much like the older THM350 or 400 transmissions. 

 

Still, the shifts should be smooth with this unit unless you have your foot in the throttle.  That will hold back (detent) the shift, causing a harder upshift due to the increased (throttle) pressure.  To provide some insight, here are the gear ratios for the 700R4 and 4L60:

  • First - 3.059:1
  • Second - 1.625:1
  • Third - 1.000:1
  • Fourth - 0.696:1
  • Reverse - 2.294:1

Upshift road speed would be governed by the axle ratio, though you can tell from engine sounds whether the transmission is hesitating or holding in a lower gear.  You'll sense the difference.  As for slipping, this is again a noticeable engine revving sound that makes you instinctively back out of the throttle!  Shifts should be distinct, though like the later transmissions you mention, there is typically a softer shift feel. 

 

Softer does not mean lag or slipping, though.  At light throttle, there will be no harshness but rather a quick and smooth upshift to the next gear.  Lag is caused by either holding in the lower gear or slip.  Holding in a lower gear results from throttle pressure issues, incorrect kickdown signals or a valve body malfunction. 

 

Overall, the 700R4 should not feel much different than a THM350 or THM400, although the ratios are different.  Again, unless the truck has a taller axle ratio, you will notice the lower first gear ratio.  You might check the axle ratio tag on the axle and also note the tire size/diameter.

 

Are you flushing the converter with that fluid and filter change?  Is this being done at a commercial shop with a flushing machine?  Ideally, before flushing, you want to check the content in the transmission pan for any metal, nylon, friction material or other signs of wear.  Some of your great pictures would be worth viewing...

 

Moses

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We replaced the filter and fluid yesterday and the test ride was the kind you like to take after improvements to you vehicle ...very noticeable changes resulting in a NICE ride. The debris in the pan was very small and soft and difficult for me to identify, pictures will come later as I'm away from home. Moses thank you for your detailed answers I always learn in a post from you.

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Jeff...From what you shared about the color of the fluid, the transmission sounded due for a fluid change and filter.  You're good at assessing a vehicle, this one seems like "good mileage"... great father/son project and place to start!

 

Glad nothing ominous turned up in the transmission oil pan.  This should be an accurate read, the fluid had been in there quite a while...You'll go over the rest of the truck...

 

Moses

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My son is serving in the Navy, currently in Charleston SC recently he had some bad luck and I found a '91 c 1500 Chevy for him to drive.  The only problem was that the truck was 1150 miles away in Texas.  My wife and I had limited time to travel and bought the truck and started on a very long test drive... Honestly I took almost every tool I owned in anticipation for a break down of some type but was pleasantly surprised to travel the entire distance trouble free.  The transmission was one of the most important potential problems as it had some shifting issues that I mentioned previously.

 

I forgot to get a picture of us after spending an afternoon working on the truck and the next day it was raining, but I did get a photo of the oil pan and sediment.

 

post-140-0-96050400-1419215191_thumb.jpg

 

I really think this transmission had been serviced at least once in its life.

 

post-140-0-74672300-1419215219_thumb.jpg

 

I'm back home now and my son is enjoying the truck I'm sure you will hear from us again.

 

Jeff

 

 

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