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1994 GMC 2500 5.7l Questions

rlb

rlb

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After about a month of looking for a truck/suv for light towing and hunting duty..

I ended up with a '94 GMC Suburban 2500. 4x4 with a 350. 250,000 miles, transmission has 75k on it. All the maintenance was recorded and everything is up to date. No leaks I could find, drives well. Truck is clean and rust free. Good tires, heat and AC work. I negotiated a deal for $1800, picking it up this evening

Looking for some info on the truck. What transmission, axles are on it? (hoping its got the 80 series transmission) Its 4x4 with the shifter on the floor. I believe its got 2 hi, 4 hi and 4 lo. What are the procedures to shift? Is the center diff open when in 4 hi? Locked in 4 lo?

I've owned several jeeps but this is my first GMC/Chevy. Couple of small issues i'll need to fix. The thermostat is stuck open and will need to be replaced, and one of the side mirrors is busted so i'll need to fix that. The AC/Heat blend doors make a bunch of racket ( loud ticking noise in certain positions) and i'll need to get after that eventually. Steering is sloppy, box will need to be adjusted.

Thanks in advance.
 

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With the HVAC you will want to remove the glovebox and check for a 2 wire connector with red and black wires that powers the HVAC. it's known to melt. Next check all your grounds including the one by the trans dipstick. Adding a large ground strap from the frame to the engine and body from the engine always seams to help.

Ticking doors usually mean the door motors have failed. They have a position sensor in them that wears out. But, check/add the grounds first.
 
WarWagon said:
With the HVAC you will want to remove the glovebox and check for a 2 wire connector with red and black wires that powers the HVAC. it's known to melt. Next check all your grounds including the one by the trans dipstick. Adding a large ground strap from the frame to the engine and body from the engine always seams to help.

Ticking doors usually mean the door motors have failed. They have a position sensor in them that wears out. But, check/add the grounds first.
Click to expand...

Thanks. So the doors do appear to work, they quit ticking when I turned the heat on and cranked up the blower motor. The heat worked, however it never got more than a little bit warm because the t-stat is stuck open and the coolant temp never got over 150 degrees. The doors tick before the heat was turned on, and they went back to ticking (for about 15 seconds) after I shut the engine off and pulled the key from the ignition. Still think its a melted connection or bad ground? Sounds like the door motors to me.
 
I have a 94 K2500 pickup. I'm assuming the 4X4 will be the same since mine is also the floor shift, not the electronic shift. The TC is an open only unit. You can shift on the fly from 2hi to 4hi, and back. With the auto tranny you need to be in park, at minimum, to shift into 4lo as the input shaft into the TC needs to quit turning when doing the shift, otherwise it tends to grind.

The front axle will automatically lock in electrically when you shift into 4 wheel drive. There is a heater module on the axle that activates the shifting, so the locking action will usually take a minute or so to actuate. I have heard of a few owners, usually those who do snowplowing, who have had the actuator disconnect the front axle when ice or snow builds up around it and cools it off.

I don't know what year the change took place, but later year models have a motorized axle locking mechanism which is more definite in the locking action than the early ones. There are two different fixes for the actuator concerns/problems. One is to update to the newer motorized actuator. The other is to go to an aftermarket mechanical actuator called a Posi-Lok. There is a manual knob, installed under the dash, which pulls/pushes the axle actuator by way of a cable connection. The posi-lok is what I have done.

The brakes and axles on the Burbs were usually way different than the comparable weight capacity pickups. I've seen both 8 lug semi-float axles, and 8 lug full float axles under Burbs in wrecking yards. Being a 2500 you most likely will have the 3.5 by 13 inch brakes, as found on the one ton pickups, although it is possible to have the 2.5 by 13 inch shoes. I have also heard of, but not seen, 2.5 by 13 inch shoes with the 3.5 inch brake drum. Because of the several possibilities here it would be best for you to pull a rear wheel to visually find what you have.

I'm going to have to guess on the transmission. If the motor was a 6.5TD the tranny definitely would be the 4L80E. Since it is the 5.7 and 2500 I'm thinking it will also be the 4L80E, but you are going to have to crawl under for a visual inspection.

Don
 
Thank you both for the information.

On the front axle, by locking - do you mean the differential will lock?

Any telltale signs for determining the transmission type. A data plate somewhere?

I'll post some more pics tomorrow when I pick the truck up.
 
Most likely you have a 9.5" 14 bolt semi floater rear axle, NP241C transfer case(it is locked anytime it is in 4X4 meaning the front and rear wheels are locked together directly via a chain, no diff in it), and a 4L60E trans. GM didn't start putting many 4L80E's behind the 350 in 2500's until 96 when the VORTECS came out, up till 95 most 2500's got the 4L60E trans and 3500's pretty much all got the 4L80E trans.
 
Thanks. Good info on the transfer case. So from what I understand, the front and rear are locked together in any 4x4 mode and the front diff locks as well.

I'm also interested in adding a transmission temperature gauge. I don't plan on doing a lot of heavy towing, but that's something I like to put on all of my vehicles. I might add an external cooler if it does not already have one. Any suggestions on the gauge location and placement would be appreciated. I'll search for a write up on the topic.

Also, what reputation does the 4L60E have in terms of reliability. My uncle had pretty bad luck with transmissions on his 2004 suburban, don't know if its the same transmission or not.
 
I have no mechanical knowledge to share, as these guys covered everything you could want to know (and more), but I wanted to say that I love those stock hubcaps. Sounds like you got a great deal on the truck.
 
rlb said:
Thanks. So the doors do appear to work, they quit ticking when I turned the heat on and cranked up the blower motor. The heat worked, however it never got more than a little bit warm because the t-stat is stuck open and the coolant temp never got over 150 degrees. The doors tick before the heat was turned on, and they went back to ticking (for about 15 seconds) after I shut the engine off and pulled the key from the ignition. Still think its a melted connection or bad ground? Sounds like the door motors to me.
Click to expand...

More than likely it is the motors. But, the grounds on these trucks will give you all kinds of problems and it is possible for the grounds to be your issue. I have fixed a few odd things on my truck with grounds and the experience on here is the same for others. The grounds include the 2 wire HVAC connector behind the glovebox melting down. Again the grounds are easy, cheap, and usually help something. I have taken to adding a ground strap from the blower motor to the body the reduce the load on the 2 wire connector. All vehicles have their specific issues and this body style is grounds and that HVAC conector being trouble areas.

If it was mine I would be pulling the 350 and dropping a 454/502 big block in with the 4L80E. The 350 has to work too hard in a 3/4 ton. You already know the story on that trans - Don't ask too much of it and don't tow in OD. Nice rig even with a 350 as they were like McDonald's cheeseburgers with millions of them made, simple, and run a long time.
 
When they say lock, they mean like locking the hubs to engage 4x4. Not a locking differential.

Check your glovebox for code G80, that would mean you have a locking rear differential.

Would be identical to the transmission in your uncle's 04 Suburban if it was the 4L60E. Find the code in the glovebox for that as well, it generally starts with M, then google it. Or you can look at the pan and compare it to what you see online. We had to put 2 in our 02 over 170k, one under warranty. They can be upgraded when rebuilt. Heat really is a killer though. Easiest way to add the trans temp gauge is put it right in the pan. Then add an auxiliary cooler after the cooler in the radiator. That will extend the life a lot
 
WarWagon said:
More than likely it is the motors. But, the grounds on these trucks will give you all kinds of problems and it is possible for the grounds to be your issue. I have fixed a few odd things on my truck with grounds and the experience on here is the same for others. The grounds include the 2 wire HVAC connector behind the glovebox melting down. Again the grounds are easy, cheap, and usually help something. I have taken to adding a ground strap from the blower motor to the body the reduce the load on the 2 wire connector. All vehicles have their specific issues and this body style is grounds and that HVAC conector being trouble areas.

If it was mine I would be pulling the 350 and dropping a 454/502 big block in with the 4L80E. The 350 has to work too hard in a 3/4 ton. You already know the story on the story on that trans - Don't ask too much of it and don't tow in OD. Nice rig even with a 350 as they were like McDonald's cheeseburgers with millions of them made, simple, and run a long time.
Click to expand...

A mildly built 350 with swirl port heads can hold it's own against a stock 6.5, so I wouldn't just write it off. And the 4L60E can be a good trans, it just hates heat. I towed my 01 donor truck home with a 93 light duty 2500 CHEVY truck with a 350/4L60E combo, and I didn't feel like it was underpowered drastically(but it was a mildly built 350). If built right both the engine and trans can last a LONG time, AND tow good while passing gas stations(something the 454 aint gonna do).
 
DieselCash said:
This is one of the black boxes that could be making the clicking noises. I had the same problem and made a write up of it.

http://www.thetruckstop.us/forum/showthread.php?33272-Recirculation-Actuator

Wish you luck, it is not easy to get too. :thumbsup:
Click to expand...

I looked in that thread and your description of the problem sounds exactly like what I am experiencing with this one.

Thanks for the compliment on the truck, I like those hubcaps too. You'll see that the passenger side front hubcap has a dent on it. Found one on ebay for $40 will be replacing that... The 350 is probably a good choice for me. The most I plan to tow with this truck on a regular basis is about 1500 lbs.

I will be ordering a transmission temperature gauge and an external cooler once I pick the truck up tonight and get a look at the fittings + measure the amount of space I have available to mount a cooler.
 
picked it up this afternoon

After a long stint on the highway, I pulled off and was sitting at a stoplight. Oil pressure was pretty low, almost in the red when at idle. The check gauges light was flickering on and off. Came back when I gave it a little gas. I never got it good and hot when I test drove it so I didn't notice.

Oil level is fine. I stopped and bought an oil pressure sensor and a bottle of Lucas oil stabilizer. Looking at the truck in my driveway, it was obvious the oil pressure sensor was recently replaced. So I added half of the bottle of Lucas oil. Oil pressure is much better now, about 20psi at idle and 40 or just above when driving.
 
Sorry, I should have been more clear with my post. By locking the front I meant the same as locking in the hubs on the older axles, not a diff lock. These front diffs, in stock configuration, are open gears only. Since these rigs don't have locking hubs the terminology doesn't really transfer over from the solid axles.

If you slide under your Burb and look at the front diff you'll see it is totally different than what you are used to. The diff is bolted to the frame cross member with open axle shafts out to the steering hub, just like a front wheel drive car. The front axle locking mechanism (does a similar job as the solid axle locking hubs) is inboard, and actually in the diff housing right where the axle enters the diff housing.

Now, about your oil pressure problem. I'm not familiar with the gasser oil pressure sensors like I am with the ones for the 6.5 TD motor. My personal experience is that only the AC Delco brand oil pressure sensor will give accurate readings. The sensor for the 6.5 TD has two circuits in it. My oil pressure circuit was still working properly, but the second circuit had quit working so I replaced the sensor with a non AC Delco unit. My oil pressure gauge readings immediately went down by about ten pounds. Within a few weeks I again replaced the sensor, but this time with an AC Delco unit. My gauge readings went back up to where they had been before. Others on the 6.5 forum have had similar results with aftermarket oil pressure sensors.

I used to tow a 17' travel trailer with a 97 Astro van. That van had a 4L60E tranny. I still have that van, though I no longer tow with it. During the several years I towed with that van I never had any tranny problems. I still have that van, at over 240K miles, and it is still on the original tranny. So, with your expected towing weight, I don't think you'll have any problems.

Don
 
Yikes...

GM makes a nice 383 stroker to replace the stock engine in these...

One thing to make note of actually: I've heard some of the aftermarket OPS's aren't super accurate. Might not be a bad idea to take the one you got back and find the best deal on an ACDelco and/or Delphi (Check amazon, Rock Auto, etc) and swap that in.
 
Well the TBI trucks use the same POS OPS as the 6.5, and it also powers the fuel pump just like the 6.5's. With as many miles as it has though, theres a good chance it needs rings and bearings. Get a good actual gauge on it and see what the oil pressure actually is on it. I would want to see at least 8-10 PSI at a HOT idle, and about 25-35 running down the road.
 
Thanks for the tip on the AC Delco pressure sensor. As I mentioned earlier I added some Lucas Oil Stabilizer, half a quart. Its now showing 15psi at a hot idle, running about 40psi on the highway.

Soon i'll change the oil and put a mechanical gauge on there to see what its actually running at. I think my uncle has one.... Will let ya know how that goes. I prefer not to run additives in the engine or transmission as a band aid fix. However this motor has good power, runs smooth, no leaks and does not smoke or appear to burn any oil. If Lucas oil allows me to get another 50,000 miles out of it i'm cool with that.

Right now i'm searching for a pre-heater hose to go from the exhaust manifold to the air intake. Thought they ditched these when carburetors became a thing of the past. The guy who did the state inspection this morning pointed out it was missing. Only thing keeping it from passing.. d'oh

I haven't had a chance to look for the transmission type yet. Too busy last night fixing a side mirror and doing other minor things to get it ready for inspection. Right now my priority is getting it inspected, putting in a new thermostat... then moving on to getting rid of the steering slop, put some new shocks on it, and add a transmission cooler and temperature sensor.
 
If the oil pressure is a bit low, I recommend doing your oil changes with a good quality 10W30 oil and a bottle of STP oil treatment. The STP stuff is a cheap additive that will help to boost the oils viscosity when hot. And I have personally seen Castrol make 8-10 PSI more idle oil pressure on a worn engine than the generic store brand stuff. Had a 97 JEEP that kept popping the check gauges light as oil pressure was dropping below 7, changed to 10W30 Castrol and a bottle of STP and it help 18-20 for idle oil pressure for another 60K miles until I scrapped it.
 
Thanks.. yeah my last 3 jeeps have gone the way of the scrapper, all with over 200,000 miles. My other car right now is a 2004 Acura MDX with 210,000 miles. I feel like I am running an old folk's home for cars and trucks. Some TLC, a couple good years and then off to the scrapper.

My passenger side mirror is cracked. If you look at the pic in my original post you can see what style they are. Below is a link to a mirror that's listed on rock auto for my year/make/model. Will these bolt in place of the side mirrors I have? They are cheaper and I like the classic look. I was thinking about ordering a pair and replacing both side mirrors, instead of just the broken one.

http://www.rockauto.com/catalog/moreinfo.php?pk=972418&cc=1302411

And thanks a ton to everyone who has replied, you all are a big help!
 
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