Needed a diff, coming home with a 90 R20

[Jsfucttruck]

I have been on the hunt for a specific rear diff. Looks like it is coming home with the rest of the truck attached.
1990 GMC R20 with a tbi454/4l80e 3.73 14bff with a g80 Trac loc. It is pretty heavily optioned. Silverado package, ac f/r, power doors, deep tint, barn doors, tow package, and a few others. I think it is just missing a two-tone paint option and rear heat.
According to the seller it ran and drove a yr ago when parked.


We shall see tomorrow when I go to pick it up.

This will be my first 2wd that I possibly plan on keeping.

[Jsfucttruck]

I also have the option to get this 90v2500. I will know more about it tomorrow.
This one was towed in. Seller never tried to start. Both of these were to be donor trucks for projects .

[68Timber]

Sweet. I used to see C20 Burb's on CL pretty regularly but they've been scarce lately. Save 'em both! lol

[Jsfucttruck]

Originally Posted by 68Timber

Sweet. I used to see C20 Burb's on CL pretty regularly but they've been scarce lately. Save 'em both! lol

I don't see BBC tbi burbs that often, they never offered a 4x4 tbi 454 burb from the factory. This one came up at the right $$$$.
I was literally looking at it as a parts burb. I can't do it.

The other one has the dash out of it. Doesn't run. Has some hoky looking open air filter setup and the tbi unit looks like dog sh*%.



I went today and rode with a bud with a truck/trailer to pick up my new addition. Of course I forgot all my tools, but not a new 1000 cca battery. Good thing too. It had no battery. After a quick inspection and battery installation, I hoped in. All the lights/gauges came on!
No crank. It seemed to get juice to all the right places. I crawled under the burb with a screwdriver and attempted to jump the terminals. To my suprise, a Ford starter relay/solenoid was mounted to the engine crossmember. Stock solenoid on the starter was still there. I crossed the connections on the Ford solenoid. Boom! Running.

It runs pretty rough, there is no check engine light on while running. The fuel could be up to 4yrs old. It looks to have a new catalytic converter on it. Abs light is on, brakes go to the floor.

I am going to swap out the old fuel and toss a tune up at it. The tires are pretty cracked so I will swap a stock steel rim set on with some okay street tires. Does anyone know where to get the stock gmc hub caps? I only have Bowtie ones.

I do have a billet grille for this Burb. I have been sitting on it for yrs. They are a little bit different than the Bowtie version.

[Jsfucttruck]

Here is the Gmc blank, I guess it's not what I remember. I had it on our 91 Chev. It didn't quite fit right so I took it off before I was going to break it.

[Jsfucttruck]

So the guy I got it from had a few other trucks for sale.
He has a 82 c10 with a 6.2/700r4. Short box, 4" -6" drop.
A 73-75 c20 with a factory BBC. I wish I took a pic of it. It has fabtech tube front upper and lower a arms, lifted spindles and 35"s. It has changed the direction of this possible build.

He had a similar setup as in these pics. There pretty much is a complete parts list on what to use, if you have a 1\2 ton. I have a 3/4.

It seems the reason for this is that no one was producing lifted spindles for the higher gvw 2wd 88-98 trucks and burbs with 8 lugs.

Well I found a company that makes a 4.5" lifted spindle for the 8 lug version!
I am for sure looking into this lift!

[Ski-me]

Isn't 91 the only year of the 4l80e tranny? 2wd and/or 4wd? I didn't think it was available in 89-90....just TH400.

[Jsfucttruck]

Originally Posted by Ski-me

Isn't 91 the only year of the 4l80e tranny? 2wd and/or 4wd? I didn't think it was available in 89-90....just TH400.

I thought the same.
It has a 1990 production date, and the build code calls for it.

I am looking at buying a 91 V2500 for parts, that has a th400, I thought all the 3/4 tons in 91 had 4L80e's!

[Jsfucttruck]

Current line up...

[Ski-me]

Originally Posted by Jsfucttruck

I thought the same.
It has a 1990 production date, and the build code calls for it.

I am looking at buying a 91 V2500 for parts, that has a th400, I thought all the 3/4 tons in 91 had 4L80e's!

Unless someone swapped it out.....

By factory, I'd expect to see all 3/4 ton suburbans with a 3 speed shifter. In 91 I'd expect to see the "OD" 4th gear on selector (4l80e).

[68Timber]

The 4L80E could have been an option with TH400 standard issue like the 700R4 was to the TH350 back then.

[Irish1941]

Jeff is correct!
1991 was the first yr for the 4L80 and it was only available in 2500 with 2wd & 4wd. TH400 was no longer offered.
700R4 was for 1/2t trucks.
Odd side note, the 4spd manual was no longer an option in Suburban for '91.
The 1 ton V3500 pickup still got TH400 though.
Have a read...tells you more than you ever wanted to know about '91 Subs...
https://www.gmheritagecenter.com/doc...t-Suburban.pdf

And is link for '90. You can compare what GM dropped off the option sheet.
https://www.gmheritagecenter.com/doc...t-Suburban.pdf

[Jsfucttruck]

Originally Posted by Irish1941

Jeff is correct!
1991 was the first yr for the 4L80 and it was only available in 2500 with 2wd & 4wd. TH400 was no longer offered.
700R4 was for 1/2t trucks.
Odd side note, the 4spd manual was no longer an option in Suburban for '91.
The 1 ton V3500 pickup still got TH400 though.
Have a read...tells you more than you ever wanted to know about '91 Subs...
https://www.gmheritagecenter.com/doc...t-Suburban.pdf

And is link for '90. You can compare what GM dropped off the option sheet.
https://www.gmheritagecenter.com/doc...t-Suburban.pdf

Aaahhhggg! I am so blind. The vin says 91....

Sorry guys.


I better go and snap a pic of the other ones sheet. The guy I got this 2wd kept saying 1990. I am still waiting on the title from him. He lost it and is getting a duplicate mailed to him.

I guess I didn't look at the door plate. There is none. Must have been repainted at some point.

[Jsfucttruck]

The other one is a 89v2500, with a th400. I am hoping it has a electronic speedo to rob parts from.
Hmm another G80 3.73 diff, too bad they stopped putting the 14bff in 4x4 burbs of the 80's. Same with the BBC. My 78 K20 Burb came with a 305/465/205 and 10/14bff.

I am no a fan of the semi float 14. I have never had one hold up.

[Jsfucttruck]

I started collecting parts for the 2wd lift. I am basing it off a 2wd 94 3/4 pick up and burb with the higher gvw, 8 lug, 8600 gvw.
At the moment I have.
#K6344 Moog ball joint
#K6445 Moog ball joint
#Set 45 Timken bearing/race
#15103S/15243 Timken bearing/race
#4740 Timken wheel seal
#106.03700 Centric brake pads
#121.66014 Centric Brake rotor (88-95 8lug 2wd)
#141.66020 Centric Caliper
#141.66019 Centric Caliper
#05145 Dorman spindle nut/washer/pin
There are a few variables in the parts and I believe I will have to wait until I get spindles to properly set everything up.
The rotors have some differences in the options
There are 2 listed for the higher gvw trucks,burbs.
121.66014 or 121.66027 both are 8 lug and for 8600 gvw.
This still leaves a few items. Lift spindles, brake hoses, shocks, and tre's. I hope to have the rest of the small parts soon. Not sure when I can afford the spindles.


First pic is 121.66027
Second pic is 121.66014
Both options use the same bearings.
Looks like I got to find some 9/16 studs to swap in too.

[Ski-me]

The 89' will be cable driven speedo. 90 and 91 are the electronic speedos....you can tell the electronic ones as they have extra tick marks for the numeric speeds.

But, you can convert the 89' to electric with some work. Just not a lot of usable parts going the other way. I took the tailshaft off a 90-91 tcase, the pickup sprocket and installed on my 89 tcase. Then ran wires up to dash, repined cluster and done.

[Jsfucttruck]

Originally Posted by Ski-me

The 89' will be cable driven speedo. 90 and 91 are the electronic speedos....you can tell the electronic ones as they have extra tick marks for the numeric speeds.

But, you can convert the 89' to electric with some work. Just not a lot of usable parts going the other way. I took the tailshaft off a 90-91 tcase, the pickup sprocket and installed on my 89 tcase. Then ran wires up to dash, repined cluster and done.

Dang it. My wife's truck is a 91 v1500 Burb with a 700r4/241c and I have not been able to figure out a way to test the components to track down the problem.

Basically, the Speedo some times works, sometimes under throttle it will work and then not when off. Other times,it works vice versa. It causes the lockup to not function.

I repaired a damaged ground wire that piggy backs to the intake and it worked flawlessly for 8 months. It is now back to its inconsistency.

I have replaced the speed sensor on the t- case and repaired/replaced a good amount of the brittle harness from the sensor up past the bellhousing.
No change.


I was hoping to swap out the problematic 700r4, with a th400.

Guidance on repair or swap?

[Ski-me]

^^ Does the cruise control work consistently? Any check engine lights? Any ABS brake light stay on in dash?

That sensor also drives/provides signal to the speedo, computer, cruise control and abs module (90-91 rear abs only). If those are working all well and no codes, then that would point towards the speedometer gauge itself.

[Jsfucttruck]

Originally Posted by Ski-me

^^ Does the cruise control work consistently? Any check engine lights? Any ABS brake light stay on in dash?

That sensor also drives/provides signal to the speedo, computer, cruise control and abs module (90-91 rear abs only). If those are working all well and no codes, then that would point towards the speedometer gauge itself.

The cruise control does not work. The linkage rod is missing and the loop on the vacuum pod is busted.
The abs light came on around the same time as the speedo issue. The brakes did weird things. I unplugged the abs mod by the master cylinder
No more light or weird pulsing brakes.



Let me ask you another random q. Can I swap in a metal master cylinder(91r2500) from the stock plastic master cylinder(91v1500)? It has a 14ff in place of the 10bolt. The bore sizes are different.

[Ski-me]

Drac Unit....

[Jsfucttruck]

Originally Posted by Ski-me

Drac Unit....

How do I test the drac for the pulses with a multimeter. I know very little about electrical.


Thanks for the explanation.

[Ski-me]

I don't know about swapping the master cylinders, but I would guess no. I would get the correct one with the correct vehicle. With that said, I have my 14 SF in the rear of my 1/2 blazer and works just fine. But most of the braking is in the front anyways so not as important.

I don't know how to test, but I'd assume a multi-meter could tell you some info. Alternatively, you could find another DRAC unit in junkyard and see if anything gets fixed. If you have the ABS error and speedo error, then maybe a replacement would help. A good check and pretty cheap if you can find one. I want to say Astro vans also have usable DRAC units....but could be off on ratio's on speedo accuracy.

I got a used one, added some selectable switches, and now I can dial in my speedo spot on, with axle ratio, tire size, etc. Works great!

Where there is a "red switch box" use to just have a straight piece of wire (or two) for vehicle specific axle ratio and tire size from factory.

[Jsfucttruck]

Thanks for the info. I will get to that project later.



Back on track here.

When I first ran and drove the tbi 454, it died a few times, had a huge miss, and had no power. I found worn out plugs, corroded cap & rotor, and two broken ignition wires. I did a full tune up. Cap, rotor, wires, plugs, air and fuel filters, pcv, and a oil change. It idles smooth and runs much better.
I am in the process of fixing exhaust leaks. I found a few busted exhaust manifold bolts, it turned out that they had snapped off in the head. I am very limited on space, but I hope a bud could come over and try to weld some nuts onto them to attempt removal. I am going to remove both side manifolds and surface them before I reinstall them with fresh gaskets.

Also found a loose control arm bolt.

[Jsfucttruck]

I decided to do a few things since i am so wound up in projects and I need something to drive to KOH in the first week of feb.

I ended up pulling the valve covers and serviced the trans. I added a drain plug and the original style reusable gasket. I will take the valve covers to works to the parts washer and I can surface the exhaust manifold.

Tomorrow night I have to install a injection pump, mechanical lift pump and feed line on my 6.2 Burb.