89 GMC Cummins conversion

[COS399]

Here is my 1989 GMC CC SRW 4x4 build. It had a 6.2 liter diesel and a TH400 transmission. I am putting in a 24 Valve Cummins and a 4L80 transmission. Over the last two weeks I have managed to pull the old running gear and put in the new running gear. One unexpected hurdle I ran into was that the NP205 transfer case would not bolt up to the 4L80 due to the longer transmission pan making contact with the transfer case. I had to order an adapter that spaced them a little over 1/2" apart. Just to be safe I also grinded a bit on both the transmission and transfer case just to give it a little extra room.

I ordered a 6" lift kit to ensure that the new crossmember did not contact the front differential and was surprised when it increased the ride height by 7" in the front and 7 1/2" in the rear. I was told it would settle a bit once I get a few miles on it.

Here are a few pictures for now.

[COS399]

Here is a few more pictures.

[COS399]

Slow progress on my project. It sure is nice to have friends with a large heated garage, overhead crane and a pit to use when your doing one of these projects. I got the suspension lift almost done, just need to build some brackets to secure the metal brake line extensions. I am waiting on my driveshafts to come back, since I had to shorten the rear and lengthen the front. In the meantime I am working on the wiring and trying to figure out what to do about the Dodge radiator not having a transmission cooler built in to it. I am leaning towards a 6.0 liter Ford diesel trans. cooler. I have plenty of room behind the grille since I mounted the intercooler on the back side of the radiator support.


I didn't start this build like most showing you pictures of what I started with so here you go.

[COS399]

Here are some interior pics.

[COS399]

I found a used seat and a dual headlight radiator support and parts to eventually get rid of the large single headlight.

[COS399]

Well I decided to just run an auxiliary transmission cooler with a fan and a deeper transmission pan. I ordered them last night and they should be here by the end of the week or first part of next week. If it doesn't cool it down enough Plan B will be to go back with the GM radiator when I switch out to the new radiator support to install the newer dual headlight front end. I will post more pictures when I have it ready to start up.

[Greenlee]

Cool build!

[nekkidhillbilly]

i actually like the single lights in the 88-91 trucks dont see many fixed up with that set up

[COS399]

Greenlee - Thanks for the acknowledgement! I wasn't sure if everyone was tired of another CC Cummins conversion build thread.

[COS399]

Originally Posted by nekkidhillbilly

i actually like the single lights in the 88-91 trucks dont see many fixed up with that set up

I will probably leave the singles for a little while. My first 1981 C20 had the single lights and I thought about leaving it that way, but once I had to do the lift kit for the cross member clearance the build kind of changed directions. I know some people may think it is actually a Chevrolet because of the Grill and tailgate but it is a GMC. I am looking for a GMC grille and tailgate. And it might be kind of fun to leave the old 6.2 Litre emblem on the tailgate.

[Hankster]

I actually bought a rad support and grill like that for my 86 RCSB c10, it is painted black, you don't see many of them thats why i got it, I think they look good! Oh BTW Nice truck! What size/kind of tires do you plan on running?

[COS399]

Originally Posted by Hankster

I actually bought a rad support and grill like that for my 86 RCSB c10, it is painted black, you don't see many of them thats why i got it, I think they look good! Oh BTW Nice truck! What size/kind of tires do you plan on running?

Not sure yet. If I wouldn't have had to lift it then I would have stuck with the stock tire and wheel combo with the dog dish hubcaps. Now, I am leaning toward 33" or 35"s. Still not sure on the wheels though.

[COS399]

Originally Posted by Hankster

I actually bought a rad support and grill like that for my 86 RCSB c10, it is painted black, you don't see many of them thats why i got it, I think they look good! Oh BTW Nice truck! What size/kind of tires do you plan on running?

Hankster - read your build thread and I hope you are getting better. Thanks for your compliment on my new project truck. I like seeing that a person your age is interested in these trucks instead of something newer that everyone has. Good luck with your build.

A week before I bought this truck I traded a Honda 300EX for a 1978 GMC CCSRW K30 with no motor. Trading is the way to go if you can do it. Oh the guy also threw in an extra front D60 and FF14 rear axle in the deal.

[COS399]

Slow progress in the last week. I got my new TCI deep transmission pan and finally got it installed. In order to drop the old pan I had to remove the cross member and mount. While I was at I went ahead and replaced the mount with a new one. The last two days have been frustrating trying to modify the transfer case shifter lever to work now that the transmission has been moved back 4 1/4" due to the 4L80 being longer than the TH400. After numerous failed attempts and countless hours wasted at trying to make the shifter work in the original location, I finally decided to move the hole in the floorboard over. I am attaching pictures of the before and after pictures of the TC shifter lever. My drive shafts came back from the shop and I got the rear ones installed. I was waiting to finish the TC shift lever done before installing the front shaft.

[nekkidhillbilly]

Originally Posted by COS399

I will probably leave the singles for a little while. My first 1981 C20 had the single lights and I thought about leaving it that way, but once I had to do the lift kit for the cross member clearance the build kind of changed directions. I know some people may think it is actually a Chevrolet because of the Grill and tailgate but it is a GMC. I am looking for a GMC grille and tailgate. And it might be kind of fun to leave the old 6.2 Litre emblem on the tailgate.

when i do a dmax one prob next year im going to try and find a diesel gate and grill emblem off an old 5.7 olds powered truck if i can. i love the singles in the early 80s trucks never liked the stacked lights as good.

[Pullin81]

Nice truck man I was curious how the 4l80e would hold up to the cummins... Be interested to see how it works for you... The more I think about it the more I wish I would have gone with an automatic transmission... I see your from north mt I ride down threw Montana when I head south on the bike in the summer... Keep up the good work

[COS399]

Pullin81 - thanks for the compliment. I was in Great Falls the other day and Ken drove me around in a 12 valve 4l80 that he was working on for a customer. He was still tweaking it a little bit but it seemed like a good set up. I still have the donor truck and a 47RE if I decide it was a bad ideal.

[1774btcrew]

Curious what you are doing about mounting the TPS and what you are using. On my 4l80e Cummins conversion I first used a GM TPS that came with the Compushift controller. It was a constant problem and I was replacing them often. I ended up switching to a Ford TPS that I mounted on top of the pump. Seems to work well but need a little more dialing in.

[COS399]

Originally Posted by 1774btcrew

Curious what you are doing about mounting the TPS and what you are using. On my 4l80e Cummins conversion I first used a GM TPS that came with the Compushift controller. It was a constant problem and I was replacing them often. I ended up switching to a Ford TPS that I mounted on top of the pump. Seems to work well but need a little more dialing in.

I just tapped into the TPS on the IP, I was told it was all I needed to do since I have the 24 Valve.

[Dieselwrencher]

Very nice work and cool project! Ken and Dawna definitely are great people and will help you out no problem. Those 24v's are easier to wire up than most think. It's just more work is all. I bet you'll like the 4L80E once it's set up right. A good TC and VB and it should be great. Can you just tap into the APPS on the 24v for the PCS for the TPS input? That's what I was able to do on a 5R110 set up I did. If you get bored, we have a diesel conversion section in the engine forum with a lot of great builds in there. Anyhow, keep up the great work!

[COS399]

Originally Posted by Dieselwrencher

Very nice work and cool project! Ken and Dawna definitely are great people and will help you out no problem. Those 24v's are easier to wire up than most think. It's just more work is all. I bet you'll like the 4L80E once it's set up right. A good TC and VB and it should be great. Can you just tap into the APPS on the 24v for the PCS for the TPS input? That's what I was able to do on a 5R110 set up I did. If you get bored, we have a diesel conversion section in the engine forum with a lot of great builds in there. Anyhow, keep up the great work!

I believe that is what I tapped into. To be honest there was already a wire tap on the harness coming from the IP. As for the diesel conversion section, I found that after posting this build where I did. I am not sure whether I can move it or if I need to. I love this site and all the projects and information that everyone shares.

[brimc]

Awesome job man. Can't wait to see how it turns out!

[COS399]

Thanks guys. I am working on mounting the trans cooler and should have it fired up this weekend.

[COS399]

My transmission cooler showed up and after planning on mounting behind the grill, I read the instruction sheet and read that it was a "puller" and not a "pusher." I therefore had to come up with a new game plan on where to mount it. I think in the end this is a better location anyway. I built the cooler holder, wired it up, heat wrapped the exhaust down pipe and mounted the right front fender. The inner fender would not bolt up because it was fighting with the intercooler tube for the space. After getting out a hammer and dolly the inner fender decided to play nice.

[Dieselwrencher]

Nice work on mounting your trans cooler! When I did the 89 burn I did, I put it right there too.