Converting 89' Cable Speedometer to 90'-91' Electronic Speedo - Long Write up (Pics)

[jeff in co]

Because my cable driven speedo was bouncing around and after replacing all kinds of parts, I decided to convert my 89’ Speedometer over to the 90’-91’ Electronic Speedometer. The overall procedure isn’t too bad but, of course, I had some problems along the way. Hopefully lessons can be learned from my mistakes.
I hit the junkyard and found a speedo housing from the T-case off of a 90’ blazer and the tone ring off the transfer case shaft.

I also grabbed the 90’ instrument cluster

The DRAC box

I also grabbed the wires that run from the speedo sensor to the firewall. Try and get as much wire (the twisted pair) as possible. It goes from the sensor all the way up into the engine compartment and then into the passenger side firewall where it goes to a connector by the ECM area. I just clipped it before the connector and that gives plenty of wire for the retro.

It’s fairly straightforward. Remove driveshaft and tailpiece housing off T-case.

Remove first snap ring which is a BEAR to get off. I did try and find the largest snap ring tool I could find and it did work…..just took some effort. This first snap ring is the thickest one and the most difficult. Once that is removed, the speedo housing can be removed. Just use the large prying tabs to gently remove it from the T-case.

Once you’ve gotten the Housing, another snap ring needs to be removed. A little tricky but this one isn’t as thick and difficult. Finally, take the blue speedo gear for the cable driven unit.

View attachment 328622

View attachment 328623

View attachment 328624

View attachment 328625

View attachment 328626

View attachment 328627

View attachment 328628

 

[jeff in co]

I ordered a 7 Pin rocker switches from eBay to modify the DRAC box. They are super cheap and you generally get a 5 pack for $5…..just be sure you are buying from within the US. Probably 90% of them are in Japan and the shipping for those items may take literally a month before you get it!

I took the speedo to a local speedometer shop and asked if they could dial in the correct mileage on the truck for me (324,200). After a little bit of twisting and aligning, he got it all dialed in for me. Very Cool! He decided to charge me $10 for the job……Sweet!

Next is the installation of the electric housing onto the T-case. As you can see, the 89’ gear is the same width as the 90’ gear so pretty cool.

The New gear goes on and then the snap ring.

Now the first problem. Torque is important when installing. Yes, it’s aluminum and yes it can be brittle. Not knowing the housing should actually just slide on with little effort, I thought I probably have to just start cranking down on the bolts evenly until it got seated. THIS is WRONG!

Behind the gear is this Grey piece that is related to the oil pump and it has 4 notches. These notches need to be lined up in the same notches in the new housing. Well, I didn’t line it up and started cranking. One tab got out of alignment and I kept cranking. Well, I cracked the housing tab and also snapped off the pump tab! SO FRUSTRATING! Now I have to start over.
Off to the junkyard again. Found a new housing and now, also the oil pump plate thing!

I originally was hoping that a later model year might work if I found another 241c T-case. What I found was that the newer T-cases are rotated about 180 degrees from the older style so as far as I can tell, only the 90'-91' housings will work off of Blazers and Suburbans.

So, lets do this over again! But this time, I had to take a third snap ring off too.

The plate I had to replace was the one labeled "16209" in this picture:

I replaced the oil pump plate but I did see the small tube inside so be sure that these two are connected before screwing back in.

This time I took my time and first did a dry fit for the housing…..sure enough, it lined up perfectly with the tabs so I added the RVT silicone and gently placed the housing on by wiggling it just a little at the very end to be sure the tabs were correctly seated.

I torqued the bolts to 20 ft-lbs. Installed the large snap ring again, the tailshaft housing and torqued to 20 ft-lbs too.

I'm not positive on this torque setting. I found the 20 in an Advanced Adaptor SYE installation procedure but I also saw 35 ft-lbs in my Chiltons.....although it wasn't specific to the tailshaft housing so if anyone knows for sure, I'd love to get the correct one

 

[jeff in co]

Next was to run the wires up to the instrument cluster area. Pretty easy and nothing to it.
I took the DRAC and used some sort of “de-soldering” type braided wire I got from Radio Shack. Pretty cool how it just sucked up the solder. Once all 14 holes were clear, I installed the new rocker switches and finished up with some solder on each pin. Worked out great!

While I had the cluster out, I decided I might as well paint it chrome as well. I took everything out, washed it, lightly sanded it, and then hit it with a chrome spray paint. Looks great!

I also did the back side of the gauge cover. First, without taping it off….

And now on a second cover, I taped the backside. Definitely worth taping off!! Any tiny amount of overspray could be removed with goof-off or in my case, a little bit of gas. Results are good.

Wiring up the DRAC is pretty straight forward. The existing, green speedo box had power coming in so I just used that same positive and negative leads. The other two were for the computer (brown) and the cruise (yellow). A good, clean tie-in.

Pink (12V Ignition)
Black (Ground)
Brown (ECM input)
Yellow (Cruise input)

I was originally going to take the printed circuit from my 89’ cluster and just move it over to the 90’ cluster. Well, the rectangle area for the speedo was larger than the 89’ so the printed circuit just wouldn’t physically fit to the back side. Dang, so much for easy! I will now need to re-pin my connector in the truck itself.

Final cluster before going in truck.

 

[jeff in co]

Setting up the DRAC was pretty straightforward as well.

Based on the axle ratio, tire circumference and the 7 Pin Dip switch, you should be able to dial in the speedo pretty good.

Here is a great site to dial in the DRAC correctly.

http://www.tbichips.com/drac/

Using the formula from the above website with 3.73 gears, 109" tire circumference (35" BFG's), I get a ratio of 0.67756.

Based on his chart, my dip pin setting should be: 1 1 0 0 0 1 1

 

[jeff in co]

So, now all I have is re-pinning the connector.

Anyone have a simple way of doing it? I'm just concerned about mixing them up......

 

[Russell]

Ah, crap! I got your pm on my phone and forgot to reply when I was back on my computer again...

The best way to go about it is to trace the circuit sheet on your old cluster to start. Write down what every wire is connected to and re connect it back up in the right slot on the new cluster.

I am not positive, but the newer cluster may have a different connector than the older ones. You will wind up with a few spare power and ground wires after re-pinning things. There may be a few duplicate wires that loop around on the old connector and come straight back out again. You can just clip the extra power / ground wires and any duplicates

 

[jeff in co]

No problem man. I am starting to trace everything now and will see how it goes. I was just checking to see if you had already established where everything went but no biggie.

I sure hope the connectors are the same though. that would suck having to go back out and get a separate plug for the 90-91 cluster.....

 

[jeff in co]

Success!!

I basically traced each wire and then decided to just label each one....just in case. Each one had a designated letter and then I used our label maker to keep things straight.

To remove the pin, just take a small screwdriver, insert below the pin and bend the tab up. Once out, bend it back down and re-insert. Worked great.

Here is my pin diagram:

And my labeled connector:

Speedo moving down the road and looks pretty close right now.... (disreguard the trans indicator).

And finally, with the lights on in the dark....nice improvement on the backlighting too!

Last but not least, I tried the cruise and it held perfectly!~

Great success!

 

[USSkoval]

I've been looking for a write up like this for years! Nobody ever goes into detail like you are, thanks!

 

[hillbilly deluxe]

Nice writeup!

 

[AaronJen]

That is AWESOME! Thanks for taking the time to do that. I have my dash all cut out in my '90 and a nest of wires hanging there, waiting for my DIY4X dash and Autometers to go in. I have searched and searched for what you posted with no luck, and I am worse than electronically challenged, so you just saved me who knows how much headache and $$$. Thanks again for your efforts.

 

[Russell]

Should submit this one as a tech article! Very well done!

 

[jeff in co]

Thanks Russ for all your help! I couldn't have done everything, especially the wiring without your input. That really helped.

I took it out on the freeway today and it's just as smooth as silk. Can't believe the difference from the cable driven unit. Cruise is also very steady and no longer pulses up and down. And finally, the tranny is shifting properly, too. No longer shifting in and out on the torque converter.

One note that I now have figured out....because the light bulbs in the newer clusters use a smaller diameter twist socket, some of the old items don't twist in.

Namely: the 4wd light and, if you still have it, the cigarette lighter cable.

When you are at the junkyard getting the instrument cluster, get those two....or at least the 4wd light socket. This one is different than the others...... it has two wires coming out of the socket which goes down to the switch on the transfer case.

I don't know about the cigarette thing. I got another ashtray last year and don't recall which year I got it out of....possibly a 90 or 91. Anyways, mine actually DID fit into the new cluster whereas before, it wouldn't go into the 89' one. I finally got lucky on something.


***********

Oh yea, anyone know the torque settings for the new housing and the tailshaft housing that I put on the T-case???
I used 20 ft-lbs but don't know if 35 ft-lbs is the proper one on not?
Thanks!!

************

 

[imiceman44]

So if I was to do this for my 89, I have a complete 91 truck that I parted, it was a crew cab and the engine tranny and tcase are gone but all the wiring and the cluster are still there.
Would it be easier to grab the whole cluster, and would I need to reset the pins on that to match my 89 truck?

Thanks Russ for all your help! I couldn't have done everything, especially the wiring without your input. That really helped.

I took it out on the freeway today and it's just as smooth as silk. Can't believe the difference from the cable driven unit. Cruise is also very steady and no longer pulses up and down. And finally, the tranny is shifting properly, too. No longer shifting in and out on the torque converter.

One note that I now have figured out....because the light bulbs in the newer clusters use a smaller diameter twist socket, some of the old items don't twist in.

Namely: the 4wd light and, if you still have it, the cigarette lighter cable.

When you are at the junkyard getting the instrument cluster, get those two....or at least the 4wd light socket. This one is different than the others...... it has two wires coming out of the socket which goes down to the switch on the transfer case.

I don't know about the cigarette thing. I got another ashtray last year and don't recall which year I got it out of....possibly a 90 or 91. Anyways, mind actually DID fit into the new cluster whereas before, it wouldn't go into the 89' one. I finally got lucky on something.


***********

Oh yea, anyone know the torque settings for the new housing and the tailshaft housing that I put on the T-case???
I used 20 ft-lbs but don't know if 35 ft-lbs is the proper one on not?
Thanks!!

************

 

[jeff in co]

So if I was to do this for my 89, I have a complete 91 truck that I parted, it was a crew cab and the engine tranny and tcase are gone but all the wiring and the cluster are still there.
Would it be easier to grab the whole cluster, and would I need to reset the pins on that to match my 89 truck?

You should be able to use the 91' cluster but you'll need to get the tailhousing and the toning ring off the transfer case of a 90-91 to make it work all together.

Also, you will need to re-pin your 89' connector to match the 91' cluster....they are different.

 

[imiceman44]

You should be able to use the 91' cluster but you'll need to get the tailhousing and the toning ring off the transfer case of a 90-91 to make it work all together.

Also, you will need to re-pin your 89' connector to match the 91' cluster....they are different.

I know about the tail housing stuff, just wanted to see if the cluster complete would make things much easier or not.

 

[jeff in co]

The electric speedo won't fit into the 89 cluster as the cutout section is bigger in the 91. So you definitely will need the 90-91 cluster......

 

[Fatrat]

Well, i saw this post during my search and rather than starting another decided to comment and ask on this one. First off i know the post is a bit old, but great write up, i just did my brothers 87 and this was great help. The detail and step by step could not have been better. Thanks

Now my dilemma is, that on my 1990, i have the stock cluster and an NP241C with electric speedo, the problem is the blazer is carb with no ECM. is there a way to wire up the speedo. PO decided to remove all FI and its wiring, then abandoned the project.

On my brothers 87 the cluster has the speedo that is cable driven but also has the small green box on back that connects to the DRAC. Is there a way to wire this speedometer to an electric speedo NP241C.

Maybe an adapter for either. Both blazers are carbed no computers

 

[dyeager535]

Cluster/speedometer doesn't need an ECM. VSS signal comes from the t-case, to the DRAC, the DRAC feeds cruise, speedometer, ABS, and ECM if used. DRAC could be used solely to feed the speedometer if that's all you needed.

"Mechanical" speedometer (if I understand) for the '87 cannot be wired up to work with the NP241 VSS. The buffer (green box) is used as an output for ECM and cruise. It must be connected to the mechanical speedometer to operate. Best bet I guess is the '89 NP241 that was still cable driven, or better yet (and more easily found) just convert it over to the '90/91 cluster.

That answer your questions?

 

[Fatrat]

You hit it right on. I was taught by old schoolers and they always avoid efi and electrical stuff. Good for the race cars they build. I like to learn new things and keep up with times. Efi for off-road is awesome so I'll be doing that next year. Thanks for the quick reply ill post my progress later.
Time to hit the junk yard for that 89 housing or 90 cluster, whichever comes first.