How-To: 88-91 speedometer calibration 25085515 91' tach upgrade

[tommie]

Ok,

This is my 1st attempt at a complete write up, so bear with me on this progress. I have a 90' model chevrolet fullsize, and wanted to upgrade to a tach dash. I already had a tach dash from a truck I parted out, and it was a 91' model. After searching the internet and several of the forums that I am on, I found many articles about swapping in a 92-94 tach dash. Many of the articles are like this one:
http://www.someotherplace.com/guides...p/cluster.html

That information is great, if you are changing a 88-91 truck over to the 92-94 external DRAC and tach upgrade. That is NOT what we are talking about here. We are talking about swapping for a 91' tach dash or just how to modify an 88-91 internal DRAC without using the GM 25085515 kit.

I am dealing with swapping in a 91' tach dash into a 90' so this is what we are going to focus on. From everything that I have read, the 88-91 trucks have an internal DRAC module located in the cluster, and the 92-94 trucks have an external DRAC. The connector for the cluster is different between these year model ranges, and is the easiest way to identify the year range of cluster.

this is the 92-94 connector

this is the 88-91 connector

When swapping a 88-91 dash for an 88-91 dash the only issue is how to reprogram the DRAC if needed for a different tire size, or different rear end gear ratio. Lets say for example you were putting a tach dash in a truck with a 235/75 R 15 tire with a 3.08 rear end gear ratio, and the dash came from a truck with a 3.42 gear with a 235/75 R15 tire.
GM's answer for this was the 25085515 calibration harness that was used to "nuke" or "pop" the internal DRAC resistors found in the 88-91 cluster and then allow the technician to recalibrate the speedometer using a programming clip inserted in the back of the cluster to compensate for tire size and gear ratio.



Wow, this seemed like a great way to do this, and it seemed to me that this would be an easy way for me to put my 91 tach dash in my 90. I called the local GM dealer and asked about the 25085515 calibration harness. I was told that it was discontinued... I asked about this, and he told me that he could look on the computer and find another dealer that had one, he gave me the information on the closest dealer (in another state) that showed to have one, but I started thinking that there has to be another way to do this....
Then I started this thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=312063

With help from members of a couple of different boards, and reading several posts on the internet (all to be named later) I found some good advise, answers, clues... It was just up to me to me to try it... and see if it would work...

The 1st thing I did was to take my tach dash apart, and see if I cold tell how it worked....most of that is covered in this thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=312063

One of the things that I was able to notice, was that the connector used for the calibration chips was wired into some "resistors" labeled F301-F307

This is where the thought prcess and experimentation came in (ha ha, more like trial and error!!!)
The connector that is used for the calibration chips when using the 25085515 calibration kit, has 8 pins. 7 of these pins attach to the "resistors" labeled F301-F307 the 8th pin is connected to what was determined later as a ground. I then used an OHM meter to measure the resistance of the F301-307 resistors and found that:
F301 continuity
F302 Continuity
F303 Continuity
F304 Open Circuit
F305 Continuity
F306 Open Circuit
F307 Continuity

When looking on the chart pictured above, this matched up with a 3.42 rear gear and 235/75/R15 tire. This was a match for the truck I pulled the cluster from. It seemed that GM had these "resistors" installed, and either made them open circuit or allowed them to have continuity to allow the programming of the tire size/rear end ratio.
This is nothing new, it seemed like the same information that the External 92-94 DRAC guys discovered...

Anyway, I decided to modify my "resistors" F301-307 to match the pin out for the 3.08 rear gearand 235/75 R15 tires. I have soldered on 2 circuit boards so far, and I am no expert on circuit board soldering techniques... So I decided to remove the "resistors" in the following locations:
F302,303,304,305,306
I was trying to get from this:
F301 continuity
F302 Continuity
F303 Continuity
F304 Open Circuit
F305 Continuity
F306 Open Circuit
F307 Continuity

to this:

F301 Continuity
F302 Open Circuit
F303 Open Circuit
F304 Open Circuit
F305 Open Circuit
F306 Continuity
F307 Continuity

[tommie]

After I removed the resistors, I used a "jumper" between pins 6 and 8 (ground) of the calibration chip connector on the back of the cluster so that I would have continuity on pin 6


Then I swapped the wires on my stock connector for the instrument cluster to upgrade from a non tach to a tach dash by following steps 7,8,9,&10 from this site:
http://www.koolcartoons.com/lowdown/...91cluster.html

I also had to cut a notch in the plastic dash next to the cluster plug to be able to remove the cluster plug from the dash:

you can see how the plastic dash was cut to the left of the dash to allow removal of the plug for the instrument cluster.

Ok, now I was ready to install the dash and check things out. MAKE SURE THE INSTRUMENT CLUSTER IS FULLY PLUGGED IN AND BOLTED DOWN BEFORE YOU START THE TRUCK BACK UP!!!!!! (Thanks ChevyTech for helping out with this one also)

If you do not have a good connection on the cluster, the gauges may not work correctly, or as I did you may not have a good connection and set some RWAL ABS codes, and cause your brake light to flash and drive you crazy into thinking something is wrong with the dash itself, or the wiring......

Now was time for the test drive, all of the gauges worked, but the speedometer did not seem correct to me....
this is when I looked at this site:

http://614streets.com/drac.html

There is a lot of good information on this site about determining the exact ratio (pin location) based on tire size and rear gear ratio, I compared the divide ratio that I was using from the GM chart above and I used the calucator on this page with my rear end gear ratio and actual measured tire circumference and came up with a divide ratio of .676270. This was my next try, ( I used a jumper wire on the back of the instrument cluster to obtain this setting)


The speedometer seemed to be closer, but it was still too slow.. Seems the lower the divide ratio, the faster the speedometer will spin, and the higher the divide ratio, the slower the speedometer will spin.

So for the 3rd try I switched to a .661133 divide ratio. This is where it was for a 50 mile trip to a car show last night and it seemed to be pretty close. It will take some time to determine if this is a 100% correct or not, but it seems very close right now.

A few other things I wanted to mention:

1. You could just remove all of the F301-F307 "resistors" and do all adjustments from the rear of the cluster, OR you could solder in a DIP switch similar to how the guys do the 92-94 DRAC modifications:
http://www.someotherplace.com/guides...p/cluster.html

2. when you have the cluster out, it is a good idea to replace the backlight bulbs used for lighting up the cluster at night 6 bulbs, 194 peanut bulbs, less than $10 at any auto parts.

3. If you wanted to you could use this site, to adjust the odometer reading of your new cluster to match your old odometer reading:
http://www.fullsizechevy.com/forums/...odometers.html

I would like to thank everyone who helped me try this, Chevytech, Someotherguy, and other board members who commented also.

I have also read, and thought about information listed in the following websites, and some of their information was used in part on this write up and how to, i would like to pass credit along to each of these websites also:
http://www.koolcartoons.com/lowdown/...91cluster.html

http://www.454ss.com/Articles_new/tweek/DASHB.asp

http://67-72chevytrucks.com/vboard/s...d.php?t=178366

http://www.fullsizechevy.com/forums/...odometers.html

http://www.someotherplace.com/guides...p/cluster.html

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

http://67-72chevytrucks.com/vboard/s...d.php?t=312063

http://614streets.com/drac.html

http://www.philter.com/cgi-bin/drac.pl
The last one listed seems to have the pin #'s reversed, I would only use this one for the calculator function, then refer back to the 614street chart for the chart and pin location (I agreed with ChevyTech on this one)

[tommie]

I found another article related to this in a magazine I had at home

page2

page3

page4

page5

[tommie]

page 5 2nd piece

and page 6

[Spray-Bomb]

So lets see some pics of your 91 tach dash in your truck

[ChevyTech]

Nice write-up.
This is the best collection of photos, information and links on this topic that I have seen.
This will help many people.

[JohnO356]

Yeah, good going, Tommie.

[jeffspower]

Thanks for doing all the homework on this! I swapped my '88 rear end gears from 4:10 to 3.21's. Needless to say, my speedo is about 15 MPH off. I want to swap in a '91 tach cluster when I find one cheap, and get the speedo right also. I figured it was going to be a guessing game since the chart doesn't show 3.21's- but it looks like I should be able to calculate it with the info you have above.

Maybe I'll experiment on one of the ugly sweep clusters I have laying around

[jeffspower]

I was just thinking... I think I'll try the dip switches, but put it on a wiring harness so you can put the cluster in and still have access to the switches. Make the harness long enough to hang out of the dash when assembled , and just tie it up under the knee panel after you have it set. I can see where this would be handy for people that run different tire sizes at different times. Or if someone is running a quick change rear end

[tommie]

Originally Posted by jeffspower

I was just thinking... I think I'll try the dip switches, but put it on a wiring harness so you can put the cluster in and still have access to the switches. Make the harness long enough to hang out of the dash when assembled , and just tie it up under the knee panel after you have it set. I can see where this would be handy for people that run different tire sizes at different times. Or if someone is running a quick change rear end

Wiring up an external DIP switch would be ideal !!!

use the charts on this page
http://614streets.com/drac.html
and you have settings for almost any tire.gear combination.

[tommie]

Ok as someone requested I took a pic of the guages after the install....

[Matt Mann]

how difficult was it to correct the odometer mileage?

Or did you just decide to note the change on the title?

[tommie]

there is a link for correcting the mileage, it is easy,

http://www.fullsizechevy.com/forums/...odometers.html

i just set the mileage on the new cluster to the same mileage as the old cluster

[Matt Mann]

that's cool...

Just the info i need...

I have a '92 silverado 1500 stepside, with 350 V8 and stock driveline.

Factory built the truck with a 85 mph non tach gauge cluster, with 107,xxx miles but, I managed to find a 120 mph tach cluster used that reads 239,xxx miles....

I want to keep the paper straight, although I plan on driving the truck into the ground. I would like the new functionality of the tach cluster. (It would resolve the defective 'trip' odometer problem)

I was always taugh it was always legal to make whatever repairs are needed to keep a vehicle operable as it was intended as it was orginally built. Niow on the other hand, FAILING to disclose that you made repairs is the problem.

In other owrds, it wasn't that used car dealers were rolling back miles, that was the problem, however FAILING to disclose they made repairs, or worse yet actually denying that repairs were ever done was the problem.

Regardless of what the odometer reads, as long as the repair is noted prominantly either on the doorjamb or the title, I would think that it is legal.

That's why used car dealers make buyers sign mileage waivers. So, that if it ever does become an issue, regardless of what it was theyre covered...

That's my .0835 pesos, your kilometers may vary.....

[mike88chevy]

Does anyone know what 2 pins are connected to the calibration plug? i cant find one so i wanna make my own.

[JohnO356]

There are several posts on here concerning calibration. Search under user name "Tommy" for one good thread and there is another thread on here telling you how to wire your own plug.

Sorry I don't have time to search, but it's here (I think) lol, or search the "interior" forum on Full Size Chevys.

[mike88chevy]

ok. i was hoping that if someone had the plug they could tell me. but thanks ill search his posts.

[mike88chevy]

never mind i just bought one.

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[tommie]

If you have any questions, just post them up, I will try my best to answer them. I am usually on here 2-3 times a week....

[mike88chevy]

I found the connector, cost me $80.00 and it was used from the dealer and didnt come with the chips. hope i can get my money back out of it.

This is what i come up with pins 4-10 goes to + and pin 11 goes to - ground..... That resets the programable chip, then your ready to put the 8 pin chip in.

[ChevyTech]

Mike thank you for the information and drawing.

I never bothered to take note of what the connections went to on the calibration harness when I had them in my hands and before they were no long available.

I am surprised you found the harness.

[mike88chevy]

yea me 2. your welcome wish i could have found this out b4 i spent $80.00

[tommie]

good info..

thanks for adding.

[scotts62]

Very cool write up, glad its a part of the FAQ now!

I need to do that to my crew cab sooner than later.. i really want the dash from my old '95 in there but dont think its possible to just swap the insides and still use the old style Plexiglas face.?

[johnbrown]

Hi every body
I know this is old thread but I have 90 chevy with a bad oil pressure gauge but I can get a 91 with a tach my question is if changing the wiring on plug
this cluster will work ?