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