Need help with ECM

[midnitewarya]

I have a 1991 Blazer 1500. I've been running 35's with 4.56's now for about a year, and as I'm many of yall know, my speedo has been all out of wack. I've been told that to recalibrate it, as well as my tranny, that I need a new ECM. I've looked all over for an ECM for my 91, but like almost every part on the 88-91's, it doesn't exist. Can anyone help me out with this problem. I know I must be doing some damage to my transmission when I'm going 75 mph or faster.

Also, I've taken the shell off about a thousand times, and as a result, there is squeaking coming from every corner of the truck. Also, there is a rattle above the driver seat under the plastic that I cannot seem to kill. Has anyone else has problems getting the shell back on to where it is quiet?

Any help would be great. I'm currently rebuilding the 91, it was stolen and totally stripped out in AZ last year. If anyone has seen some Gray Cloth bucket seats for my model run, that would be awesome.

 

[riggz]

You should be able to find a used ECM from a local junk yard for around $30-40. As for the squeaking, could just be something not tightened down all the way or a loose seal.

 

[Fierospeeder]

there should be people on here who know more about this.


You dont need to change the ECM. This would be a larger issue with an OBDII system. All you need to do is change the speedometer drive gear on the transmission to give the proper speed. This will also provide feedback to the computer when to lock up the torque converter.


You can bring your truck in to a trans shop and ask them to adjust the trans for shift points due to the tire difference.

 

[nvrenuf]

You do NOT need a new ecm to reprogram your speedo. Since your truck has an electric speedo there is a part behind the radio called a DRAC that can be recalibrated to compensate for the tire/gear differences. There are sites that offer these parts for about $100 and there's also lots of DIY tech articles available for you to do the fix too.

 

[dyeager535]

drac post

 

[midnitewarya]

Thanks

Thanks so much all. The info on the DRAC cleared things up for me, hopefully by the end of the weekend I'll be running smooth again.

 

[Jonny-K5]

just curious what size tires and gears were on the truck when stock?
i was lucky, i had 29" tires w/ 3.73 gears. 35's and 4.56 takes the same speedo calibration

 

[midnitewarya]

I was originaly running 3.17 with 31's. It was an odd combination.