Problems with speedometer

[dieselguy9311]

So i was tuning a lbz truck today and it had 40 in tires on it. went through the process of changing the info so that it would be right. I got it dead on but problem is that as soon as the speedometer would get over 65 mph it would just drop to zero. then when we slowed down once we got to 25 it would pick it back up and then start to work again till 65 again. i thought that maybe the cluster was going bad but i reset his speedometer back to what it was when he showed up and we ran it to 100mph no problem never kick off or anything. has anybody else had this problem and if so have you been able to fix it. i have done tons of truck none with tires quite this big but never ran into this problem. thanks for reading hope you can help.

[dieselguy9311]

also i have had problems with abs light kicking on after i tune trucks is there a trick to this. they go away after awhile but customers always complain and would be nice if it didn't happen. seems real hit and miss on which trucks do it.

[THEFERMANATOR]

BOSCH ECM's don't play well with the ABS when you go over a 35" tall tire calibration. You can set it for a 35" tall tire and call it as close as your going tto get it to work without the ABS getting PO'D.

[Motorzane]

Have you tried messing with the gear ratio instead of the revs per mile?

[dieselguy9311]

sorry for taking so long. no i have had the abs lights come on even when i never even mess with the tire calibrations. and yes i did try with the gear ratios as well. seemed that no matter what i did once i started getting close just would cut out. thanks for the idea though. i take it nobody has actually had this problem. thefermanator i take it you have dealt with tall tires before have you ever ran into the speedos dieing like that.

[ScarabEpic22]

You cant program the ECM with anything larger than 35" tires, you WILL get a ABS light. This is a Bosch issue, not a GM issue so it's a quirk that wont/cant be fixed, unfortunately.

[akaponch]

I tried it on my trucks and you just have to keep it at 35 inch tires for everything to work without troubles.

[dieselguy9311]

ok thanks for getting back with me good to know i can relay the information.

[killerbee]

It is probably a cluster issue.