Georgia Fiero Club Forum

All Things Fiero => Tech Tips, Tech Questions => Topic started by: GTRS Fiero on August 17, 2019, 05:27:59 pm

Title: VSS confusion
Post by: GTRS Fiero on August 17, 2019, 05:27:59 pm: My speedo has been 10 MPH off since the change to the 4T60. It was correct on the THM125C.

We tried a different VSS. No change.

The ECU and speedo both show the same speed. Since I was getting my gauges calibrated, included the speedo. No change.

The VSS gear is blue. Today, I replaced the blue gear with a white (or beige) gear, which does have an additional tooth.

Are you with me, so far?

Get this. No change.

Color me confused.
Title: Re: VSS confusion
Post by: scottb on August 17, 2019, 05:54:44 pm: What is the percentage of change in the gear ratio between the transmissions?
Title: Re: VSS confusion
Post by: GTRS Fiero on August 17, 2019, 05:57:29 pm: Um, both supposed to 3.33, but I may have misunderstood your question.
Title: Re: VSS confusion
Post by: scottb on August 17, 2019, 07:39:01 pm: Thats what I was curious about...... the only thing that I can think of is the transmissions and speedometers were calibrated for different size tires. In my mind you will have to adjust the gears by the same percentage as the tire size difference. ....... in others words if the tire difference is 15% you have to adjust the tooth count by 15%. It makes sense in my mind
Title: Re: VSS confusion
Post by: GTRS Fiero on August 17, 2019, 08:24:30 pm: The thing is, I increased the tooth count. No change to the speedo.
Title: Re: VSS confusion
Post by: Fierofool on August 17, 2019, 08:34:42 pm: On the manual trans, one tooth only changes it 2 or 3 mph at 60.
Title: Re: VSS confusion
Post by: GTRS Fiero on August 17, 2019, 08:37:19 pm: There are only 2 speedo gears for the auto. I'm using the one with the most teeth.
Title: Re: VSS confusion
Post by: GTRS Fiero on August 18, 2019, 01:18:26 pm: It looks like a 12% error.
Title: Re: VSS confusion
Post by: Fierofool on August 18, 2019, 01:43:01 pm: That's not an electronic transmission? You're running a 7730 ECM? Wonder if the table can be reprogrammed for your tire size. You would need to know the original tire size of the car the trans came from, then find that tire size calculator on PFF to match the diameter, or reprogram to make the adjustment.
Title: Re: VSS confusion
Post by: GTRS Fiero on August 18, 2019, 01:45:47 pm: I have Ryan working on that angle.
Title: Re: VSS confusion
Post by: GTRS Fiero on September 22, 2019, 09:58:55 am: Quote from: Fierofool on August 18, 2019, 01:43:01 pm
That's not an electronic transmission? You're running a 7730 ECM? Wonder if the table can be reprogrammed for your tire size. You would need to know the original tire size of the car the trans came from, then find that tire size calculator on PFF to match the diameter, or reprogram to make the adjustment.

Ryan says no. :(
Title: Re: VSS confusion
Post by: GTRS Fiero on September 22, 2019, 10:00:20 am: I want to do everything possible to avoid using the problematic Dakota Digital box.

I think that the problem may be in the VSS unit itself. I'm going to try using the original VSS with the new gear.
Title: Re: VSS confusion
Post by: GTRS Fiero on September 25, 2019, 07:13:46 pm: I'm wondering if I have the wrong VSS.
Title: Re: VSS confusion
Post by: Fierofool on September 25, 2019, 09:22:21 pm: Is the one from your transmission interchangeable with that from another series transmission?
Title: Re: VSS confusion
Post by: GTRS Fiero on September 25, 2019, 09:24:10 pm: Dunno. The Fiero VSS is rather unique. I can't pin that down.
Title: Re: VSS confusion
Post by: Fierofool on September 25, 2019, 09:59:34 pm: Have you looked at one from the Grand Am, Beretta, Skylark, Cimmaron, Cutlas Ciera? They all used the TH125c transmission. I know Mel's 93 Grand Am and Ray's Skylark of the same era had that transmission. I was thinking you had upgraded to a later transmission.
Title: Re: VSS confusion
Post by: GTRS Fiero on September 25, 2019, 10:01:21 pm: I did. 4T60. The VSS from the 4T60 does not work for the Fiero, apparently.
Title: Re: VSS confusion
Post by: GTRS Fiero on September 30, 2019, 10:33:34 pm: Apparently, there are gross and fine adjustments on the 7730. I believe that the fine adjustment needs to be modified to get the correct reading.

F85gtron, any comment?