CTYANK2

OK, I just had my 2:59 rear replaced with a 3:07 in my 89 automatic. The speed sensor specific to the 3:07 rear is a discontinued part.

After a lot of calls I ordered a used speed sensor that is supposed to be specific for the 3:07 rear, but when I got it and looked it over it is identical to the speed sensor for the 2:59, including the only numbers on it, "34-39".

I expected the 3:07 speed sensor to have different numbers on it but it has the same numbers (34-39) and looks identical.

So how do I know this is the right speed sensor for the 3:07 without installing it and checking speedometer readings at speed?

CFI-EFI

The difference isn't in the calibration, it is in the fit. The 3.07, and lower ratios, VSS has the hole in a different location to move the larger (more teeth) driven gear further away from the drive gear. The calibration is in the tooth counts, not the VSS. If you have changed the gears and goes together, it is right. If the speedo is off, it's not the VSS, it's the gears you used.

RACE ON!!!

MrRenoman

I think you still have the wrong sensor for the 3.07 and lower application and have one for the 2.59 and higher. The reason I say this is because the sensor I removed from my '85 with 2.59 is also a 34-39. I recently read an article on the "net" where the poster suggests simply rotating the speed sensor 180 degrees in order to use the existing sensor for a 3.07 and lower gear. I examined my sensor and realized that it just might work. It may require modifying the hold down clip (trimming the fork ends) so the clip would keep the sensor from rotating. I haven't tried this, I did post this same message on another post concerning the lower gear sensor and it being hard to find. Examine your sensor and you'll see that the gear is offset from the center axis. Rotating it 180 degrees moves it in the opposite direction which may put the gear further away from the drive gear. I'd like to see this installed and feedback on whether it worked or not.

Art

Funny, the previous post I mentioned the above in was a previous post by CTYANK. Oh Well, isn't it the mind that goes first?

Here's the link again to that net message. To find the info, scroll down a ways.
http://home.alltel.net/wayne/faq.html

CTYANK2

Thanks Mr Renoman, although I may be loosing my mind, I do remember your proposed solution. Since I thought the correct VSS was coming to me there was no need to try the solution.

My question here is if I have the 2:59 VSS next to the 3:07 VSS how do I tell them apart?

Or, as CFI-EFI is saying, are the VSS identical on C4s except for fitment and the gear?

CFI-EFI

The gear and the VSS are two separate items. It is the hole in the VSS that locates the gear that will be different.

RACE ON!!!

CTYANK2

Thanks. Do you mean the hole in the gear, not the "hole in the VSS"? (Or the part of the VSS that the gear slides on to?)

ps: I have the correct gear for the 3:07, I'm just trying to figure out the VSS.

MrRenoman

I hope the early C4 sensors and the later sensors look alike. Mine has a half-moon shaft that goes into the driven gear. If you remove the gear from the sensor and look down directly at the shaft and main body you'll see it is offset from the from the large flat surface that has a notch (at least mine has a notch). The offset is even greater for the sensor used for lower gears.

Art

CTYANK2

Thanks Art.

Both of the VSS have no notch on the shaft. I slid the speed gear (2:59) on both half moon shafts and the gear slides down to the flat surface, same position both.

Since the VSS I bought looks identical (even the color) to my 2:59 VSS, has the same numbers, and the gear fits in the same position on both I will conclude that they are both for 2:59s and the one I bought couldn't be for a 3:07 rear.

I'll be calling the seller tommorrow to arrange a return.

CFI-EFI

Hole is the wrong word. I am picturing a conventional speedo gear sleeve. The two different sensors locate the driven gear on different centers, relative to the hole in the tailhousing. It is much like having 2 and 3 series diff cases for different ratio ring and pinion gears. The larger driven gears for 3.07 and lower gears have to be moved further from the drive gear, to fit. Is that clearer?

RACE ON!!!

CTYANK2

Yes, I think I've got it !