I need to verify that I'm getting a "pulse" signal from "B8" of the PCM (CKT # 817). The Service Manual states that I should be getting 6.5V w/ key ON, but I am NOT. Is there any difference in checking for Volts, than checking for "pulses"?

Here's my situation:

I've got a '95 LT1/4L60e installed in a '76 Vette and had originally wanted to keep the stock gauges, so purchased an aftermarket tailshaft housing that allowed the use of a mechanically-driven speedo...well, the SOB's sent me the completely incorrect Drive and Driven gears for my tire and rear-end combo (3.36 on 275/60/15's) and my speedo is about 30% off! They sent 21-Tooth Drive and 39-Tooth Driven, but my calculations indicate they should have sent an 18-Tooth Drive and 45-Tooth Driven (17 and 42 is pretty close as well).

I needed to remove my rear-end for bearing replacement, so decided this was the time to get at the tailhousing and correct the problem...well, finding these gears for the 4L60e (which didn't come w/ mechanical gears) seems to be a problem, although I thought it was the same as the 700-R4. (3 Chevy dealers were very kind and searched, but couldn't provide)

So, I can either get a gear-reduction box or go electronic on the speedo. I've already purchased a MECHANICAL Auto Meter Phantom speedo (to match my center gauges), so does it go up on eBay???

Prior to all of this, I was getting DTC 97, which involves CKT # 817, so turned-off that using TunerCat. Is this "turning-off" the signal on "B8" or just not setting a DTC? CKT # 817 seems to be an "out" signal, so never understood why the PCM would be getting information "in" from it...???

Sorry so long, just wanted to be sure all info was out there!

Thanks

 

I think I will try to answer your question. If I am interpeting your problem correctly, it seems that you are having a problem with CKT 817 which is the VSS signal to the electronic speedo in either a F or a Y body. This signal is also used by the ABS and Traction Control features of both car lines.

This signal is generated by the PCM based on the incoming signal from the PM Generator (VSS) at the transmission. The incoming signal to Pin A32 is a sine wave signal, the signal coming from CKT 817 is a square wave signal generated by the PCM.

Turning off the diagnostic in the PCM code does not disable the output of CKT 817. However if there is no VSS input to CKT 400 at pin A32 then there would be no output from CKT 817.

Now my questions is, are you even using a VSS input? It appears that you are using a mechanical interface at the tranny tail shaft. If this is the case then the PCM does not have a speed input. If you had this VSS sensor in place and working, you could use a device called a "cable X". It is made by Abbot Electronics, it takes a electrical (VSS Pulse) signal and coverts it to a mechanical cable drive to drive your mechanica speedo. Or you can just use the VSS signal, to drive a electronic speedo, then do the speedo recalibration in the PCM code.

 

As far as the speedo is concerned, No VSS signal; it's completely mechanical. The tranny adaptor has provisions for both; the mechanical speedo gear drive and the VSS (CKT's 400 & 401, Yellow & Purple). The PCM is receiving the info thru CKT 400, as I'm getting data when logging.

Questions about CKT 817 ("B8") and CKT 400 ("A32"): I either read or was told that CKT 817 is 4,000 pulses per mile, while CKT 400 is 128,000 pulses per mile... which of these operates most electronic speedos (in particular, the Autometer Phantom 5")? Is it a setting that can be changed (like tachs w/ choosing no. of cylinders)?

Thanks SO much for the info.

Have a safe Holiday weekend!

 

Actually the output from CKT 817 is the same as what the input is at CKT 400. The only difference is that the PCM changes the signal from a sine wave to a square wave. I beleive it is generated by an open collector circuit. Either signal should be able to drive a electronic speedo. I do know that the OE GM instrument cluster likes a clean square wave, I tried running an OE dash using a speedo calibrator unit from JET and it would not drive the speedo as it was not a true square wave. It was a noisy signal, and while the speedo would work a lower speeds (lower freq output) at higher speeds it would go nuts.

I can tell you if it doesn't work with the PCM output at CKT817, you could parallel the VSS output at the CKT 400. The Phantom speedo should have a high impedance input circuit that should be able to read the VSS signal without degrading the signal to the PCM.

 

Cool! I'm glad to have options, just in case there is something funny about my CKT 817, I'll tie into CKT 400.

OK, looks like I'll be visiting Summit.com... and will have a mechanical Autometer Phantom 5" Speedo available AFTER electronic speedo is in and working correctly!

Thanks again...your knowledge of Everything-LTx is amazing!

 

Don't give up on the mechanical speedo yet. I bet the 4L60E tail shaft is the same as the 4L60 and 700R4. Your 4L60E already has drive and driven gears installed. You just need to change them out for the correct ratio.

You can purchase the drive and driven gears from your local GM parts department or from Transmissioncenter.net
http://www.transmissioncenter.net/sp...n_______va.htm


18 tooth DRIVE gear part number 8640518
42 tooth DRIVE gear part number 25522491

[700R4 Speedometer Calibration] – Changing the drive and driven gears
http://forums.corvetteforum.com/show...572&forum_id=3

 

I would have put money on that bet too! A local Chevy dealer did have the 17-tooth DRIVE gear in stock (8640517), so had them pull it...it had a smaller inside diameter and needed the clip to retain it, while mine is a metal press-on gear similar to item #68-2 from the link you sent (near the very bottom, under "TH400 parts"). Outside diameters seemed to be the same. Then I was told that the carrier for the 40-45 tooth driven gear was a discontinued item, so just said "screw it".



I bet Transmission Center could have helped, but I have already put the 21-tooth back on and re-installed the driveshaft and rear-end. I can still go w/ a gear-reduction box, so the mechanical isn't totally out of the question just yet!

Thanks for the reply!

 

DR'76, that is good information to know about the 4L60E. Did GM make the shaft larger or smaller?
I just removed a Stewart-Warner speedo gear reducer unit because it seems to be worn out.

Mike

 

Miker,

Do you have the I.D. dimension for the plastic drive gear numbers you referenced? I found something in my garage that is starting to make sense of this situation, and that's another Reluctor Gear.

This is making me think that I was sent a Reluctor Gear, a Spacer, A Drive Gear, AND a "sleeve" to increase the diameter of the output shaft (as seen in the pic in my post above, going from left-to-right). I have NO IDEA as to why I would have another Reluctor Gear lying around...this is the only "electronic" tranny I've ever owned.

If this "sleeve" was needed to increase the diameter so the "new" reluctor gear could be used, then I'm going to say that the part numbers you referenced "probably" would have worked! The plastic drive gear I did look at seemed to have an Inside Diameter of about 1/8" or so smaller than the metal drive gear I'm using, so if that "sleeve" wasn't there, mybe it would have gone on the bare shaft???

If you have that inside diameter measurement, let me know, I'll see if it's the same as my "old" reluctor gear that originally came on the 4L60e.

 

The I.D. measurement is 1-3/16"

Keep us posted.

Mike

 

That's what it is...1 - 3/16".

I bet I was sent another Reluctor gear that allowed me to reposition it, which required the use of the "sleeve". (Looking at my pic again a few posts up, where the Drive gear is located is exactly where the Reluctor gear was originally positioned.) So you can see that it had to move forward almost an inch. This allowed the driven-gear housing to go into the location of the VSS. My "new" tailshaft housing has a VSS on the passenger-side, which is aligned with the new forward position of the Reluctor gear.

Man, that was certainly a head-scratcher, but now all makes sense... except for why my memory is so poor - it was only 2 years ago that I did this!!!

Thanks again for the research and replies!