Thinkin im going to be need a VSS. Now which one do i go with?

[rokcrawlin85]

I've been reading up on this site about VSS and how it supposedly made idle and tuning much easier for some... Is it really a noticable difference at idle? Mine currently wants to stumble and die when coming to a stop, (not everytime but often) amoung other issues that i just need to get tuned. I'm currently using the 8747 ECM. (I know slow and ancient but i'm not after peak power just good and smooth running for my 4 wheel toy)
With the VSS, how do i know how many PPM's i need to make up for 4.10 gears and 37" tall tires? From what i can tell i need a cable to electric VSS, that will wire directly into the ECM, and still have the cable? By the way this is on a TBI converted '85 K5 Blazer with a 700R4. Feel free to point me to a link that i've overlooked that explains all this...

[gregs78cam]

I'm currently using the 8747 ECM. (I know slow and ancient but i'm not after peak power just good and smooth running for my 4 wheel toy)

Upgrading to a later PCM is not just helpful in tuning for more power, it makes tuning in general easier by giving you more data points to look at when making tuning decisions. And also gives you many more parameters to get things just right.


With the VSS, how do i know how many PPM's i need to make up for 4.10 gears and 37" tall tires? From what i can tell i need a cable to electric VSS, that will wire directly into the ECM, and still have the cable? By the way this is on a TBI converted '85 K5 Blazer with a 700R4. Feel free to point me to a link that i've overlooked that explains all this...

Regardless of tire size and gear ratio, the ECM wants to see 2000 Pulses per mile from the VSS. If your speedometer reads correctly, then you should have approximately 1000 cable revolutions per mile. That would mean you need a 2 pulse per revolution sender. If your speedo does not read correctly then I would suggest getting it right by changing the speedo gears in the trans, or just go junkyardin' and find the appropriate cable multipiers, then just set the jumpers in the DRAC/VSSB to the right positions for your application, which you would have to do in either case.

[rokcrawlin85]

then just set the jumpers in the DRAC/VSSB to the right positions for your application, which you would have to do in either case.

I'm new to this whole electronic VSS thing... How do i do this and and what exactly is changing these settings doing for the ECM? DRAC is some sort of buffer correct? Which is only used if i run a speedometer with the signal running from the VSS to the guage and then to the ECM?

[EagleMark]

Yes the DRAC/VSSB is a little white box with electronics in it with sets of jumpers which correct output for speedometer, ABS, Cruise control and transmission speeds. Corvette even had a radio volume hooked up to this to rasise and lower volume with speed.

Try this thread for more info.
http://www.gearhead-efi.com/Fuel-Injection/showthread.php?641-VSS-Speed-Sensors

Since you have a 1985 Blazer there's a good chance your speedometer already has this VSS on it? Then it goes to DRAC/VSSB. If not you may be able to JY a speedo to your truck from 1987 to 1991 Blazer/Suburban.

[PJG1173]

basically you solder 7 pin dip switches into the vss control board that is attached to your pcm. by setting the sequence of on/off switches it will alter the pulses it sends to the pcm. I did mine in about 45 min following these instructions http://www.tbichips.com/drac/

[Nasty-Z]

Since you have a 1985 Blazer there's a good chance your speedometer already has this VSS on it? Then it goes to DRAC/VSSB. If not you may be able to JY a speedo to your truck from 1987 to 1991 Blazer/Suburban.

Your cluster should have already / or have the provision for the optical eye speed sensor , simply install the sensor into the cluster (or check if it is already there) , 3 wire hook up if it is not already done , +12v ignition power , ground and a siginal to the ECM. The speed sensor buffer is already atttached to the optical eye so there is nothing else to wire in .

NAPA P/N VSS203 if you want to see one in person , if you don't have one in the truck already you might have to pirate a J/Y for the connector as I have not found them aftermarket.

FWIW , this sensor will also supply a siginal to the later '7427 PCM if you decide to upgrade , as long as you stay with the 700R4 and do not run an electronic trans.

HTH

TOM

[rokcrawlin85]

Thanks guys! seems easy enough to do. Ill have to pull my cluster out and see what i can find out. Now as far as benifits? ... Noticeable gains with idle and tuning?

[EagleMark]

I had a 1227747 TBI conversion on a 345 IH Scout II and ran for a year tuned. Added a VSS and everything got better! Greg had done a 16197427 PCM and ran without VSS and said same thing, much better with!

[rokcrawlin85]

I've been searching and searching about the DRAC. I believe i got that all sorted out in my head. But now... Can i pull a gauge clucter, speedo and DRAC from a '90 burb or truck and just swap out the speedo i currently have with the new one? Do those speedo's use gears to send signal, or do they use the pulse rings?


I haven't looked for that optical eye yet, but i'm highly doubting ill have it since i have a cable driven spedometer gauge. So im betting on having to go the DRAC route. That and from what i've read, the DRAC makes it easy to change the speedometer reading for gears and tire size whenever they change.

[Six_Shooter]

You don't need a DRAC or VSSB for the C3 style ECMs, when using an optical trigger, or the Painless Performance style VSS.

If it was me, I'd look at getting an optical VSS from a similar year full size truck with cruise, S-10s up to at least 1991 had this style VSS, and could be used in your cluster, I'm sure. Even if there are not factory provisions for it, you can make the provisions.
These VSS attach to the back of the speedo, and use the metal arms that are part of the speedo gear as the reflective trigger.

[EagleMark]

I've been searching and searching about the DRAC. I believe i got that all sorted out in my head. But now... Can i pull a gauge clucter, speedo and DRAC from a '90 burb or truck and just swap out the speedo i currently have with the new one? Do those speedo's use gears to send signal, or do they use the pulse rings?


I haven't looked for that optical eye yet, but i'm highly doubting ill have it since i have a cable driven spedometer gauge. So im betting on having to go the DRAC route. That and from what i've read, the DRAC makes it easy to change the speedometer reading for gears and tire size whenever they change.Nasty Z sadi you could add it to your speedo, I have replaced the speedo, CK5 may be able to tell you if you can use the entire cluster. It's the same thing as Six Shooter is talking about.


You don't need a DRAC or VSSB for the C3 style ECMs, when using an optical trigger, or the Painless Performance style VSS.

If it was me, I'd look at getting an optical VSS from a similar year full size truck with cruise, S-10s up to at least 1991 had this style VSS, and could be used in your cluster, I'm sure. Even if there are not factory provisions for it, you can make the provisions.
These VSS attach to the back of the speedo, and use the metal arms that are part of the speedo gear as the reflective trigger.Your right he does not need DRAC/VSSB with C3 but he has a 700R4, so does he need it for that to work TCC lockup correctly from bin adjustments?

[rokcrawlin85]

Ok, so this optical eye thing, how does it output a speed signal to the ecm? I know that one of the wires will be pinned into the ECM but how does it read how fast the actual vehicle is going? From the speedometer cable that goes to the VSS? I only ask because my speedometer cable has been burned through from the exhuast, and no longer reads on the guage, and i need to get it replaced.

[EagleMark]

I don't know if what I am mentioning and Six is mentioning are same thing. But the ones I have seen that may already be in your truck is in speedo. The way Six was talking it goes on speedo cable? Yes speedometer cable would have to be working.

[Nasty-Z]

The OP already has what he needs , there is a place in his original cluster for the optical eye , one screw holds it in . I'll run $5 that it is already there :innocent2:

If not , my previous post gave the NAPA part # , as well as the wiring for it .

TOM

[EagleMark]

That year truck he has was CCC in many states. I have found the VSS in carb trucks too of that year as GM must have already had speedos made or ran out of ones that did not have it.

[gregs78cam]

I've got one of those optical speedometer mount deals in the shop. They are interesting.

[Six_Shooter]

Yeah, the optical sensor mounts to the back of the speedo, is what I'm talking about.

I'm using one in my Datsun. I had to modify the speedo to do so, FWIW.

The Optical VSS, needs simply an ignition connection, a ground and the output connected to the proper input on the ECM.
http://domestic.3400z24.com/sixshooter/My vehicles/Datsun/Under hood/ECM and Wiring/Speedo/optical VSS.jpg

The silver part that is between the aluminium back piece and the odo in the picture, spins with the speedo cable, which reflects the IR light emitted by the senor and is read back by the detector. This is a Datsun speedo, but the GM speedo works on the same principle.
http://domestic.3400z24.com/sixshooter/My vehicles/Datsun/Under hood/ECM and Wiring/Speedo/speedo head top.jpg

Close up of the optical VSS head:
http://domestic.3400z24.com/sixshooter/My vehicles/Datsun/Under hood/ECM and Wiring/Speedo/optical VSS head.jpg

This was a test fit into my modified speedo, just to illustrate the location:
http://domestic.3400z24.com/sixshooter/My vehicles/Datsun/Under hood/ECM and Wiring/Speedo/optical VSS test fit.jpg

[EagleMark]

That one post is worthy of a writeup! :thumbsup:

[rokcrawlin85]

Thanks guys for all your help!!! This has been great. I'll be working on my K5 this weekend and ill take a peak and see if i have the optical there and if not, i know the part number i need. Ill be picking up a speedo cable too. I guess ill have to do a little math to figure out what speedo gear i need to swap in to get my speedometer reading right so the ECM is seeing the right numbers. If i remember right i think it was reading 7 MPH slow when i was running 35" tires, now 37"ers... So for correct readings it wants to see 1000 revolutions per mile as the driveline spins around correct?

[kunsan1987]

I have 2 versions of this buffer.the 3 wire version was for cruise control and is 2k pulses.the 4 wire one has 2 outputs .both are 2k pulse.this pic is the 4 wire the yellow wire is for ecm,the brown is for cruise control.pink/blk 12 power,blk/wht is ground.

2046

[rokcrawlin85]

These posts with pics really clarify what i'm looking for and how to get it wired right!!! Thanks

[EagleMark]

Yeah thanks Six and kunsan1987 for the pictures! :thumbsup: