VSS on TBI???? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

breezy · 11-10-2010, 11:07 PM

I'm still gathering info on my TBI swap. Almost there except for VSS.
Has anyone used a VSS from a 91 caprice. It is cable driven. Does the
signal have to be perfect. I'm not needing anything from the VSS except
a pulse for ECM. Basically to tell the ECM that truck is moving. Thanks

**LEEVON** · 11-10-2010, 11:50 PM

I did some research on it awhile back when I had an '87 silverado, are you using the vss unit on the tailshaft and the stock ECM? I know the pulse rate differs among models/years. They have some vss coverters at www.jagsthatrun.com

**LEEVON** · 11-10-2010, 11:51 PM

I did some research on it awhile back when I had an '87 silverado, are you using the vss unit on the tailshaft from the carprice and a truck ECM? I know the pulse rate differs among models/years. They have some vss coverters at www.jagsthatrun.com

breezy · 11-11-2010, 12:06 AM

I am using the VSS on the tailshaft. I know they sell these, but I would rather use a gm piece that I can buy at work. The caprice had a cable driven VSS that has a built in sensor for ECM. I'm finding out I may have to wire in a buffer or DRAC. I was hoping someone had figured this out without spending more money.

69sixpackbee · 11-11-2010, 12:25 AM

Use the one from JTR. I did and it worked flawlessly wit a TBI and a EBL modified '7747 ECU.

Bud

breezy · 11-11-2010, 12:30 AM

I know those probably work. I want a direct plug into my tranny.

**LEEVON** · 11-11-2010, 12:57 AM

I know it doesn't help but I remember reading a thread where a guy actually built a DRAC. What year ECM are you using? They're cheap can you just get one that is compatible with a '91 caprice?

breezy · 11-11-2010, 01:05 AM

That's what I'm loking into. I have the info on fixing the drac with dip switches. I just need to find the wiring diagram for VSS to DRAC to ECM? What I don't understand is the painfull harness or others have two wires for the VSS when the computer only shows one input for VSS signal. I'm confused??

breezy · 11-11-2010, 01:12 AM

I have seen this on several places also. This is from stealth's website.
There are four types of VSS signals required by the engine computer (ECM):
1. The two-pulse (2000 pulses per mile) square wave (D.C. current or direct current) used on all TBI engines through 1992, all computer-controlled-carbureted engines, and on 1985–1989 TPI engines.
So if I read that correctly the 91 caprice with TBI should have a 2000 pulse per mile VSS. Somewhere else i read that 2000 pulse per mile equals 2 pulses per revolution which is what I need for the ECM 7747. I'm really thinking I just need some kinda signal to tell the computer I'm moving. I don't want the ECM to think I'm doing like 600 MPH the ECM may shut down for safety. Although I don't belive the early ECM's shut down like the newer ones. I know I had a 89 S-10 that would never quit until you floated the valves.

breezy · 11-11-2010, 09:42 PM

So here is what I found out today. I called a GM tech that has done these before. Ask him why I could not use the oem VSS from a 91 caprice and wire it to the ECM like painfull wants. He told me that if I was not using the ECM to control anything like TCC or lockup this should work fine. Said to ground one wire from the VSS and send the other to the high side input on the ECM. Don't no if it will work but I'm going for it on this project. If anyone has any input chime in.

11-10-2010, 11:07 PM	#1
breezy Registered User Join Date: Oct 2010 Location: Palaskifield,Mo. Posts: 71	VSS on TBI???? I'm still gathering info on my TBI swap. Almost there except for VSS. Has anyone used a VSS from a 91 caprice. It is cable driven. Does the signal have to be perfect. I'm not needing anything from the VSS except a pulse for ECM. Basically to tell the ECM that truck is moving. Thanks

11-10-2010, 11:50 PM	#2
LEEVON Moderator Join Date: Jul 2006 Location: Ozark, MO Posts: 4,888	Re: VSS on TBI???? I did some research on it awhile back when I had an '87 silverado, are you using the vss unit on the tailshaft and the stock ECM? I know the pulse rate differs among models/years. They have some vss coverters at www.jagsthatrun.com __________________ '20 Silverado Trail Boss ~ '17 Tahoe ~ '79 K15 Sierra Grande ~ '76 Blazer 2wd ~ '71 Cheyenne swb ~ '55 Pontiac Safari ~'50 3100 bagged ~ '80 Wife ~ Late model kids

11-10-2010, 11:51 PM	#3
LEEVON Moderator Join Date: Jul 2006 Location: Ozark, MO Posts: 4,888	Re: VSS on TBI???? I did some research on it awhile back when I had an '87 silverado, are you using the vss unit on the tailshaft from the carprice and a truck ECM? I know the pulse rate differs among models/years. They have some vss coverters at www.jagsthatrun.com __________________ '20 Silverado Trail Boss ~ '17 Tahoe ~ '79 K15 Sierra Grande ~ '76 Blazer 2wd ~ '71 Cheyenne swb ~ '55 Pontiac Safari ~'50 3100 bagged ~ '80 Wife ~ Late model kids

11-11-2010, 12:06 AM	#4
breezy Registered User Join Date: Oct 2010 Location: Palaskifield,Mo. Posts: 71	Re: VSS on TBI???? I am using the VSS on the tailshaft. I know they sell these, but I would rather use a gm piece that I can buy at work. The caprice had a cable driven VSS that has a built in sensor for ECM. I'm finding out I may have to wire in a buffer or DRAC. I was hoping someone had figured this out without spending more money.

11-11-2010, 12:25 AM	#5
69sixpackbee Registered User Join Date: Jan 2001 Location: Sabinal, Texas Posts: 1,706	Re: VSS on TBI???? Use the one from JTR. I did and it worked flawlessly wit a TBI and a EBL modified '7747 ECU. Bud

11-11-2010, 12:30 AM	#6
breezy Registered User Join Date: Oct 2010 Location: Palaskifield,Mo. Posts: 71	Re: VSS on TBI???? I know those probably work. I want a direct plug into my tranny.

11-11-2010, 12:57 AM	#7
LEEVON Moderator Join Date: Jul 2006 Location: Ozark, MO Posts: 4,888	Re: VSS on TBI???? I know it doesn't help but I remember reading a thread where a guy actually built a DRAC. What year ECM are you using? They're cheap can you just get one that is compatible with a '91 caprice? __________________ '20 Silverado Trail Boss ~ '17 Tahoe ~ '79 K15 Sierra Grande ~ '76 Blazer 2wd ~ '71 Cheyenne swb ~ '55 Pontiac Safari ~'50 3100 bagged ~ '80 Wife ~ Late model kids Last edited by LEEVON; 11-11-2010 at 12:58 AM.

11-11-2010, 01:05 AM	#8
breezy Registered User Join Date: Oct 2010 Location: Palaskifield,Mo. Posts: 71	Re: VSS on TBI???? That's what I'm loking into. I have the info on fixing the drac with dip switches. I just need to find the wiring diagram for VSS to DRAC to ECM? What I don't understand is the painfull harness or others have two wires for the VSS when the computer only shows one input for VSS signal. I'm confused??

11-11-2010, 01:12 AM	#9
breezy Registered User Join Date: Oct 2010 Location: Palaskifield,Mo. Posts: 71	Re: VSS on TBI???? I have seen this on several places also. This is from stealth's website. There are four types of VSS signals required by the engine computer (ECM): 1. The two-pulse (2000 pulses per mile) square wave (D.C. current or direct current) used on all TBI engines through 1992, all computer-controlled-carbureted engines, and on 1985–1989 TPI engines. So if I read that correctly the 91 caprice with TBI should have a 2000 pulse per mile VSS. Somewhere else i read that 2000 pulse per mile equals 2 pulses per revolution which is what I need for the ECM 7747. I'm really thinking I just need some kinda signal to tell the computer I'm moving. I don't want the ECM to think I'm doing like 600 MPH the ECM may shut down for safety. Although I don't belive the early ECM's shut down like the newer ones. I know I had a 89 S-10 that would never quit until you floated the valves.

11-11-2010, 09:42 PM	#10
breezy Registered User Join Date: Oct 2010 Location: Palaskifield,Mo. Posts: 71	Re: VSS on TBI???? So here is what I found out today. I called a GM tech that has done these before. Ask him why I could not use the oem VSS from a 91 caprice and wire it to the ECM like painfull wants. He told me that if I was not using the ECM to control anything like TCC or lockup this should work fine. Said to ground one wire from the VSS and send the other to the high side input on the ECM. Don't no if it will work but I'm going for it on this project. If anyone has any input chime in.



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