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Generating a VSS signal
- Thread starter Justin V
- Start date
What are you trying to do? GM used a fair number of different VSS outputs/setups over the years.
You need the 2000PPM (pulse per mile) setup.
Easiest would be running the '82-89 C/K/R/V speedometer with VSS, but outside of that you'll definitely need something like that Rostra unit. There might be a way to change what the TBI ECM is looking for in terms of signal, but it would require a new PROM if even possible.
I'd want to talk to Rostra first, there seems to be great ignorance about the trucks when it comes to EFI. The cars (older GM EFI) all ran 4 or 8 pulse units AFAIK, while the trucks were 2000PPM. I don't recall seeing 2000PPM units like that Rostra, but they may be out there. I just did a quick search, and couldn't find one.
Easiest would be running the '82-89 C/K/R/V speedometer with VSS, but outside of that you'll definitely need something like that Rostra unit. There might be a way to change what the TBI ECM is looking for in terms of signal, but it would require a new PROM if even possible.
I'd want to talk to Rostra first, there seems to be great ignorance about the trucks when it comes to EFI. The cars (older GM EFI) all ran 4 or 8 pulse units AFAIK, while the trucks were 2000PPM. I don't recall seeing 2000PPM units like that Rostra, but they may be out there. I just did a quick search, and couldn't find one.
Well, quick math says roughly 2230 revolutions of the driveshaft per mile with my gears and tires. 8 pulses per revolution means means way too many if I need 2000 pulses per mile...
I use the optical VSS sensor/buffer setup on the back of the speedo for the TBI swap in my 85. I tried on if the inline advance adapters units that drives off the speedo cable on the t-case but it wasn't very reliable, the drive pin came loose and nothing worked.
Swapping in a later cluster (or at least VSS speedo) is probably the cheapest most reliable solution
Swapping in a later cluster (or at least VSS speedo) is probably the cheapest most reliable solution
Optical VSS on the back of the speedometer is definitely the easiest way to go for these trucks.
offroad design offers rear tail housing mod with parts to make 205 vss plug-n-play with gm parts .
http://offroaddesign.com/catalog/NP205VSS.htm
http://offroaddesign.com/catalog/NP205VSS.htm
90-91 electric speedo only on our trucks from what I have always been told .
You mean for electronic speedo?
'89 back to early eighties (?) used the shadow wheel in the speedometer, correct?
ya full electric speedo 90-91. 80's some had the shadow wheel . I have seen some NOT .
We've started to lean toward keeping a VSS since the later model stuff (don't know about TBI) does a good job of keeping the engine idle up so it doesn't die when you brake and steer. Lots of stand alone systems don't do this but compensate in other ways. Our stand alone TBI systems always worked well without the VSS but it could have been from other factors. My edelbrock multipoint system has never been a problem either but we've seen problems in our 7.4L '99 truck when the VSS quit and gen 3/4 stand alone stuff too.
Generally with TBI that seems to be the only complaint (problems with stalling under deceleration), if there is one.
As a potential side benefit, from my experience fuel economy can be greatly increased (~10%) by allowing the ECM to cut fuel on decel, which doesn't happen without VSS. I don't know TBI as well, but TPI of the same vintage uses the VSS signal to determine when to cut fuel to the engine on deceleration, so it's possible TBI does too, since stalling under decel is sometimes a symptom of missing/inoperative VSS.
VSS with TBI does not seem to have as much of an impact on the idle quality, but there are probably a host of reasons why that may be the case.
As a potential side benefit, from my experience fuel economy can be greatly increased (~10%) by allowing the ECM to cut fuel on decel, which doesn't happen without VSS. I don't know TBI as well, but TPI of the same vintage uses the VSS signal to determine when to cut fuel to the engine on deceleration, so it's possible TBI does too, since stalling under decel is sometimes a symptom of missing/inoperative VSS.
VSS with TBI does not seem to have as much of an impact on the idle quality, but there are probably a host of reasons why that may be the case.
To note on that:
I had the stalling on decel issue with mine (VSS removed from program)
The only malfunction was that it didn't like the reading on the TPS when closed. Once that was corrected, I've had no drivability issues
Had a long conversation with Troy at Howell about it. When they remove the VSS from the program, it relies on the TPS movement (or not for cruise lean out) to replace the input of what would be the VSS
But with our need to have a chip burned for a custom tune, changing the speedo is certainly the best option
I had the stalling on decel issue with mine (VSS removed from program)
The only malfunction was that it didn't like the reading on the TPS when closed. Once that was corrected, I've had no drivability issues
Had a long conversation with Troy at Howell about it. When they remove the VSS from the program, it relies on the TPS movement (or not for cruise lean out) to replace the input of what would be the VSS
But with our need to have a chip burned for a custom tune, changing the speedo is certainly the best option
First time posting, ever, to anything, but my 465/205 swap out and VSS signaling issues recently had ME (not the truck) running in the red so I felt compelled to offer some input.
Sorry for the long dissertation. I figured I would break down my final solution which worked out for me and will hopefully help some others not run down the same 2000PPM rabbit hole that I just came out of.
Did your 1990 TBI come out of a rig with a NP241 and a DRAC?
Are you transplanting a DRAC to provide signal to your TBI, EGR etc. only while using a mechanical speedo or are you also transplanting an electronic speedo into your 79 as well?
I don't believe you are looking for a 2000PPM signal unless you are only looking to run a stock electronic speedo from a 1990 (or whatever other year might have the 2000PPM signal electronic speedo) I believe there are other discussions in here which clarify what years have which speedos
The reluctor ring & sensor on the 1990 NP241 is 40PPR (40 Pulse Per Driveshaft Revolution) The electronic speedo itself may need a 2000PPM signal but that signal is converted via the DRAC. As you have already done you need to convert your tire size & gearing to determine what the correct PPM / PPR will be to provide to the DRAC. Once the DRAC is satisfied with the correct signal it will do it's own work and redistribute the correct signaling to the other devices.
This was my breakdown to convert the approximate PPM / PPR signal that I needed.
5280' per mile divided by tire travel per revolution (9.16' for 35" tires) = 576.41revolutions per mile x 3.42 (gear ratio) x 40 (factory PPR) EQUALS 78,852 PPM.
You've already done your math and came up with 2230, now you just need to multiply that by how many PPR your factory VSS was sending to the DRAC originally; I.E. multiply 2230 x 40 (factory PPR) = 89,440 PPM.
Dakota Digital SEN-01-1 w/ SGI-5E Universal Speedometer Signal Interface worked for me. However, I don't think you would want the SEN-01-1 unless you are changing to the 1990 DRAC fed speedo. On your 79 I think you would need the SEN-01-4160 which is a sensor with a speedo cable bypass/pass-through which will allow for you to run the VSS along with your stock speedo cable & speedo (by the way, this is what the Rostra unit appears to do although the Summit Q&A is somewhat misleading, whereas Q/A 1 states it will allow for a mechanical speedo to be run while Q/A 2 states that it will provide signal to an electronic speedo; which technically is correct but there are aftermarket VSS units that allow for mechanical bypass/pass-through to run your stock mechanical speedo and also aftermarket VSS units to run an electronic speedo without cable pass-through) By running the pass-through cable VSS you will have A.) Stock mechanical speedo WITH B.) a VSS to provide signal to your DRAC which would then provide input to the TBI, TC lockup, EGR etc. The SGI-5E Universal Speedometer Signal Interface will allow for you to electronically modify the manufacturers setting of 8,000PPM being sent from the (SEN-01-1 or SEN-01-4160) VSS to a high speed signal of 128,000PPM. Once the signal is converted to 128,000PPM the SGI-5E will then allow for you to calibrate the signal back down to the 78 - 80K needed. To clarify, this device is labeled a Universal Speedometer Signal Interface but what it really does is electronically increases or decreases the rate of signaling from the aftermarket VSS sending unit; therefore I wouldn't get hung up on the equipment description, just be aware of what the device will do. In other words, I like to think of it as a Universal DRAC Signal Interface. And yes I know I started to sound like Ricky Bobby for a second there. In any case, whether you run a Dakota Digital, Rostra or JTR speed sensor I think you will need an interface such as this to be able to modify your pulse settings from PPR / PPM.
A few other notes,,,,,
- If your running a 205 the ORD setup that sweetk30 linked to is probably the way to go, it takes out all of everything described above and would reduce this entire write up to "Install the kit, Connect the sensor, Pedal on the Right"
- I don't know what trans your running or if you need signal to a tc lockup but keep that in mind. I'm running a 465 so didn't have that concern and just cut that sensor out.
- In terms of confirming final signaling calibration I don't know how you would do that without an electronic speedo hooked up after the DRAC. All I know is that my DRAC is hooked up, my speedo works and is close to accurate, and I'm not dropping to 2-300 RPM on decel and idle (which was happening prior to installing and adjusting this equipment) I'm sure that there are some smarter guys on CK5 that can provide some higher level input on how to confirm that the signaling calibration is spot on without using the primitive Pete method of reado the speedo and go.
Sorry for the long dissertation. I figured I would break down my final solution which worked out for me and will hopefully help some others not run down the same 2000PPM rabbit hole that I just came out of.
Did your 1990 TBI come out of a rig with a NP241 and a DRAC?
Are you transplanting a DRAC to provide signal to your TBI, EGR etc. only while using a mechanical speedo or are you also transplanting an electronic speedo into your 79 as well?
I don't believe you are looking for a 2000PPM signal unless you are only looking to run a stock electronic speedo from a 1990 (or whatever other year might have the 2000PPM signal electronic speedo) I believe there are other discussions in here which clarify what years have which speedos
The reluctor ring & sensor on the 1990 NP241 is 40PPR (40 Pulse Per Driveshaft Revolution) The electronic speedo itself may need a 2000PPM signal but that signal is converted via the DRAC. As you have already done you need to convert your tire size & gearing to determine what the correct PPM / PPR will be to provide to the DRAC. Once the DRAC is satisfied with the correct signal it will do it's own work and redistribute the correct signaling to the other devices.
This was my breakdown to convert the approximate PPM / PPR signal that I needed.
5280' per mile divided by tire travel per revolution (9.16' for 35" tires) = 576.41revolutions per mile x 3.42 (gear ratio) x 40 (factory PPR) EQUALS 78,852 PPM.
You've already done your math and came up with 2230, now you just need to multiply that by how many PPR your factory VSS was sending to the DRAC originally; I.E. multiply 2230 x 40 (factory PPR) = 89,440 PPM.
Dakota Digital SEN-01-1 w/ SGI-5E Universal Speedometer Signal Interface worked for me. However, I don't think you would want the SEN-01-1 unless you are changing to the 1990 DRAC fed speedo. On your 79 I think you would need the SEN-01-4160 which is a sensor with a speedo cable bypass/pass-through which will allow for you to run the VSS along with your stock speedo cable & speedo (by the way, this is what the Rostra unit appears to do although the Summit Q&A is somewhat misleading, whereas Q/A 1 states it will allow for a mechanical speedo to be run while Q/A 2 states that it will provide signal to an electronic speedo; which technically is correct but there are aftermarket VSS units that allow for mechanical bypass/pass-through to run your stock mechanical speedo and also aftermarket VSS units to run an electronic speedo without cable pass-through) By running the pass-through cable VSS you will have A.) Stock mechanical speedo WITH B.) a VSS to provide signal to your DRAC which would then provide input to the TBI, TC lockup, EGR etc. The SGI-5E Universal Speedometer Signal Interface will allow for you to electronically modify the manufacturers setting of 8,000PPM being sent from the (SEN-01-1 or SEN-01-4160) VSS to a high speed signal of 128,000PPM. Once the signal is converted to 128,000PPM the SGI-5E will then allow for you to calibrate the signal back down to the 78 - 80K needed. To clarify, this device is labeled a Universal Speedometer Signal Interface but what it really does is electronically increases or decreases the rate of signaling from the aftermarket VSS sending unit; therefore I wouldn't get hung up on the equipment description, just be aware of what the device will do. In other words, I like to think of it as a Universal DRAC Signal Interface. And yes I know I started to sound like Ricky Bobby for a second there. In any case, whether you run a Dakota Digital, Rostra or JTR speed sensor I think you will need an interface such as this to be able to modify your pulse settings from PPR / PPM.
A few other notes,,,,,
- If your running a 205 the ORD setup that sweetk30 linked to is probably the way to go, it takes out all of everything described above and would reduce this entire write up to "Install the kit, Connect the sensor, Pedal on the Right"
- I don't know what trans your running or if you need signal to a tc lockup but keep that in mind. I'm running a 465 so didn't have that concern and just cut that sensor out.
- In terms of confirming final signaling calibration I don't know how you would do that without an electronic speedo hooked up after the DRAC. All I know is that my DRAC is hooked up, my speedo works and is close to accurate, and I'm not dropping to 2-300 RPM on decel and idle (which was happening prior to installing and adjusting this equipment) I'm sure that there are some smarter guys on CK5 that can provide some higher level input on how to confirm that the signaling calibration is spot on without using the primitive Pete method of reado the speedo and go.
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