What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Electric Speedo VSS

Psymantis

Psymantis

Member
Joined
Apr 14, 2019
Posts
7
Reaction score
2
Location
Colorado
Hi all, I have a 1987 4x4 Blazer with original ECM and 700R4 trans that I am swapping the original gauges out on to new Auto Meter gauges. I have nearly everything connected and the pulse generator installed running from the trans to the new electric speedo but I'm not sure how to properly connect the electric speedo to the ECM for VSS. I still have the VSS buffer in the truck but obviously the old optical sensor running from a mechanical speedo to it isn't going to do anything for me with the new electric speedo.

If I cut the brown wire coming out of the old VSS buffer (which from what I've read is the VSS out to ECM) and connect it directly to the "Out" tab on the back of the Auto Meter speedo will it send the correct signal to the ECM for VSS? Auto Meter says the signal from the "Out" tab on their speedo is +5 volt DC Square wave and "may be able to be used as a VSS signal with some OEM ECM's".

I've called Auto Meter and they didn't know what the correct way is to make the connection to the ECM. Any help is much appreciated.
 
The GM ECM is looking for 2000 pulse per mile (PPM). I don't believe that is what autometer is saying their output is.

I wouldn't just hook it up and hope it works. What input does the Autometer require from the pulse generator? There were a few factory options for signal generation, I'm not sure if the car VSS setups can be made to work in the transfercase (as I recall those used pulse generators, if jagsthatrun.com still exists, they had a VSS section that covered GM VSS quite well, but ignore the truck portion, it's incorrect), the truck NP241 from 90-91 with VSS gets you a DRAC, which has a multitude of outputs, I want to say at least 2k, 4k, and 128k PPM.

The autometer doesn't have provisions to feed the GM cruise control of your vintage? That's the same signal the ECM needs.
 
Psymantis said:
Hi all, I have a 1987 4x4 Blazer with original ECM and 700R4 trans that I am swapping the original gauges out on to new Auto Meter gauges. I have nearly everything connected and the pulse generator installed running from the trans to the new electric speedo but I'm not sure how to properly connect the electric speedo to the ECM for VSS. I still have the VSS buffer in the truck but obviously the old optical sensor running from a mechanical speedo to it isn't going to do anything for me with the new electric speedo.

If I cut the brown wire coming out of the old VSS buffer (which from what I've read is the VSS out to ECM) and connect it directly to the "Out" tab on the back of the Auto Meter speedo will it send the correct signal to the ECM for VSS? Auto Meter says the signal from the "Out" tab on their speedo is +5 volt DC Square wave and "may be able to be used as a VSS signal with some OEM ECM's".

I've called Auto Meter and they didn't know what the correct way is to make the connection to the ECM. Any help is much appreciated.
Click to expand...

i use an autometer Speedometer with VSS output and connected it directly to the vss wire on the ECM. Works fine. The ECM is a 7747 TBI from I think 1987 or 89. Not sure if they use different outputs on different Speedo’s though.
 
bp71k5 said:
i use an autometer Speedometer with VSS output and connected it directly to the vss wire on the ECM. Works fine. The ECM is a 7747 TBI from I think 1987 or 89. Not sure if they use different outputs on different Speedo’s though.
Click to expand...
bp71k5 - Do you remember what the pulses per mile or pulses per revolution was on the transmission pulse generator you used? Auto Meter said that whatever PPR/PPM the pulse generator is putting out that will be the same as what comes out of the speedo. The pulse generator I have is putting out 16 PPR.
 
dyeager535 said:
The GM ECM is looking for 2000 pulse per mile (PPM). I don't believe that is what autometer is saying their output is.

I wouldn't just hook it up and hope it works. What input does the Autometer require from the pulse generator? There were a few factory options for signal generation, I'm not sure if the car VSS setups can be made to work in the transfercase (as I recall those used pulse generators, if jagsthatrun.com still exists, they had a VSS section that covered GM VSS quite well, but ignore the truck portion, it's incorrect), the truck NP241 from 90-91 with VSS gets you a DRAC, which has a multitude of outputs, I want to say at least 2k, 4k, and 128k PPM.

The autometer doesn't have provisions to feed the GM cruise control of your vintage? That's the same signal the ECM needs.
Click to expand...
dyeager535 - I just came across this module that DD offers https://www.dakotadigital.com/index...id=1192/category_id=287/mode=prod/prd1192.htm for converting the pulse output to 2000 PPM. I read thru the manual and I'm thinking something like this would work if I can't just connect the output on my speedo directly to the ECM.

Does this look like it would work to you?
 
For $100, and yet another component, I'd go VSS/DRAC if possible.

If people have been able to run the signal wire direct to the ECM, then Autometer should sure as hell be able to tell you what the output is.

TBI isn't real picky, people that don't run VSS usually only notice some tendency to stall on deceleration without it, so maybe if it's getting SOME kind of signal, it works good enough? I really don't know. But given that the GM systems have been around forever, and are extremely well understood, the manufacturer should definitely be able to tell you X will work with Y, etc. It doesn't get much simpler than TBI with a '7747 ECM.
 
Psymantis said:
bp71k5 - Do you remember what the pulses per mile or pulses per revolution was on the transmission pulse generator you used? Auto Meter said that whatever PPR/PPM the pulse generator is putting out that will be the same as what comes out of the speedo. The pulse generator I have is putting out 16 PPR.
Click to expand...
I used a painless performance vss adaptor that plugs into the transfer case where the old mechanical speedometer plugged in.
Psymantis said:
bp71k5 - Do you remember what the pulses per mile or pulses per revolution was on the transmission pulse generator you used? Auto Meter said that whatever PPR/PPM the pulse generator is putting out that will be the same as what comes out of the speedo. The pulse generator I have is putting out 16 PPR.
Click to expand...

I think I used a Jagsthatrun vss. Don’t remember which one and they look a lot more expensive than I remember. I could be remembering wrong though. They had several different options for pulses. I’d try summit and see what they have.
 
dyeager535 said:
For $100, and yet another component, I'd go VSS/DRAC if possible.

If people have been able to run the signal wire direct to the ECM, then Autometer should sure as hell be able to tell you what the output is.

TBI isn't real picky, people that don't run VSS usually only notice some tendency to stall on deceleration without it, so maybe if it's getting SOME kind of signal, it works good enough? I really don't know. But given that the GM systems have been around forever, and are extremely well understood, the manufacturer should definitely be able to tell you X will work with Y, etc. It doesn't get much simpler than TBI with a '7747 ECM.
Click to expand...
Thanks for all the help dyeager, I appreciate it. I'll see if I can find a DRAC. I'll probably end up going with whatever option I can get delivered the soonest so I can use my truck again.

Auto Meter says that whatever the pulse count is coming from the generator on the trans is what their gauge will output. Since the pulse generator I have is 16 PPR I'm nowhere near that 2000 PPM you mentioned so I'm guessing I have to go either DRAC or use that Dakota Digital module.
 
bp71k5 said:
I used a painless performance vss adaptor that plugs into the transfer case where the old mechanical speedometer plugged in.


I think I used a Jagsthatrun vss. Don’t remember which one and they look a lot more expensive than I remember. I could be remembering wrong though. They had several different options for pulses. I’d try summit and see what they have.
Click to expand...
Thanks for the info bp71k5, I'll see what I can find.
 
I just read some of his reviews.....I’m all for saving money but I value good customer service a lot more than a few dollars.
 
Jumppr said:
I just found these guys. If you’re using a 205 this should work.

https://behemothdrivetrain.com/coll...s/products/np205-rear-output-retainer-vss-kit
Click to expand...
Thanks for the response Jumppr. I have a 208 in my Blazer.

I called Dakota Digital about that module I posted a link to and they said that it would definitely work for my application of converting the 16 PPR to 2000 PPM. Amazon had it for $94 and Prime shipping so I got it and installed it 2 days ago. So far everything is working great.
 
You must log in or register to reply here.

Latest Posts

Top Bottom