Can someone help me sort out my speedo problem please? Wiring issue

[Mr. Smith]

The problem is the speedo on my HPT software (or any other scanning software it seems) runs about 13x slower than my actual speed / car speedo on dash. My car speedo works perfectly.
If I'm doing 70, VCM Suite will be saying 5.
If I'm doing 7, VCM Suite will be saying 0.
So if I log a 0-100 run, my speedo will just go from 2 to 3 to 4 to 5 to 6 to 7 to 8. That's it.
I changed the speedos Math formula.. x'ed it by 13. So now it goes 26, 39, 52, 65... but that still sucks.

My car is a 30yo GM with digital instruments (dash) from factory. No ABS. Factory hydraulic auto trans.


Here's some wiring info:

LS1 PCM

RED pin 20 -- VSS Low signal -- Tan wire

RED pin 21 -- VSS High signal -- Blue/White wire

RED pin 50 -- VSS (digital) signal -- Violet/White wire


Cluster plug

C15 -- Speed sensor Low Input -- Purple (purple to VSS)

D11 -- Speed sensor High Input -- Yellow (to tan/white to VSS)

D12 -- Vehicle speed Output -- Brown/White from the dash to Brown to the ECM

D3 -- Speedo ground -- black/white


I've tried every wiring combination from what I remember. It will either read 13x slower on the scanner, or no reading at all.
The only thing I can think of is one of the cables might need a resistor.
Anyone know?

[gofastwclass]

What vehicle are you working on? Where is the vehicle speed coming from that the digital dash works and the PCM is incorrect? What transmission? Is the PCM controlling the transmission?

The PCM is looking for an alternating wave (can't remember sine or square) on the VSS high and VSS low supplied by the input speed sensor on the transmission tail shaft or transfer case. If you can't supply this input to the PCM, it won't reliably be able to calculate vehicle speed.

Vehicle speed is calculated from tire height, gear ratio and tooth count on the exciter (reluctor) wheel in the transmission or transfer case. If one of those numbers is incorrect the speed reading will be off by that percent.

[Mr. Smith]

Thanks.

What vehicle are you working on?

My car is a 30yo GM with digital instruments (dash) from factory.

Where is the vehicle speed coming from that the digital dash works and the PCM is incorrect?

Here's some wiring info:

LS1 PCM

RED pin 20 -- VSS Low signal -- Tan wire

RED pin 21 -- VSS High signal -- Blue/White wire

RED pin 50 -- VSS (digital) signal -- Violet/White wire


Cluster plug

C15 -- Speed sensor Low Input -- Purple (purple to VSS)

D11 -- Speed sensor High Input -- Yellow (to tan/white to VSS)

D12 -- Vehicle speed Output -- Brown/White from the dash to Brown to the ECM

D3 -- Speedo ground -- black/white

What transmission? Is the PCM controlling the transmission?

Factory hydraulic auto trans.

30yo GM

car

The PCM is looking for an alternating wave (can't remember sine or square) on the VSS high and VSS low supplied by the input speed sensor on the transmission tail shaft or transfer case. If you can't supply this input to the PCM, it won't reliably be able to calculate vehicle speed.

So how do I supply this input to the PCM?

Vehicle speed is calculated from tire height, gear ratio and tooth count on the exciter (reluctor) wheel in the transmission or transfer case. If one of those numbers is incorrect the speed reading will be off by that percent.

So how can I make my 30yo car get all those numbers/readings and accurately?

[Mr. Smith]

I'm thinking if I add the tyre height and gear ratio into HPT, then its speedo might work properly

I doubt it.

[Jon S.]

My car is a 30yo GM with digital instruments (dash) from factory. No ABS. Factory hydraulic auto trans.

Do you mean a non-electronic automatic (like a 700R4), in your 1986 car ?

LS1 PCM

RED pin 20 -- VSS Low signal -- Tan wire

RED pin 21 -- VSS High signal -- Blue/White wire

RED pin 50 -- VSS (digital) signal -- Violet/White wire

What is generating the VSS signal ?

Cluster plug

C15 -- Speed sensor Low Input -- Purple (purple to VSS)

D11 -- Speed sensor High Input -- Yellow (to tan/white to VSS)

D12 -- Vehicle speed Output -- Brown/White from the dash to Brown to the ECM

D3 -- Speedo ground -- black/white

Is the cluster using the same VSS generator that is feeding the PCM ? Is this speed signal being routed through a DRAC or VSS Buffer ?


Posting your tune, tire size, and gear ratio may also be beneficial.

[Mr. Smith]

Do you mean a non-electronic automatic (like a 700R4), in your 1986 car ?

Yes.

What is generating the VSS signal ?

The VSS and trans, I believe

Is the cluster using the same VSS generator that is feeding the PCM ?

Yeah I think so. VSS wired straight to my cluster (like factory).. then 1 of the 3 PCM speedo signal wires tapped into VSS wire (that goes to my cluster) or 1 of the 3 cluster plug signal wires. Can't remember.

Is this speed signal being routed through a DRAC or VSS Buffer ?

I don't think so, unless GM used something like that from factory 30 years ago. I certainly didn't wire up a "DRAC" ( ) or Buffer.

Posting your tune, tire size, and gear ratio may also be beneficial.

I can't tune my PCM as it's locked. Will hopefully be able to next week.

Thanks.

[Mr. Smith]

I just found this VSS wiring.jpg pic I made a couple years ago..
I believe that's still how my wiring is now.

[Jon S.]

Thanks for posting the photo ... it was helpful.

To answer your question from post #6 - Yes, GM vehicles in the 1980s and early 1990s used a DRAC Module (earlier models) or a VSS Buffer Module (later models) to achieve the correct output to the speedometer. There were different DRAC / VSS Buffer modules for different transmission, gear, and tire combinations.

If I had to guess, I would hazard that the PCM is getting the VSS signal AFTER it has been processed by the DRAC or VSS Buffer, and that the PCM is then processing (recalculating) the speed from this source, rather than from the "raw" pulses from the transmission mounted VSS.

The speedometer in the cluster only needs a single feed - 4K for most later speedometers, often 2K some earlier speedometers.

In this type of conversion, I wire the transmission mounted VSS signal directly to the PCM. Then I wire the 4K output from the PCM (C2 pin 50) directly to the Speedometer (isolate or remove the DRAC / VSS Buffer).

Originally the VSS Low & High signals would have went to the DRAC / VSS Buffer, and then this device would have output a signal on a single wire (not 2 wires) to the Speedometer.

[Mr. Smith]

Thanks for your time and help.
I will try what you wrote when I swap my PCM for another, hopefully in the next few days.

Btw, I found this in my speedo notes (regarding my car's factory speedo):
"The sine wave input comes from the VSS at 4000 pulses per mile and a square wave goes out to the ECM and cruise control at half the frequency (2000 pulses per mile)."

and this is a good read regarding LS speedos..
http://www.pirate4x4.com/forum/gener...eedometer.html


If we can adjust the VSS signal in VCM Editor, then I'll give that a try first. Would be great if I can fix my problem by just typing in "2000" in a VSS PPM section, instead of ripping out my PCM/interior for the 73rd time + trying to get to my VSS wiring and re-wiring...

Holy shit.. I just checked.. we can.. will give it a go asap.

[Jon S.]

Your PCM has 2 available vehicle speed outputs.

C2 (Red) pin 50 - 4K output - normally to the Speedometer

C2 (red) pin 49 - 128K output - normally to the EBCM (ABS Brakes).

In conversions that are DBW, the PCM communicates vehicle speed with the TAC Module via the UART comms.

In conversions that are DBC, the Cruise Control Module does not use UART comms, and must be fed a vehicle speed signal.

I wire C2 pin 50 directly to the speedometer.

I wire C2 pin 49 to the Cruise Control Module, and change the output from 128K to 4K via tuning software.

[Mr. Smith]

Your PCM has 2 available vehicle speed outputs.

C2 (Red) pin 50 - 4K output - normally to the Speedometer

C2 (red) pin 49 - 128K output - normally to the EBCM (ABS Brakes).

In conversions that are DBW, the PCM communicates vehicle speed with the TAC Module via the UART comms.

In conversions that are DBC, the Cruise Control Module does not use UART comms, and must be fed a vehicle speed signal.

I wire C2 pin 50 directly to the speedometer.

I wire C2 pin 49 to the Cruise Control Module, and change the output from 128K to 4K via tuning software.

Thanks.
I'll first try tuning it with VCM Suite. If I can adjust the PCM signal/pulses then for sure I can fix my speedo that way and will save me from doing a lot of work/re-wiring...


Old notes of mine in case it helps anyone or if anyone's interested..
Pin 50 to high input + Pin 20 to low input = speedo not working
Pin 50 to high input + Pin 20 to earth = speedo not working
Pin 21 to high input + Pin 20 to low input = speedo working and scantool speedo reading slow
Pin 21 to high input + Pin 20 to low input + Pin 50 to D12 = no difference or scantool speedo not working

[Mr. Smith]

Sorry, never mind. My mistake.
I'm going to multiply my "Pulses per Mile" figure by 13.
Thought I had to divide it by 13 which would've put me under the minimum of 1,625.. and possibly fried my PCM.
What happens when you don't sleep enough..

[pontiacjeff]

Did you ever figure out this issue? My speedo (2002) reads double. 2008 ecm.

[Mr. Smith]

Did you ever figure out this issue? My speedo (2002) reads double. 2008 ecm.

Not really. I tried playing with the speedo settings with HP Tuners.. didn't really do anything.
My problem seems to be my wiring.. what Jon S. was saying above. Just haven't had the time and energy to sort it out.

[pontiacjeff]

I'm afraid my speedometer is a wiring issue, too. Nothing I do with the tune affects the speed reading (double actual speed)