aftermarket speedo troubleshoot
 

Looking for advice for troubleshooting why I've got no speedo…

I've got the factory PSOM unit (from a '95) wired in between VSS signal coming from rear-end and the Speedway Motors electronic speedometer. What/where should I check with a multimeter — voltage coming from VSS, or PSOM? If so, what am I looking for?

Thanks in advance.

CHECKING THE SPEED INPUT SIGNAL to the PSOM.
NOTE:
Only wiring harness end of connector is to be probed.

Connect a Digital Volt-Ohmmeter set to AC to Pin 4 (speed in +) and Pin 5 (speed in -).
The voltage show increase smoothly and continuously from 0 to approximately 3.5 volts AC as vehicle speed increases from 0 to 48 km/h (0 to 30 mph).

OR,
If available, a frequency counter may be connected to Pin 4 (Speed in +) and Pin 5 (Speed in -).
The frequency of the signal show increase smoothly and continuously from 0 to approximately 667 Hz at approximately 48 km/h (30 mph).

CHECKING THE SPEED OUTPUT SIGNAL from the PSOM.
Operate vehicle by raising the rear wheels. Set speed at 30 mph. Use an AC voltmeter to measure the voltage between Pin 7 (SPEED SIG, Circuit 679) and Pin 2 (GND, Circuit 901/676/570).
The voltage should be reading 4-5 volts AC.

Maybe these will help to understand the system.

From the PSOM:
Attachment 277292

From the rear end:
https://i.imgur.com/TWqDcsS.jpg
Attachment 277289
Attachment 283102
/

Thanks, Bill. Both tests you outline check out. So I guess the problem must lie either between the PSOM and the speedo, or with the speedo itself (aftermarket from Speedway Motors). Had a bad tach from them, and actually _two_ bad master cylinders, so I guess I wouldn't be surprised. Hopefully it is just wiring-related, though it won't be easy getting behind the dash again now that everything is all put together. :'(

follow up question
 

Relatedly, I was able to change my PSOM to the proper constant for my tire size.

One option for this speedo is to do a manual calibration, where you can enter PPM. My question is, would that be the raw VSS PPM, which I've calculated to be 75,600 — or did I read somewhere that the post-PSOM signal is always 8,000?

Thanks in advance.

Look at the diagram Subford posted above. "PSOM converts the input from the ABS sensor to the standard 8000 PPM speed signal." (paraphrased)



https://cimg1.ibsrv.net/gimg/www.for...9660ff4803.jpg

The PSOM converts the signal from the VSS to the old 8,000 PPM that the old transmission mount VSS putout before 1992 to the PCM.

If you look at the wave form above it says that the frequency is about 120HZ @ 60 MPH.
60 MPH is a mile a minute or a mile ever 60 seconds.
Mathematically 120 times 60 comes to 7,200 or close to 8,000 PPM.

Note the above said ABOUT 120HZ. I would think it would be closer to 133HZ to give the 8,000 per mile at 60MPH.

The manual calibration will change whatever it gets from the VSS.
The output to the PCM would very in frequency as the speed changes but it will maintain the 8,000PPM.

OK, so I think I may have my speedo wired incorrectly — currently I have the two "speed sensor signal" wires from the speedo wired to LG/BK (5) and PK/O (2). I don't recall why I did it that way, but I'm guessing that instead of PK/O it should be R/PK (i.e. both leads coming from VSS). Then I could simply program the speedo PPM for the raw 75,600 value, right? I think in my head I was thinking that I had to somehow get the post-PSOM signal to the speedo, which I don't believe is the case anymore, correct?

No it is not.
Your speedometer should be hooked to the pin #7 GY/BK and pin #2 PK/O the PSOM output pins.
The VSS input should be hooked to pin #4 R/PK & pin #5 LG/BK.

X2 on Bill's response.

D'OH! Conflicting responses (hate when that happens!) :-X0A6 :)

Bill, if I do it your way, then I presume I would use the 8,000 PPM value?

Yes but you will only see the 133HZ at 60MPH.
You will get 8,000 pulses for each mile that you travel.

Double post, Deleted.

Rats... still no luck.
This is the annoying aspect where you wonder if the unit is defective (like the first tach they sent me) or I still have something haywire. :-huh

FWIW, the speedo instructions state that the polarity shouldn't matter for the two VSS signal wires — think that is true?

I am not sure what model you are working with.
Can you post the model number of the unit.
Also the URL when I can look at it.
If you could send me a PDF or images of the instructions that it came with as there is not any online.
My email address is at the bottom of all my posts.

Thanks, Bill. (Interestingly your email doesn’t show up in your posts when using mobile device to access FTE?)

speedo:
https://www.speedwaymotors.com/Omega...Set,43710.html

instructions: (mine is the first example listed, 4 3/8”)
https://static.speedwaymotors.com/pdf/New_Omega_Ins.pdf

It does say something about adjusting sensitivity, which I haven’t changed yet.

Appreciate your expert insight.