Yellowbox Speedo wiring help

[Martyb]

So I had 3:55’s put in several months ago and just got around to messing with the speedo box. I found pin 27 in the middle pcm connector(white/tan wire), cur the wire and connected the blue wire from the speedo box to the cut end of the white/tan wire that’s going to the pcm. Then I connected the white wire from the speedo box to the other side of the cut white/tan wire going to the trans. Found pin #2 on the black pcm plug and tapped for a power source and connected the red wire from the speedo box. Black wire grounded. Took the car for a spin and the speedo did not work at all. Thoughts??

[Martyb]

The instructions that came with the unit say the signal wires are yellow and green, mine are blue and white. Possible I have them backwards and blue goes to speedo side of cut wire and white goes to pcm side of the cut wire?

[My98RT10]

Maybe this helps:

https://driveviper.com/forums/thread...ghlight=yellow

My assumption is you swapped the signal wires, provided power and ground are correct.

[Martyb]

So, power and ground are correct, I switched the white and blue wires coming from the box and still nothing. It’s like it’s in a locked mode or a defective unit. I get the flashes at start up but nothing else.

[My98RT10]

Is yours to be programmed via dip-switches or via 2 buttons ("+" and "-")? If it is the latter one, how did programming go? I remember having some difficulty setting it up correctly. Maybe you have accidentally set a ratio that leads to a measured speed of zero...

Else, you might have not picked the right signal signal wire...

[Martyb]

I have the two button unit, I didn’t touch the programming as the instructions said it was preset for 111% correction, which is just about what I need. And I spliced the speedo sensor wire back together and the speedo worked fine, so I’m guessing I have the correct wire(pin27).

[My98RT10]

I don't remember if mine was preprogrammed...

- what is the exact model of your yellow box (mine is 2012VR)?
- my instructions say the speedo signal is pin28, but it maybe a typo in your post?
- have you done the 100Hz output test and what is the result?

[Steve-Indy]

This is a familiar story in that a friend had similar problem with the Yellow Box many years ago on a Gen II Viper...printed directions vs. actual connections to make it work. Sorry that I cannot recall the details.

PLEASE NOTE: The factory Service Manual for the 1998 Viper shows the following for the PCM C2 connector pins:

Pin #27...WT/TN...Speedometer signal

Pin #28...WT/OR...Vehicle speed sensor signal

I hope this helps.

[My98RT10]

Sorry, I wasn't precise in my wording. The yellow box needs to be spliced in between the sensor in the tranny and the PCM and not between PCM and Speedometer. Hence the instructions referring to PIN 28 seem to be correct. If OP spliced it in between PCM and Speedometer that could be the culprit.

[Martyb]

So maybe I should be using pin28 instead of 27? I’ll give it a shot and see what happens.

Appreciate the assistance gentlemen!

[Martyb]

Pin 28 did the trick! Now speedo at 50 mph puts the car at 63 mph on gps. Let the math equation fun begin😊

[My98RT10]

Glad it works now. Good luck with the programming

[Martyb]

Glad it works now. Good luck with the programming

Thanks again for the guidance!

[Martyb]

The ground I used was the chassis ground on the frame just below and in front of the pcm, guessing that is what you used as well? Unit is not allowing me to make any adjustments and the mfg thinks it may be the ground I used(which doesn’t make sense to me).

[My98RT10]

Chassis ground should be ok. Also, since the box is working there is no issue with the ground in my opinion. I remember the programming procedure to be very awkward. Does it at least go into programming mode? My model requires pushing down "+" and "-" simultaneously for 5 seconds until both lights start to flicker...

[Martyb]

Chassis ground should be ok. Also, since the box is working there is no issue with the ground in my opinion. I remember the programming procedure to be very awkward. Does it at least go into programming mode? My model requires pushing down "+" and "-" simultaneously for 5 seconds until both lights start to flicker...

Both lights flash at start up, and then green stays on constant. Pushing both buttons for 5 seconds does nothing. The needle on the speedo also starts to bounce at anything over 60mph.

[My98RT10]

What is the model# of your yellowbox? Mine is 2012VR and I don't know about other models, they might be programmed differently!?

But what you discribe at startup is what mine does as well. The green light should then start to flicker when when wheels are turning, indicating the reception of input signal from the sensor.

Do you have the instructions and what do they say how to program it?