F40 with 4.9...VSS Question (Page 1/3)
Spadesluck FEB 03, 01:25 AM
I know some on here have some 4.9 knowledge so I figured I would ask. I am close to putting the 4.9/F40 combo into my car but want to wrap up some small wiring issues that I have not addressed yet. One being the VSS from the F40 to the 4.9 ECM. I know the F40 puts out a 60k ppm signal. I am looking at running a Dakota Digital inline from the F40 to the 4.9 ecm high and low inputs. Then then I was going to run the "corrected" (was just going to get a prebuilt piece from Red Devil River) 4k ppm output from the 4.9 ecm to the fiero VSS High input (C203 position G). Does that sound correct? Additionally, do I need the VSS Low input on the C203 connected to ground since I am using the dakota digital (this is the part I am most confused on)?

I have looked at all the 4.9 wiring threads and have a decent grasp on what is going on, I just need some additional clarification. Thank you in advance.
Will FEB 03, 09:34 AM
The Fiero speedometer needs a sine wave signal. If you're going to run the Fiero speedo from the 4.9 ECM speedometer output, you'll need to build a signal conditioning circuit to turn the square wave output from the ECM into a sine wave signal.

Here's how to build the circuit: https://www.fiero.nl/forum/Forum2/HTML/139415.html

Alternatively, use the aftermarket stuff to produce the right signal in the first place. That might involve having a Dakota Digital unit with two different outputs... one to the ECM and one to the Speedo.
J Gunsett FEB 03, 09:49 AM
I did answer your PM. Here is how I had mine wired up.



Jack

[This message has been edited by J Gunsett (edited 02-03-2022).]

lou_dias FEB 03, 10:00 AM
You don't need a Dakota Digital unit if the 4.9's ECM has a 2000ppm output and also had a divisor setting. You just need a way to convert the signal to an analog wave as Will mentioned.
I believe there is a forum member that sells this simple circuit.

What I did with my 7730 conversion in my v6 is set the internal divisor to compute the INTERNALLY CALCULATED speed to 60000 and instead of using the 4000ppm signal out to feed the VSS, I used the 2000ppm signal. This essentially acts as an extra divisor of 2. Then I set the external divisor to 7 or 8 depending in tire diameter and add the circuit to convert it from digital to analog.

No Dakota Digital unit necessary. This has been working flawlessly for over a decade.

Now I don't have the 4.9's ECM pinout in front of me nor do I recall if it has those same divisor settings as the Fiero and 7730 ECMs but I would suspect that it does.

To be clear:
The 7730, and possibly Cadillac ECM has an internal VSS PPM count for the transmission that is used to calculate the car's current speed. For the Fiero/getrag it was 4000 already, the Cadillac and 7730 expect a 24000 number as I recall. This would need to be updated to 60000 for the F40. Then they use a divisor to divide down to 4000 PPM for speedometers....and there is also a 2000PPM output as well, not sure why but it was there on the 7730. This VSS divisor number is typically 6 to divide a 24000 pulse to 4000. If you use the 2000ppm signal instead, your speedo would read half as slow. The divisor range is from 5 or 6 to 11 as I recall but you would need a value of 15 for a 60000ppm signal. Using the 2000ppm out signal essentially gives you essentially a 30000ppm signal. Then the closest setting you can get is using the divisor of 7or 8 (7.5 would theoretically be ideal) to give you a total of 14 or 16 against the original 60000 number. Best to use a gps and drive your car with twice using each value to see which one is more accurate at 65 mph.

So you will need to update the ECM's flash memory with these values.

[This message has been edited by lou_dias (edited 02-03-2022).]

lou_dias FEB 03, 10:19 AM
I bought a premade circuit from https://reddevilriver.com/ but I don't see it listed on the site anymore. It was roughly $5 ... Might want to contact him directly about it.
olejoedad FEB 03, 10:24 AM
The caddy ECU has two 4k outputs, the buffer circuit is required to make the signal analog for the Fiero speedo.
lou_dias FEB 03, 10:37 AM
Here is the settings I used in TunerProRT using the $A1 code:



With the Cadillac ECM you'd have to use some other tuning software to set similar values. In this picture I am hovering over the divisor setting so that you can see the value of 64 represents a divisor of 7...etc...
The road speed constant is 2 settings below and you can see it's value in the other small window that is displayed there.

[This message has been edited by lou_dias (edited 02-03-2022).]

lou_dias FEB 03, 11:00 AM


I see now that it's labelled as 4000 output #2 but is it actually 4000 or 2000? On the 7730 ECM it is 2000. Seems odd that it would have a 2nd redundant speedo output... I guess I will find out for myself in about a month when I try this...

[This message has been edited by lou_dias (edited 02-03-2022).]

Spadesluck FEB 03, 12:21 PM
Lou, I have all those schematics downloaded as well. I am not sure why it has the two 4000ppm outputs. I know one member had mentioned his setup didn't work with the pin 12 and switched to pin 11, so you may be on to something.

My thing is I need to convert the F40 VSS from 60k ppm to the 24k ppm for the Caddy ECM to read. Which the Dakota digital should handle.

Then I take the 4000k ppm from the ECM output (aforementioned above) to feed the C203 Fiero speedometer connevtions using the RDR inbewtween. I am confused on the c203 high and low connections. Do both these still need to be hooked up?

Lou, are you able to just program the value in the Caddy ECM with tunerpro?

Jack, I responded to you. Thank you
Spadesluck FEB 03, 12:22 PM

quote
Originally posted by J Gunsett:

I did answer your PM. Here is how I had mine wired up.



Jack




This looks to answer my question for sure. Thanks Jack