NP231 Pulse Generator

Kulm

I am putting a 1990 SBC, with factory throttle body, a NV4500, and a NP231 transfer case in a 1950 Willys pickup. The speedometer has a 27 tooth gear I it now. My question is will the 27 tooth gear give the ECM the correct signal in low range? Any help would be appreciated. Thanks

YJSam

The 27 tooth gear will send the speed the vehicle is moving either in low or high since it is driven from the transfer case output shaft. If you're asking if it will tell the ECM you are in 4 low to compensate for a higher RPM, no it will not. I think that a different switched circuit will change that. I'm not sure which circuit does switch that however.

Magneto

Is it a Jeep 231 or a Chevy 231 transfer case?

This page may be helpful:
https://www.novak-adapt.com/catalog/electronics/sensors.html

Do you have a DRAC with your 1990 TBI PCM?

Kulm

It is a Jeep Cherokee 231 transfer case and I don’t know what a DRAC is

Kulm

I thought I read somewhere that in low range the signal from my transfer case would not send enough pulses for the GM ECM. Please forgive me if I am not using the correct terms. This is all new to me

YJSam

Kulm In low range the PPM signal would be less, correct. The original circuitry would have had a switch to change the ECM for a lower signal while in low range. The DRAC is a Digital Ratio Adapter Controller. It has the capability of taking a PPM signal and changing it to other PPM signal. The TBI PCM has a DRAC module mounted right by the PCM. Usually a small white plastic piece that has two pigtails going to it. This information may help:

https://harristuning.com/Tbi/vssb-drac-calibration/

Magneto

I wonder if the Dakota Digital SGI unit would help:

https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=1192/mode=prod/prd1192.htm

These are available from Dakota as well as Novak. They are useful for speed signal conversion tasks like this.