Frequency @ AC terminal on N3135 Regulator with 200A Alternator

[kurtvk]

Hi all,

in the process of hooking up a Tach i got from amazon.

Amazon.com

I'm trying to figure out what the relation is between the Frequency @ AC terminal on the N3135 Regulator and the actual engine RPM's.
Measuring with a scope was about 120Hz at idle (guess around 700-750 RPM) see attached picture.

This weekend i will try to measure the actual rpm with a handheld tach on a motor pulley if i can find a safe spot to do so.
or measure crank and alternator pulley and some math to calculate freq and rpm ratio


(M1123 6.5L NA) 4Speed

 

[Mogman]

I used the signal from the crank sending unit 4p/rev. I was never able to get an aftermarket tach to work on the AC tap.
(edit), the signal from the crank sensor is 5V IIRC, if you have a NA you can also use the RPM sending unit on the back of the engine 1p/rev but you will need to provide a bias voltage as it is simply a reed switch.

 

[kurtvk]

I used the signal from the crank sending unit 4p/rev. I was never able to get an aftermarket tach to work on the AC tap.
If you do run the tach on 12V as that is what the transmission TCM is using just to be safe but would likely also work on 24V, the signal from the crank sensor is 5V IIRC, if you have a NA you can also use the RPM sending unit on the back of the engine 1p/rev but you will need to provide a bias voltage as it is simply a reed switch.

Thank You that definitely helps !

Does the 6.5NA have a crank sending unit? and where would it be located?
I will check if there is a RPM sending unit on my engine (will have to take radio tray out to get access behind the doghouse cover)

 

[Mogman]

I believe all 6.5L engines have the crank sending unit although not being used on the 3 speed, if not you can add one.
After thinking about it I am running the aftermarket tach on the Detroit on 24V, there is no voltage on the tach sense line so I don't see where it would make any difference.
If you have a 3 speed try grounding one lead of the crank sending unit and connect the other to the tach

 

[FlameRed]

@Mogman is the ultimate authority as I am just a newbie here but here is my 2 cents worth

I installed the Military tach, so I paid the HMMWV tax as usual instead of using something affordable. I just hooked it up to a "run" +24V source, ground, +24V from the dash bulbs and two sensor wires. I used the sensor at the back of the engine as the source . I tapped into it using Posi-taps connectors which is not really as good as a Packard connectors. I found a picture of the hook up provided to me by another forum member so the below is not my truck and I cannot take credit. Yes, remove the radio shelf and dog house and the job is simple. Run the wires through a grommet if you drill a new hole to get it in the dash. Test and button it up.

 

[Mogman]

@Mogman is the ultimate authority as I am just a newbie here.

I installed the Military tach, so I paid the HMMWV tax as usual. I just hooked it up to a "run" +24V source, Ground, and +24V from the dash bulbs. I used the sensor at the back of the engine as the source . I tapped into it using Posi-taps connectors which is not really as good as a Packard connectors. I found a picture of the hook up provided to me by another forum member so the below is not my truck and I cannot take credit. Yes, remove the radio shelf and dog house and the job is simple. Run the wires through a grommet if you drill a new hole to get it in the dash.

View attachment 904854

You have to be very careful adding any load to the dash lamps, if the dash lamps are switched to dim, first you will not have 24V to whatever you connected and second you can burn up the dim resistor in the light switch which can cause other damage inside the light switch.