HMMWV tach on CUCV signal calculations

[Skinny]

This subject has been beaten to death but here we are once again...everyone says that it works and that the alternator has a tach lead or the STE/ICE has a port for a tach signal but no one ACTUALLY has a thread about the installation and successful operation of the M998 tach on a CUCV.

I have the M998 tach which has power/ground (24v) for the actual gauge. It also has two ports labeled positive and negative for the signal coming in. Using a signal generator, I fed it a 2v AC sine wave and it responded with a tach indication and the hobbs meter starting rotating. Going off of memory, it was completely linear and that feeding it 50 Hz it would indicate 3,000 rpms. If my calculations are correct, 50 revolutions per second is equal to 50 Hz. That also means that 1 revolution per second is 1 Hz.

Since a magnetic pickup would produce an AC sine wave, wouldn't this tach work with just simply mounting a single magnet to the harmonic balancer with a pickup? 1 revolution would create one sine wave which if I am calculating correctly is exactly what the M998 tach is looking for. Granted the pickup size, magnet size, and gap would have to be tuned because the tach really likes atleast 1v AC otherwise it starts to drop off and wiggle.

For you electrical engineers...thoughts???

 

[Sharecropper]

Thought I would bump this thread back to top since there were no replies.

I too am wondering where to obtain the tach signal from the STE/ICE port. Is the brown wire the one that comes from Gen 1?

 

[Skinny]

Lost interest and diverted to other more important projects like a 24v 12000lb. multi mount winch that uses the NATO slave port...post coming soon.

I successfully fed the HMMWV tach a signal to get it to work, still figuring out where to get the signal.

 

[Warthog]

The brown wire from GEN1 goes into the tach buffer module. Looking at diagram E-11, I don't see a clear path to the STE/ICE port.

And without knowing how the tach buffer is setup or how the STE/ICE unit reads the signal, no telling which pin is used.

 

[Sharecropper]

Thanks Warthog. I plan to start fooling around with it over the holidays and will post my findings. It seems reasonable to assume one pin of the STE/ICE plug would drive the tach. I have both the 24v M998 tach as well as a GM 4000 RPM factory tach out of a 1985 C60 diesel dump truck. This GM tach has the small fuel gauge in the bottom. If I can get the GM tach to work, I will swap it out in my cluster, which is my preference.

 

[Warthog]

Can you just tap into the brown wire at the buffer? Is that the correct signal?

 

[Sharecropper]

I think so. I will try it with the 4000 rpm GM tach and see if it works. Say, where is that tach buffer located? (stupid me)

 

[M1008BOV]

Did you finish this project up? I can't be the only one sitting on the edge of my seat waiting for results!

 

[Sharecropper]

Naw, back surgery and then a heart attack slowed me down a bit. Am planning to try to get back on it in the spring.

 

[M1008BOV]

Well then take care and heal up

 

[Ilikemtb999]

I was able to use the brown wire attached to the drivers alternator to run a vdo diesel tach, it has a built in potentiometer to fine tune it.