The basic idea of the rpm to function is to load the alternator. I.E Switch on lights fogger?? and anything electric which is rated high. So instead of having a constant load by switching on the lights etc.. i will replace it with a small device that will do the job..

Come on, there are plenty of electronic brain boxes on this forum. Am sure you can find a solution.

 

I will try and connect it to this box here and see the results

 

That's one of the down sides to that type of meter... they're pulling the rpms by looking at small spikes in the voltage supply created whenever the spark plugs fire (the same spikes us EEs work hard to remove and smooth out when we're designing electronic equipment for the automotive industry). It's simplistic in the sense that no extra wires need to be tapped, but as Trip has seen, it's also not very reliable.

They actually rely on your electrical system being noisy. In this case, the car's electrical system is too quiet until the car starts to put some oomph into it (above 3,500 rpm) or you load it so heavily (with lights and such) that the alternator sags a bit more and the spikes become more prevalent. Another downside is there is no separate wire to tap into the ECU's rpm signal. That being said, it's actually not a bad idea to tap as close to the noisy alternator as possible.

As far as placing a resistor in there, this is a bad idea all around. Not only would you be drawing that same amount of power all of the time, but the sheer size of the resistor and the amount of heat generated would be atrocioius. Let's say you wanted to drop 50A into it... you'd be dissipating 750W of power all of the time. 750W is a decent size garage heater!

 

I think I'd just live without the RPM below 3500. It's not like there is any torque there anyway.

 

Mac makes a good point. I didn't think of it like that...ummm

 

I'm an EE too but they don't want to listen to me. From what I know, the G-tech uses the alternator for RPM's, not spark plug. Your not going to hurt anything, because the alternator is wired directly to the battery. The battery will smooth out any spikes make by the alternator. There are also filter caps in a cars wiring system (especially radio power), that's why the G-tech can't read the signal.

 

Thanks for the response guys. If I tap it as close to the alternator, will I have down sides of voltage levels or will it be constant ? I wouldn't want to fry my g-tech.

 

I agree, try as close to the alternator as possible. It's not exactly a safe place to be connecting wires, but if you fuse it close to the alternator you should be OK.

The only other alternative I can see is to make a dirty power supply. That would take some work. In theory you could get as much noise as you need, but we're not talking about a $5 fix. And you'ld have to verify the G-tech will work with a noise signal matching the tach frequency instead of the alternator frequency.

 

tell me more about this dirty power supply

 

I believe Bob is suggesting making a clean power supply with a copy of the tach signal superimposed on it... a workable solution, but definitely going around your elbow to get to your A$$. Kind of defeats the purpose, though, of not needing to tap any wires to get the meter to work.