tach misreading - showing wrong rpm's

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Buschi340

Buschi340

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is that possible? currently i have a speedhut tach. On a MSD6HEMI ignition box. I am not sure if so but it is possible that the Tach is showing ie 6000 rpm but actually the engine is reving 7000 rpm?

Can I put an Autometer tach parallel on the MSD Tach port to check that?
 
My guess is yes, or you can use a multi meter with a tach function to check rpm
 
I agree, should be able to put another teach in parrelle with the other.
Buschi340 said:
is that possible? currently i have a speedhut tach. On a MSD6HEMI ignition box. I am not sure if so but it is possible that the Tach is showing ie 6000 rpm but actually the engine is reving 7000 rpm?

Can I put an Autometer tach parallel on the MSD Tach port to check that?
Click to expand...
 
Make sure your tach and ignition are set to V8.

I have a 4 cylinder dirt track car and went though this with mine.
 
Before you go to the trouble of adding a tach... How does the MSD system work? Can you plug in a laptop or other devise to see timing and ign setting real time while the engine runs? I have a speedhut tach and it is always accurate with my FAST xfi/com RPM signal. Also, does tha MSD system have a rev limiter set? I would think even if the tach was off you'd have hit the limiter if you had one.

Joe
 
Limiter was set at 7000. tach never reached that mark. But it felt like I hit the limiter.
Yes you can see rpm and timing on a laptop. And both showed the same in the lower rpm range. I never checked that in the higher range to be honest. I wish it would have a data logger.
 
I'm pretty sure you should be able to hook both up together. I believe is just puts out a 12 volt square wave with a frequency that matches RPM. At one point I had the tach line feeding a tach for me and being the RPM signal for my fuel only Megasquirt setup. It doesn't really need to provide any current, so I think it should split okay. Seemed to work well enough for me at least.
 
yes, squared Signal. so i can check in 3 ways : 2 times tach Output ( checking the tachs against each other ) and same time thru Laptop. thats good because the Laptop uses his own port. too bad there is no data logger in the MSD Software...
 
That is a bummer you can't datalog, that would help in your search for sure...
 
Buschi340 said:
what is the source of the rpm Signal? Crank or cam sensor?
Click to expand...

I would just about bet it's the crank signal that's driving the tach. Much better resolution there. I think the cam signal is more just for phasing to determine TDC of each cylinder.
 
so if the msd box is reading wrong on the crank the Signal for Timing from the cam can still be correct?
i shot an email to msd/holley tech supp but never received an answer
 
Hmm, I'm not sure on that. Since the crank still uses a missing tooth to determine #1 TDC I would think the MSD box would never sync the cam and crank signals if something was wrong with either and the box wouldn't work at all. I would probably trust what the software is saying more than a tach, but hard to say with electronics.
 
Yes, the cam sensor would time sparks, but what typically happens in computer setups is the crank sensor will look for the missing tooth first and then look for the #1 cylinder on the cam sensor to make sure they line up. If they are off the the computer sees the signals as not in sync and will typically not send spark. At least that's how the Megasquirt setup is, though I think the methodology is fairly typical for most computers. It's kind of like a test to make sure both sensors are working right that the computer does.
 
Correct. The ECU only looks for the cam signal at startup. Once it establishes its position it is ignored.

I would imagine the MSD is getting its tach signal from a coil. That seems to be a favorite aftermarket signal.
 
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