MSD Tach Signal Question

[DanStokes]

Hey Guys -

The Volvo is back with a non-functional tach. As a reminder, it was back with me for quite a while as I reworked the fuel tank for greater capacity and also because the car wouldn't run. I ran a ground from the fuel pump relay to ground and it fired right up. I didn't realize at that point that the tach was inop (the owner didn't mention it at that time).

So today I started hunting the tach wiring from the MSD 6AL back to the EZ-EFI computer. With a lot of taking apart and standing on my head I determined that the wiring from the MSD to the computer is fine. But then another question: I know MSD's make kick-butt spark but are known to have their "issues" so I'm wondering if the MSD tach out signal has taken the proverbial dump. I'm not finding any way to test that - the Holley website had some info but it was contradictory. Any ideas out there? Before I tear into the dash again (this will be about the 97th. time) I'd like to be sure that the tach is even getting a signal. I also wondered if the car stopped running in the first place because the tach signal to the EX-EFI computer signal went out. I appreciate any input on this.

EDIT: I just found the installation instructions online and it says that it outputs a 12VDC square wave. How can I check that without a scope? Also, my DMM has a frequency function - would that tell me anything I need to know?


Dan

[Monster]

The tach signal probably changes from 0v to 12v and back too quickly (too high a frequency) for your DMM to register, so a scope may be required.

[squirrel]

the frequency is 4 times the number of revolutions per second of the engine. So...600 rpm gives 10 revs per second, you'll see 40 hz on the frequency counter at idle, roughly.

try it see what happens. But why don't you have a scope? I have several, they seem to collect.

[DanStokes]

the frequency is 4 times the number of revolutions per second of the engine. So...600 rpm gives 10 revs per second, you'll see 40 hz on the frequency counter at idle, roughly.

try it see what happens. But why don't you have a scope? I have several, they seem to collect.

Dunno. Guess one just never came my way so the collection never started. I was looking at a 'scope app for my iPhone but the ones I found seemed to be slanted toward audio functions so I don't know if they'd work for me. I'll dig a little deeper. And THANKS for your input!

Dan

[DanStokes]

I just bought a small, hand-held 'scope from Ebay - $30.00 including shipping though there was a small tax on it. Seemed like a worthwhile purchase for that little $$. Calling around town I wasn't able to turn up anyone, even electronics repair shops, who had one. Evidently it's technology of the past. So from here on out if any of y'all need one I can ship it - it's all solid state and should ship as well as a DMM.

Dan

[squirrel]

so....what did you find on the tach signal? curious minds want to know.

btw I've used this scope, which is older than me, to help diagnose tach signal problems

[DanStokes]

I just ordered it a few minutes ago so 'taint here yet. Love your old scope - looks like the ones we had at EPA. Later they got little bitty ones (from Tectronix IIRC) that were battery powered and did pretty much everything the big ones did but were a lot easier to take to the test sites. The one I ordered is about the size of a DMM so we'll see if it's any good. For this job it doesn't have to be all that accurate but the signal does have to exist. Hope it gets in the mail QUICKLY!

Dan

[oletrux4evr]

That's a neat old scope.