MSD 5900, MSD Blaster Coil, and Autometer Autogage Tachometer Wiring Question

[ItrunZ]

Hello,

 

I've read through an awful lot of posts on this homepage and really love all of the information that is available. This will be my first post to request advice from the users here. If I make any missteps, please let me know. I looked through a lot of posts about tachs and didn't find anyone with my specific problem.

 

First, of course, is a bit of background. I have an early 1974 260Z with a L28E 2.8 liter inline 6. The previous owner installed the engine and a 5-speed transmission. I believe it is from a 1979 or 1980 Z. I haven't pulled block and head numbers to confirm.

 

Now, this is where it gets a bit confusing to me. I pulled the dash to repair some cracks. The dash harness had a sticker that said 1972 240Z. The car has an aftermarket MSD 5900 ignition box hooked to a MSD Blaster 2 coil. That coil runs into what appears to be a stock distributor.

 

With that setup, the stock coil, which I assume is a 1974 tach, would not register the RPM's. So, I thought I would upgrade to an aftermarket tachometer. Long a fan of Autometer, I bought their budget Autogage series 5-inch tach with a shift light from Amazon.com for $82.00.

 

I got the tach and hooked it up as directed. I confirmed with MSD and Autometer that, since the ignition box is not a capacitive discharge unit and since the 5900 ignition box does not have a tach output signal, I was fine to get the tach signal off the negative side of the coil.

 

I hooked everything up. One tach wire to a confirmed engine block ground. One wire to the negative side of the coil. One wire to light the dash. The tach didn't function. For the last wire, I was told to hook it into a 12V ignition wire. I got out my manual, followed Autometer's instructions, and identified a wire that didn't have a current when the key was off and had a current when the key was switched to on. I cut that wire open and hooked up the tach. It didn't register the RPM's when the engine was running.

 

Not understanding why the tach wouldn't work, I hooked the 12V power wire to a 'constant on' wire that wasn't hooked to anything else. I'm not sure what that wire is for. I think it might be for the clock. When I hooked the tach to that, the tach pegged at its highest RPM when the ignition was switched to on before starting the engine and registered the idle normally when the engine was running.

 

So, I'm at a bit of a loss. I don't want to drain the battery by constantly powering the tach and I don't want to install a switch on the wire that makes the tach function correctly. At this point, I have two thoughts:

 

1. The ignition wire that is "On" when the engine is running doesn't have enough juice to power the tach or my connection is inadequate. I had just cut a bit of wire out and secured the tach power wire with electrical tape. I don't want to solder wires until I know the tach functions.

 

2. There is some sort of wiring anomaly caused by the 1979 or 1980 engine, 1972 interior wiring harness, and my tach.

 

Does anyone have any suggestions? I will attach pics to show you what I tried and what worked and what didn't. Of course, if I didn't give enough background or anyone needs more information to answer, just let me know!

 

I'm really looking to getting this tach hooked up so I can cruise the streets around my house on the weekends. I'm willing to lend an inexperienced hand to anyone in the Sacramento, CA area, too!

 

If someone has a suggestion for another power wire to tie into, I think that would work, too.

 

I'll attach a few pics to further illustrate what I see happening.

 

Itrunz - Sort of!

[ItrunZ]

Sorry. I said "stock coil." I should have said "stock tach."

[vantage]

It's the connection which is the problem. Don't cut into the cable isolation, rather gently remove the crimped connector from the connector housing by inserting a very small screwdriver or pin in the upper rectangle on the front to unlock the connector. Pull it out and solder the red wire from the tach to one side of the crimped barrel, so there is enough space to reinsert the connector back into the housing.

[ItrunZ]

Thank you for the quick reply. I will give your suggestion a try and report back.

[ItrunZ]

It worked like a charm. Thank you!

 

I popped out the connecting pin and soldered the tach power wire directly to the pin. I hooked everything up, routed all the wires cleanly, and am very happy.

[vantage]

Very good, enjoy. Looks like a compe steering wheel, nice!