Need conformation on MSD & OEM Tach

[newguy]

Ok I have a 78 datsun 620 (another one) and it did not come with a OEM tach. I have found one at a junk yard and want to wire it up but the truck currently has a MSD 6AL box and a blaster 2 coil. I'm worried it might fry the tach. I ask napa and they suggest a MSD Tach Adapter.
Does anyone have the same setup (MSD) and running an OEM tach? And If so is the signal wire on the tach ran to the negitive post on the msd coil?

[datzenmike]

Tach signal wire to the neg side of coil should do it. Don't worry it won't blow up all it will 'see' is some 12 volts with sudden drops to zero when the coil is fired.

[hang_510]

inductive pick up tachs require adapter #8920 to function w/the MSD box.

[newguy]

Wait I'm confused, mike says one thing and ol,school says another. is anyone currently running there truck this way?

[newguy]

bump

[1Low620]

Originally Posted by newguy

Ok I have a 78 datsun 620 (another one) and it did not come with a OEM tach. I have found one at a junk yard and want to wire it up but the truck currently has a MSD 6AL box and a blaster 2 coil. I'm worried it might fry the tach. I ask napa and they suggest a MSD Tach Adapter.
Does anyone have the same setup (MSD) and running an OEM tach? And If so is the signal wire on the tach ran to the negitive post on the msd coil?

Napa? that's like asking a whopper flopper from Burger King about rocket science. Stay away from napa!

[freaky510]

your best answer I would say is to call

Tech Line
915-855-7123

Iam running a cheap aftermarket tach out of my box,it holds up fine and it does jump on power up like mike said

[newguy]

Ok I'm going to do it, tomorrow I'm going to wire the OEM tach signal wire directly to the neg. on the coil. This will conferm this question for others.

[hang_510]

Originally Posted by newguy

is anyone currently running there truck this way?

not in a truck, but i have BOTH in use in the sedan.

[datsun4dsc]

Is there a big gain using MSD box if you already upgraded to EI?

[hang_510]

Originally Posted by datsun4dsc

is there a big gain using msd box if you already upgraded to ei?

none!

[Z-train]

Originally Posted by datsun4dsc

Is there a big gain using MSD box if you already upgraded to EI?

No power gain IF all things are "correct".BUT,you will see easier starts,cleaner idle and the rev limiter is cheap insurance.

Now to the tach-if a 6Al box is in use you MUST wire the tach to the box.And then a "tach adapter" most likely will be needed.

[datsunaholic]

The tach adaptor is only needed if using an inductive tach- the '73-75 620 tachs are inductive (they are wired in series with the wire running from the points to the coil). The '76-79 620 tachs are wired parallel to the coil, and would just be hooked up to the tach output terminal of the MSD box.

With a MSD 6a box, you cannot use the coil neg wire, because the MSD box fires it multiple times instead os a single spark. The tach would jump around, read high, and be inconsistant.

[hang_510]

Originally Posted by Z-train

the rev limiter is cheap insurance.

OK, i left that out

[newguy]

I didn't know there was two types of tachs that came in these trucks. Is there a way to ID what type of tach i found at the junk yard without going back to the yard to see the year of the truck?
I have tried wiring it to the tach output on the MSD 6AL box and nothing.

Originally Posted by datsunaholic

The tach adaptor is only needed if using an inductive tach- the '73-75 620 tachs are inductive (they are wired in series with the wire running from the points to the coil). The '76-79 620 tachs are wired parallel to the coil, and would just be hooked up to the tach output terminal of the MSD box.

With a MSD 6a box, you cannot use the coil neg wire, because the MSD box fires it multiple times instead os a single spark. The tach would jump around, read high, and be inconsistant.

[zed]

This may be OT but I have a Magfire coil (40kV) and a Pertronix Ignitor in my L18 - they work very well, but I can't get a tach (rev counter) to work with them. I tried two aftermarket tachs - both jump constantly, read high. I tried splicing in diodes to reduce the voltage - no improvement. Strangely, both these tachs work fine on my other truck - Toyota Landcruiser...

[RoadRace]

Originally Posted by zed

This may be OT but I have a Magfire coil (40kV) and a Pertronix Ignitor in my L18 - they work very well, but I can't get a tach (rev counter) to work with them. I tried two aftermarket tachs - both jump constantly, read high. I tried splicing in diodes to reduce the voltage - no improvement. Strangely, both these tachs work fine on my other truck - Toyota Landcruiser...

i have MSD6T, pertronix and Blaster2, autometer tacho works fine. is yours dead or just behaving irratically? if its dead try hooking it up at the coil, just hold it in your hand, try to eliminate wiring.

the MSD system becomes more necessary when you have higher cylinder pressures and higher RPM, with stock compression levels and mild cams, power gains will be minimal, if any - especially when you only have 4 cylinders to fire, the stories of big gains are all from big V8's, where they have 4 more cylinders to take care of, doubling the chances of better spark, comparitively speaking.

[datsunaholic]

Originally Posted by zed

This may be OT but I have a Magfire coil (40kV) and a Pertronix Ignitor in my L18 - they work very well, but I can't get a tach (rev counter) to work with them. I tried two aftermarket tachs - both jump constantly, read high. I tried splicing in diodes to reduce the voltage - no improvement. Strangely, both these tachs work fine on my other truck - Toyota Landcruiser...

Diodes don't reduce voltage, they block current in one direction. Resistors reduce voltage (actually reduce current, but that has the effect of reducing voltage across the load when placed in series) but that won't do anything with a tach since they record pulses, not voltage. If the tach is proven functional but is still jumping around, you're getting stray signals. Those could be fixed with a condenser, but it could have a detrimental effect on the ignition system. Or it could help if the stray signals are causing other issues.

[hang_510]

Originally Posted by datsunaholic

If the tach is proven functional but is still jumping around, you're getting stray signals.

all MSD says is to check that all components are in good working order...

mine jumps ~4000rpm, about where the spark goes from multi- to single- ... no idea why either!

[hughdogz]

There are basically three different tach adapters from MSD. The correct one will depend on if your tach is current controlled, or voltage controlled.

They have a tach adapter for each, and the expensive one can run either way.

Both (8920):


8910 for current controlled tachs



8910EIS for voltage controlled tachs



I'm pretty sure you want to go with the 8910. It is ~half the price of the 8920.

[hang_510]

Originally Posted by hughdogz

The correct one will depend on if your tach is current controlled, or voltage controlled.

I'm pretty sure you want to go with the 8910. It is ~half the price of the 8920.

510 stock tach requires the 8920 (cuz it costs more )

[newguy]

Originally Posted by hughdogz

There are basically three different tach adapters from MSD. The correct one will depend on if your tach is current controlled, or voltage controlled.

They have a tach adapter for each, and the expensive one can run either way.

Both (8920):


8910 for current controlled tachs



8910EIS for voltage controlled tachs



I'm pretty sure you want to go with the 8910. It is ~half the price of the 8920.

This info is very help full. I don't have points in my dizy so can anyone tell me which one of these adpater I'll need?