will tach work with electronic distributor?

[zoltans4]

PO put in an electronic distributor, since i didn't do it i don't really have any documentation or anything about it working with the stock Tach. I know from my 240z there can be issues with the stock tach working unless you make a few modifications.

 

I have a stock tach coming i will be putting in, wondering if before hand i could get some help on this

 

thanks

[datsunaholic]

A '79 620 had an electronic distributor stock. So a stock '79 Tach would work with EI. The '73-74 one won't without some work. So is this something else?

[datzenmike]

Doug, need to know more about this. Whether EI or points isn't it just an on/off switch that grounds power through the coil? How would the tach know?

 

Also the tach. One I had was inductive, with a loop of wire around a terminal. Not connected electrically. Like this one with the Black/white wire ...

 

Then there is this one which may just have a single Green/Black wire to it.

 

 

My 710 has a matchbox on it now and I plan to add a factory tach soon and want it to work

 

.

[datsunaholic]

Dammit, you're making me get technical.

 

The upper one in the picture is an inductive tach. It reads signal through the grounding wire between the coil negative post and the points. It has to be wired in series with the points.

 

It MIGHT work with electronic ignition. I don't know this, as I've never had the inductive type. To try it, you'd have to wire it inline with the coil. I've seen some that used the (+) side of the coil- in fact it will work on either side, because all it's reading is essentially a AC wave through the wire. Polarity and current flow doesn't matter to an inductive circuit at this level- it's simply a counter. The issue here though is you HAVE to have a coil. If you have a true MSD box (not a MSD coil, which some folks call an MSD ignition) it only has a tach output port. However MSD does make an adapter to use an inductive tach.

 

The issue with an electronic ignition is that the signal from the EI box to the coil may not be strong enough to trigger the pickup coil in the tach. I generally think though that there is enough. The "work" I refer to is simply knowing where to cut/splice he wiring to install this... which you have to do with points too. Because it's wired in series, no car is "pre wired" for this kind of tach. You have to make the loop. The bad part is it adds probably 5 feet of extra wire to the ignition circuit, which can pick up its own signals if the wires make loops and such. The plus part is it's far less likely for the tach itself to affect the ignition.

 

The lower one is a signal reading tach. Almost all aftermarket ones are the same type. They read the pulses as a difference between "float" voltage and "Zero Volts" compared to the +12V main bus voltage. Internally, the tachs are pretty much the same- they read pulses and convert that to a meter deflection. They're just wired differently. The "Pulse" type tachs work just fine with points, standard EI, and even generally with an MSD box that has a Tach output port. All it needs is a switching circuit thatgoes on/off/on/off... The plus of these types of tachs is the simplicity of wiring. The drawback is that if the tach itself fails electronically, it has the capability of grounding out the ignition. The inductive type can't do that, since there's no physical hard-wired electrical connection, just a pickup coil.

 

Both kinds can affect the ignition if the added-in wiring shorts out. Had that happen to me once. MUCH less likely if you use the stock wiring.

[zoltans4]

great info, thanks.

 

i will see what year the tach is, hopefully its a later year and its an easy connection. otherwise i will chime in for help!

 

 

sean

[datzenmike]

Thanks Doug. What about the 720s and S110s (maybe the zx too)? They have a 2.2K ohm resister in line from the EI to the tach.

[DAT510]

Are you you supposed to use a tachometer filter for these? I read that it's necessary to use these with EI.

I read this in a V8 hot rod book thing

Sorry for the thread jack

[datsunaholic]

I don't know why the 2.2KΩ resistor is used, or why they didn't simply incorporate it into the tach itself. Other NAPS-Z dual-plug carbbed vehicles (the '81 510, for instance) doesn't have that, and the tach works fine with both the 8-plug NAPS-Z as well as a 4-plug, external-box '78 EI.

 

One thing I can think of is that the EI system on the 720 is so sensitive that the tach was backfeeding the EI and causing misfires. Or, the '84+ 720 with the feedback carb & ECU was fed using a different method- I know the '83 200SX had the tach run off the ECU and not directly off the EI/Coil. As long as the tach is sensitive enough to pick up the pulse, you wouldn't need a lot of current.

 

Or, as I stated before, a tach that uses a direct pulse rather than an inline electromagnetic pickup has a chance of shorting the coil. By putting in a fairly high resistance resistor that prevents it from happening.

 

Or, the tach having too much current would interfere with the radio.

[datzenmike]

I figured the tach could still 'read' the signal through the 2.2K and it wouldn't look like a short to groound from the EI side. Shrug who knows.

[zoltans4]

Okay, my tach looks like the bottom picture of Datzenmike's picture, so from your description it is just power/ground then the tach input single wire from the coil.

[datsunaholic]

Yes, but you still haven't answered what kind of electronic ignition you have. A '79 was already wired for a tach, with the plug like the one shown on the bottom. So if your wiring is still intact and operational it should be plug n play.

[zoltans4]

i havn't pulled the old dash to see if a wire is there or not.

The distributor seems to be the 'matchbox' variety.

[zoltans4]

Ok, now that parts have come in and I am getting time for the truck I have gone and looked at a few things to see where i stand on this truck.

 

I understand why the electronic distributor was 'added'. It seems my 79 has a L16 block and W53 head. Not surprised because i know these trucks have been around a while and parts get used when needed.

 

So it looks like i am down some displacement (thought it should have more umph for a 2 liter), but got a good head? If i ever wanted to i could always just get the bottom end....

[datsunaholic]

I have a W53 on a L20B. It works fine, compression isn't too high for regular grade. Only issue is the ports are a little smaller than the stock L20B one. Makes good low-end torque actually.

[zoltans4]

thanks for letting me know, so now i can simply pick up a full motor or just the bottom end and be fine.

i appreceate the help and responses