Tach info for 75 280z

[Cruzzar]

All guages were working before I made the switch to a megasquirt fi system. I found that the tach wire (blue) was shorting the ignition system out when it was attached so to get it turned I just left it off. When the engine is running it will short out the ignition if I attach it and the engine dies. I thought it might be the tach so I pulled the guage and wire plug out of the dash completely and the engine still dies if I try to attach the ble blue wire while it is running so I don't believe that the tach is the fault. Looking at the schematic in the service manual it appears that the blue wire goes to a resistor but I don't know where that may be, anyone know? 

I have removed the stock distributor and am using a turbo distributor for the crank angle sensor for the megasquirt. Can I remove the transistor ignition module that is located on the passenger side next to the fuse panel or does the tach need this to produce a signal and if so, could the module have a short in it and this is causing a short to kill the ignition?

One other thing, with the engine turned off I checked the blue wire to see if it was grounded and it wasn't , this is why I am asking about the module. When I power up the car it may be shorting out but isn't shorting when not powered up.

Any suggestions will be appreciated.

 

[SteveJ]

The stock ignition grounds at the TIU on the blue wire. If you are using a MS to fire the ignition, that means you two incompatible ignition systems connected to your car. 

Is the MS connected to the negative terminal of the coil? Post a copy of the wiring diagram for the ignition you are using.

Disconnect the TIU. Wrap the ring lugs in the factory harness individually with electrical tape. Reconnect the blue wire at coil and reconnect the distributor. See if you still have your problem.

 

[Zed Head]

3 hours ago, Cruzzar said:

Can I remove the transistor ignition module that is located on the passenger side next to the fuse panel or does the tach need this to produce a signal

Adding to SteveJ's post, re the question above - yes and no. The tachometer is on a branch off of the blue wire to the ignition module.  The resistor is in that wire.

Reconnect the wire at the coil and disconnect all of the wires at the old ignition module.  Megasquirt will probably work but the tachometer might not.  But that's a new problem.

[Cruzzar]

Megasquirt has only two wires going to the coil (plus the blue wire for the tach signal on the negative side). I have double checked that they are correct. Megasquirt  triggers the ignition from the turbo distributor module . I have completely removed the ballast resister. It sounds like the stock transistor ignition unit is killing (grounding out) the coil which is controlled by megasquirt. Since I do not have the stock distributor installed, it sounds like the transistorized ignition unit is useless unless the accompaning wires serve some purpose and it sounds like they are not--thanks SteveJ

I will try disconnecting all of the wires going to the TIU and isolate them and see if it works.

[Cruzzar]

Just as a follow up, I removed all of the wires going to the TIU and hooked up the tach and all is working. I know that it is pretty hard for an aftermarket FI manufacturer to include every bit of info in their instruction manual but somewhere at their tech line info center there should be a note that the TIU should be completely disconnected when switching to the turbo distributor.

[SteveJ]

54 minutes ago, Cruzzar said:

Just as a follow up, I removed all of the wires going to the TIU and hooked up the tach and all is working. I know that it is pretty hard for an aftermarket FI manufacturer to include every bit of info in their instruction manual but somewhere at their tech line info center there should be a note that the TIU should be completely disconnected when switching to the turbo distributor.

I figured there was a good chance the tach would work. 

I learned about disconnecting the TIU a long time ago from reading about the ZX distributor swaps. It's common knowledge with us old farts who have been on this board forever. I actually had that come up about 10 years or so ago. A friend did the ZX distributor swap on another guy's car, and it ran like crap. He brought the car over to my place, and one of my first questions was about the TIU. He quickly disconnected it, and the problem was gone.

I don't know how you have the CAS signal wired to the MS or what you did with the wires that the old distributor connected to, but somehow the TIU was getting a signal to ground out. That was what was messing you up.

The tach doesn't really care too much about what device is grounding the coil negative to fire the plugs. It's basically just looking for that pulse.

[cgsheen1]

On 3/19/2024 at 8:44 AM, Cruzzar said:

Just as a follow up, I removed all of the wires going to the TIU and hooked up the tach and all is working. I know that it is pretty hard for an aftermarket FI manufacturer to include every bit of info in their instruction manual but somewhere at their tech line info center there should be a note that the TIU should be completely disconnected when switching to the turbo distributor.

There have been numerous posts about this topic (changing ignition systems) and the stock tach. Most probably years ago but still in the ecosystem. A quick look at the EFI wiring schematics shows the economical way Nissan did the wiring to the tach. It tees off the Ignition Unit wiring rather than run separately from the ignition coil. I know that they hide it away under the dash, but why would you assume that the original Ignition Unit could just be left in place? I would think it should always be removed just as a matter of fact when you're changing ignition design. That's the way I did mine (well, long before megasquirt, but still...).

[Cruzzar]

If you look at the 1975 factory service manual under the section for the tach it shows the wiring schematic. If you follow this wiring diagram it does not show any wires (ie. blue wire) going to the TIU which is what was confusing to me as to what would cause the tach to short out the ignition. After reading SteveJ's response I went to the section on the distributor and looked at the area for just the TIU you will see it shows where all of the colored wires go that attach to the TIU, one which went to the blue wire from the tach.

[Zed Head]

That's one reason the 280Z's are great tools for learning about automotive electrical.  Everything on them is at the beginning of today's auto electronics.

Not trying to double-down on cgsheen's point but the start for modifying the ignition system would be to understand how the one you have works.  Again, no offense intended, but if you had done that you would have looked for the "make and break" device that creates spark.  The original ignition module.  If the coil primary circuit is not broken, no spark will be created.

It's fascinating stuff!  I've learned a lot by owning and working on a 280Z.  Even though I knew quite a bit before I got mine.

https://en.wikipedia.org/wiki/Delco_ignition_system

[cgsheen1]

6 hours ago, Cruzzar said:

If you look at the 1975 factory service manual under the section for the tach it shows the wiring schematic. If you follow this wiring diagram it does not show any wires (ie. blue wire) going to the TIU which is what was confusing to me as to what would cause the tach to short out the ignition. ...

Various sections show a simplified schematic for each circuit examined designed to give you a basic idea of how the appurtenance is wired. Not a detailed schematic of the entire circuit or system.

Note also that the wiring diagram - even though it's complete - doesn't actually show how each wire is actually laid out in the various harnesses. You usually have to dig a little deeper - which you eventually did...

Looking at the wiring diagram, one might say that since your megasquirt is now firing your coil rather than the TIU, that the blue wire should have been disconnected from the (soon to be abandoned) TIU itself and connected instead to the megasquirt output to the coil.

On 3/15/2024 at 8:11 PM, Cruzzar said:

Megasquirt has only two wires going to the coil (plus the blue wire for the tach signal on the negative side). I have double checked that they are correct.

I had a question about this: Megasquirt only needs the one connection to the (-) side of a single coil system in a Z since the stock B/W wire provides power at IGN ON to the (+) side of the coil. If you removed the ballast, the B/W simply needs to be transferred from the ballast to the coil (+). The B/W wire(s) in a Z are always battery voltage at IGN ON.

That may simplify your wiring - as would just using the L wire removed from the TIU under the dash to connect to the MS coil output. Then you wouldn't need additional wires to the coil (just leave in place the stock L and B/W)...

It's been a few years since I used a single coil with MS but I'm pretty sure that's accurate.

Edited April 1 by cgsheen1