Electronic Ignition Timing - Sources of Advance?

[Captain Obvious]

I'm not an ignition guy and I'm having a hard time even figuring out how to ask this question, but here goes...

What are the sources of changes to base timing in the 260-280? Is there any spark timing control done in the ignition module near the passenger's feet, or is all the timing control done in the distributer with mechanical and vacuum advance?

By that, I mean... I know they use the typical weights-n-springs mechanical advance in the distributer, and I also know they use a typical vacuum advance with an actuator on the side of the distributer, but is that it? Are those the only two sources of advance or is there additional RPM derived electronic controlled advance done inside the electronic ignition module?

And no, I'm not talking about temperature switching between the two different pickups in the auto transmission cars. That's not RPM related.

[laminator]

I think you are right, only 2 sources of timing control, otherwise the vr converter module makes the dwell issues.

[Zed Head]

280 could include the 280ZX. They went to computer controlled timing with the 280ZXes that came with the Electronic Concentrated Engine Control System (ECCS ) (I think it was for turbos only but could be wrong) and the Crank Angle Sensor (CAS).

[laminator]

81 zx turbo has trigger wheel and crank sensor, yes only 3 connectors ecu's could control timing tables on zx's.

[Captain Obvious]

Thanks guys.

Yeah, I don't know anything at all about the ZX's, so I probably should have excluded them. Although, since you can pretty much drop-in some of the ZX distributors and modules into earlier cars, I suspect that not all of the ZX's have computer controlled timing.

Not sure if I've asked the question correctly, so at the risk of beating a dead horse, I'll do it again.

On the 260 and 280, there's a pickup in the distributor that sends a signal to an ignition module near the passenger's feet. When the ignition module near the passenger's feet sees the pulse from the distributor it cuts the current to the primary side of the ignition coil which causes a spark.

That's all the ignition module does, right? There's no timing modification done in that module adding any additional advance or delay to that spark, right? It sees a pickup pulse - It breaks the primary circuit. That's all it does.

Some propagation delay is inevitable, but that prop delay is fixed and does not change on purpose based on any RPM, vacuum, or temperature conditions.

And just for curiosity, since we're talking about it... In the automatic trans modules, it switches which pickup pulse it pays attention to once the engine has warmed up, but other than that, the manual and auto are identical.

[Zed Head]

Essentially, but you left out the dwell control function. I think that Nissan calls it "duty cycle". When they went to the computer controlled timing they also transferred dwell control to the computer.

That's why you can replace the big bulky 260Z and 280Z Nissan ignition module with the small GM HEI module, which also does dwell control, as long as you don't need the pickup coil switching function. $25 instead of $300+ (for a 1976 module at O'Reilly Auto).

The 75-77 (maybe the 74 260Z too) manual transmission cars also had a water temperature controlled second pickup coil in the distributor, usage controlled by the ignition module.

[Captain Obvious]

Yes, the dwell function is controlled by the ignition module. Got it. Someone has to make sure you don't burn up the primary side of the coil while still allowing it to build up enough field to get a good spark when you break the primary. Points do that mechanically in the 240, but in the 260-280, that function is part of the ignition module.

Now when you're talking about the GM HEI module and say that it works as long as you don't need the pickup coil switching function, you're talking about the dual pickup distributors and the ability to switch timing between those two different pickups, right?

I don't know how temperature comes into it for the 280's, but I can tell you how the 260 works:

Manual-trans 260's have a single pickup distributor while the auto-trans 260's have the dual pickup distributors.

In the auto-trans 260's with the dual pickups, the ignition module uses the signal from the advanced pickup when the coolant is cold but then switches over to the retarded pickup signal once the engine has warmed up. There is a temperature control signal derived from the coolant temperature switch that comes through the EGR relay and tells the ignition module that the engine has warmed up.

On the manual trans 260's, they have just one pickup although the same temperature control signal derived through the EGR relay still goes to the ignition module, I don't know what (if anything) the manual trans ignition module does with that signal.

That, in fact, is one of the things that got me started thinking about this stuff in the first place... I've got a manual trans car with a single pickup distributor and I'm wondering what, if anything, does the ignition module do with that temperature signal. If they aren't making any changes to the timing advance based on engine temperature, then why does that signal go to the module at all? They're already using a different wiring harness for the auto trans cars, so why did they bother to include that wire to the module if they aren't doing anything with the information?

In other words, if the timing on the manual trans 260s doesn't change with temp, then why do they bother to tell the module that the engine has warmed up? Does that wire even connect to anything inside the module? Anyone ever opened one up?

[Zed Head]

Interesting. I looked at the wring diagram in the Emissions section (EC-16) and it looks like the relay supplies power to the ignition module when the key is on. Maybe it's a dual function relay. I don't really understand Nissan's relay drawings though, it shows the ign. module circuit as closed and the EGR solenoid as open, but both are on the water temperature switch control circuit. Maybe it disables the advance circuit n the automatic transmission module when the engine gets hot, and they just forgot to describe it in the FSM.

It looks redundant. Is there actually a wire there in a manual transmission model, or is it only shown in the FSM?

Re your second paragraph - yes, that's what I meant. You can save a lot of money, but you lose the use of the second pickup coil.

[Captain Obvious]

To be honest, I haven't actually looked for that wire with my own two beady eyes. It's in the FSM, but I don't know if it actually exists in reality. As for that relay, yeah, the same relay controls EGR and spark timing (on the auto).

In fig EE-60 and in the text above that figure on page EE-27 (Engine Electrical) they refer to the same relay as the "Advance Control Relay". Same thing goes for fig EE-79 on page EE-34. They call it the "Advance Control Relay".

The physical location of the relay is shown in fig ET-51 on page ET-26 (Engine Tuneup) and it also mentions that the same relay controls EGR and spark timing functions.

So, why am I calling it the "EGR Relay"? Because if you look at the wiring diagram (the whole car wiring diagram - both manual and auto), you'll see the same relay down in the lower left hand corner (just like it is in the car) and it's referred to as the EGR Relay.

As shown on page EC-16, that relay has two sets of contacts in it, one normally open, and one normally closed while the coil is controlled by the water temp switch. The normally closed set controls the signal going to the EGR solenoid valve, and the normally open contacts control spark timing. It works like this...

• When the water is cold, the water temp switch is open, and the relay coil is not energized. The NC contacts are closed (EGR disabled), and the NO contacts are open (use advanced timing pickup).
  
• When the water is hot, the water temp switch is closed, and the relay coil is energized. The NC contacts open (which enables EGR), while the NO contacts close (which tells the ign. module to use the retarded pickup).

I know how it's supposed to work, and I still find that chart on EC-16 confusing! One thing that helps is to know that the picture shows the configuration of everything when the coolant is hot.

[Captain Obvious]

So, back to the original question about the sources of changes to base timing in the 260-280... Belief so far is that the mechanical and vacuum advance functions are the only two sources of changes to base timing. There are no electronic modifications to timing other than the auto-trans cars switching to the retarded pickup once the engine has warmed up.

Anyone want to bring up any divergent ideas, or do we stick a fork in it?

[laminator]

Megajolt lite jr.