Zed Head

Seems like what you really need to do is move the power supply source for the fan. The tachometer and MSD aren't the source of the problem. It's the power supply to the tachometer and/or the fan.

It would depend on current flowing through the transistor, the one in the drawing (if that's what it is). It's that meditation law. Oooohhhmmm...

The MSD tach wire bypasses the multiple sparks. That's the reason they have it on the MSD component. Not sure what you mean by blue wire taped up by the coil. How is the MSD tach wire connected to the tachometer? If you could draw out your circuit and post it somebody might have some ideas.

If it was mine I would measure resistance to ground through the blue wire that heads back to the tachometer. Since you have disconnected the original TIU the tachometer should be the only thing on the circuit. Not sure what it should be but it might show something. Do it with the key on and off. 12 volts. Power is supplied to the positive side of the coil, 12 volts, it passes through the coil and up the blue wire, through the 2.2 Kohm resistor and on to the tachometer. As shown in the drawing. The ignition module turns the 12 volts off and on. That's what the tachometer is designed to see - a 12 volt off-on (aka square wave) signal. The MSD tachometer wire (gray wire) is supposed to produce a square wave that looks like the one created by the ignition module.

I had a weird situation on my 76 where the tachometer would not work correctly unless I connected a capacitor, actually a simple RF suppressing "condenser" from an alternator, to the coil negative circuit. That was after I converted to a GM HEI module ignition system. I don't understand why it worked but I confirmed it later when the connection broke and the tach went wonky again. The needle would just sit and quiver at the low end of the scale. I was doing the same thing you're doing, just guessing based on my own limited knowledge. My working word was "noise". Anyway, just for viewing pleasure, here is the FSM schematic of the tachometer in a 260Z. I assume that the square with the line through it represents a switch, probably a transistor, that is actuated by the square wave from the coil negative terminal. You can see that the tachometer just passes current with each pulse from the coil.

Still not clear why you decided to start putting resistance in the MSD tach output circuit. The circuit already has a 2.1 Kohm resistor in it. The tachometer is designed to be on the end of a 12 volt line from the coil negative terminal. It can't be passing much current because it would cause the coil field to stay energized if it did. Maybe your tach is damaged and your efforts are finding a way to make it work.

More. Larry B can't be wrong...

There is stuff on the internet, of course. https://advancedstructures.in/starter-motor-teardown-and-feature-study/

Because it has been shown to work, very well. There is much discussion around the EFI Z based forums about why old EFI Z's very often have lean condition intake popping problems that are then cured by adding resistance to the coolant temperature circuit. Fuel quality, electronic component drift, etc. have been proposed as potential reasons that the system doesn't work the way it was designed to. But, whatever the cause is, the added resistance is the cure. There aren't many reasonable aging reasons for resistance to drop in the coolant temperature circuit. Maybe something in the ECU changes. Who knows. But cleaning connections in the coolant sensor circuit will only lower resistance, making the air-fuel ratio leaner. I know that you moved on to the ECCS system and other more developed Nissan EFI control systems long ago. The primitive 280Z EFI system has its own idiosyncrasies. The 280Z EFI has a fever and the only cure is more resistance.

No offense cg but I think that you have committed the not uncommon error of not reading all of the previous posts in order to grasp the whole of the situation. kully is using an MSD 6A ignition, using the tach wire (gray wire) from the box to run his tachometer. Tachmatch Voltage Booster, which is inline with his tachometer. The MSD has multiple spark at low RPM so his tachometer would be all over the place if he used his coil to drive the tachometer. Apparently the gray wire wire outputs a simple 12 volt square wave to stimulate the tachometer. It's described in the MSD instructions.

This occurred to me - a GM HEI module should work as a voltage increaser. It's odd that MSD's "12 volt square wave" is not 12 volts. But, you should be able to use an inexpensive HEI module as a pseudo amplifier. Connect some small load to B and C of the module, like a light or a resistor, with a branch off to the tachometer wire, and G and W to the MSD tach output wire. The G-W circuit will be the trigger, triggered by the MSD wire, to cause the module to pass the 12 volts to the light bulb or resistor, operating the tach through the branch. Basically the MSD tach wire acts like the reluctor trigger and the light bulb acts like the coil, and the branch off to the tach is the standard tach driver wire. Only commenting while it's in my head. Something to consider if you get stuck.

Here's the MSD stuff. I've always wondered about the voltage sensing tachs. Do they just see the voltage appearing and disappearing as the coil circuit is made and broken? Or do they need the big "flyback" spike from the coil discharge? https://documents.holley.com/6425.pdf TACHOMETERS The MSD Ignition features a Gray Tach Output wire that provides a trigger signal for tachometers, a shift light or other add-on rpm activated devices. The Tach Output wire produces a 12 volt square wave signal with a 20% duty cycle. Some vehicles with factory tachometers may require a Tach Adapter to operate with the MSD. For more information on Tachometers and MSD Tach Adapters, see the Tachometer Section on page 6. If your GM vehicle has an in-line filter it may cause the tach to drop to zero on acceleration. If this occurs, bypass the filter. INOPERATIVE TACHOMETERS If your tachometer fails to operate with the MSD installed you may need an MSD Tach Adapter. Before getting an Adapter, try connecting your tachometer trigger wire to the Gray tach wire of the MSD. This output produces a 12 volt, square wave (see page 2). If the tach still does not operate, you will need a Tach Adapter. There are two Tach Adapters:

I wonder how the tach wire on the MSD box works. That's where the voltage is coming from that is then either amplified or is used to trigger the Technoverion unit. "generates a stronger signal". I went back and found the instruction manual by accident. Here's some text. Click on Voltage Boost on the sidebar in the link and it will take you down to the instruction manual. Haven't opened it yet. So exciting!!! https://www.technoversions.com/documents/TachMatchVBoostInstructionManualRev-.pdf https://www.technoversions.com/TachMatch.html#TM4 TACHMATCH VOLTAGE BOOST MODULE Most tachometers can be driven with a 12v pulse signal, such as that provided by the standard TachMatch and the MSD-6 series. However, some tachometers need a stronger signal that more closely replicates the signal generated by the coil. The TachMatch V-Boost Module takes a lower level input signal, whether it be ECU output, MSD 6 tach output, or from a standard TachMatch TM-03, and generates a stronger signal to the tachometer, similar in function to the MSD 8920. Another application is when you are changing from a coil-style ignition to a system that uses a ECU. Often, ECU's, with a 5-volt signal, will not drive tachometers designed to use a 12-volt (or higher) level signal. This unit will convert from the digital-level signal to one which will drive your tachometer, similar to the Auto Meter 9117. The TachMatch Voltage Boost Module does not do cylinder conversion. If you need that, it needs to be driven from a TachMatch TM-03. Details of it's connection can be found in the instruction manual below.

That's not where I end up. My general feeling is "don't mess with the AFM".

Where do resistors come into the picture with the voltage booster? The pull-up resistor? Are there instructions that came with the unit? Might help to take a picture and post them. I don't see anything on the web site, except the FAQ's. Probably want to tell which MSD wire you're using also. https://www.technoversions.com/TachMatch.html https://www.technoversions.com/Faq.html

The AFM vane is controlled by a spring. So you have to get in to how the force curve changes as you loosen or tighten the spring. For example, if you loosen the spring, the vane will move faster at low air flow, than at high air flow. I think, my memory of spring rates and how to work with them is fuzzy. And I don't know if a clock spring (which is what you'd call the AFM spring, I believe) is a variable rate spring or constant. It's probably out on the internet somewhere. Maybe even in an old paper book. Plus there is some preload. Some things can just be logic'ed out though. The ECU might behave as it would when the AFM is maxed out at mid-range air flow rates if the spring is loosened enough to let the vane move to its limit, even though the air flow rate is still changing, because the spring is not hindering vane movement as much. So you'd end up rich at low air flow and lean at high air flow. Because the AFM signal to the ECU has stopped changing. Or, if you tighten the spring the AFM vane might not move at all at low air flow rates. So you'd be lean at low air flow for a certain spread of air flow rate, then still lean as more air passed by but did not move the vane. Basically you change the slope of the curve. The resistor slides the whole curve over, AFM adjustment changes the slope of the curve and can create artificial limits. The ECU and AFM and injectors and sensors are all finely balanced to work together.

It's just a way to richen the mixture across the whole temperature range, and air flow range. It's a constant. Edited - I'm not sure that the actual curve is as flat at the end as Nissan shows it. Maybe it is though and with added resistance the ECU never sees a fully warmed up engine. The whole curve will get shifted to the right.

Looks like they were only used for a short time period. https://jdm-car-parts.com/products/water-inlet-lower-radiator-hose-rh-front-of-engine-datsun-280z-1975-5-1976?variant=27526042633

I think that might be the purpose. Not clear how many 280Z's or 260Z's or 240Z's had it or why nissan stopped using it. I've seen it discussed before thoguh. ZCD sells them and some people put a T in the return hose from the heater core. https://zcardepot.com/products/coolant-hose-engine-bay-t-fitting-280z-280zx Here's a good place to browse if you want to see what things are supposed to look like. Sounds like you have an aftermarket AC system. https://bringatrailer.com/listing/1975-datsun-280z-22-13/

Your post seems to have flipped the cause and effect relationship. Coolant temperature affects sensor resistance. But sensor resistance does not affect coolant temperature.

The 1976 FSM has a complete and separate chapter dedicated to the AC system. The 1975 FSM does not appear to have it. Probably worthwhile to download the 1976 manual and take a look.

A picture of the AC compressor would probably tell something. And the throttle linkage. The factory setup has a vacuum controlled idle speed enhancer for when the compressor is loading the engine. The AC condenser in front of the radiator would also be a clue.

In the early days he was just a guy trying to start a business. It was small. Unfortunately for many small business owners they often try to get big and take on debt that has to the repaid. Give him a chance to respond. He used to be active on the site. You can find his name and comments in past posts. @zcardepot.com

Here's an old interesting thread about the Euro turbo. Apparently not widespread. Made for the "Autobonds". https://www.zcar.com/threads/euro-280zx-turbo.309088/