Getting a 240Z tacho to work with an RB25DET engine

[jamo240]

Hey gang...I thought I would do a separate post so it's easy to find in future for people installing RB25 engines (or in fact any 6 cyclinder engine using multiple coils), and wanting to get their standard 240Z tacho to work (may work for 260Z too...not sure if they trigger off the positive side of the coil).

 

From what I read on the web before I did my installation, I could not find any solution to driving the 240Z tach from a modern engine management system (such as Nissan ECCS or Apexi FC), as those systems have a dedicated tach driver output that operates a tach designed to pick up the pulses it outputs. A 240Z tach cannot recognise these pulses, and is designed to pickup the activity on the positive side of the ignition coil. In the standard 240Z, the coil feed either goes through a ballast resistor if fitted (when key is in the run position), and then through the tach, or in the crank position it bypasses the ballast resistor but still goes through the tach so that the coil gets full battery voltage for starting. In both cases, the coil supply goes through the tach and that's how it picks up the number of sparks and displays that to you in RPM via the needle on the tach.

 

Most solutions I saw involved replacing the tach with one that works with the Engine Control Module (ECM) output, or attempting to convert the 240Z tach to accept the ECM output. While this all works, it is unnecessary, and assumes the only way to get a tach signal is to work with the ECM output.

 

The other way is to supply the six ignition coils via the standard 240Z coil supply. The way this is done, is via the standard RB25 ignition relay. When you wire up an RB25 engine, you will find in the R33 skyline engine harness that there are two relays near the ECM that are often referred to as the 'relay twins'. They are identical in configuration, and one of them (the ECCS relay) supplies power to the ECM, while the other one (the Ignition relay), provides power to the ignition coils. That is it's only function. Both relays are energised, and in turn grounded by the ECM so they turn on, when the ignition key is turned on (an 'ignition on' feed to the ECM is the reference it uses to 'wake up', and ground the ECCS and IGN relays).

 

The relevant circuits are:

 

AF04: Red wire - power to IGN relay switching coil when key is on

AF49: Orange wire - grounds IGN relay switching coil and 'turns it on' when ECM is powered up via circuit AF36

DA31: Black/red wire - power to relay switching circuit when key is on (this is the 'normally open' circuit in the relay)

AF50: White wire - power output to ignition coils once relay is energised/switched on

 

If you take your black/white coil feed wire from the standard 240Z, and feed circuits AF04 and DA31 (you can supply them both off the same wire), then effectively you are powering the 6 coils via the original 240Z coil feed (in effect a series circuit from 240Z B/W coil feed to DA31 then to AF50), and are replicating the standard 240Z condition, only you are powering 6 individual coils firing one cylinder each, instead of one coil firing six cylinders....electrically, it's the same thing.

 

When you fire up your RB25 (or any other six cylinder engine using this configuration), your 240Z tacho will work, and hence you can retain the look and feel of the original. If you combine this with the VL turbo vehicle speedo cable output (from the gearbox), you can retain the look and function of the original 240Z instruments for both tach and speedo.

 

I will attach a circut diagram once I figure out how to compress the pdf scan I did of the circuit diagram described in these words.

 

Cheers

 

 

Jamo

 

 

[jamo240]

Here is a cct diagram of how it works.

 

Cheers

 

Jamo

[zzzzed]

sticky!!!!!!!!!!!!!!

[jamo240]

Not sure what Sticky is Mick, but hope the post is of service to you!

 

Jamo

[zzzzed]

Haha yes it surly is. I mean sticky it or pin it so it stays at the top of the list in electrical systems so we

Don't have to search for it I. The future. Please admins???

[jamo240]

One final thought to retain the standard instruments. For oil pressure, simply use your L series oil pressure sender unit screwed into the RB25 port in the aluminium oil filter body (it's the same thread), and for coolant temperature, use the sensor in the inlet manifold cooling jacket out of an RB20 engine (it is the same as the sensor in a number of other Nissan's built through that period). It is a direct replacement for the standard RB25 sensor, but has a response curve that jives with the 240Z gauge.

 

If you follow these instructions, you can retain all of the standard 240Z instruments without modification.

 

Cheers

 

Jamo

[sanGK]

Hey!

 

Thanks for this thread! I've had my rb25det removed for some major chassis work on my 260z 2+2 that was just bought.

 

The automotive dude said that the existing engine wiring loom is not in good shape anymore, and recomended that it be changed.

 

Would you be able to advise me on whether it is possible to customize the wiring, or where I can get an engine loom that is in good shape for a reasonable price?

 

I have no clue about engines, but I do know that the previous owner had done alot of mods to the car to fit the rb25det inside. The dash gauges are all from the 260 itself from what i know.

 

Hope that my questions dont seem too obscure for you.

 

Thanks!

[jamo240]

Hi SanGK

 

You can get used engine harnesses for RB25Det's on ebay. Just make sure you get one for the right series of engine you are using (Series 1, 2 or NEO). Your auto electrician should be able to do the adaptation into the car if they're prepared to nut it out. It is not a particularly difficult engine to graft into a Z car.

 

Cheers

 

Jamo

[sanGK]

Thanks for the advice:-)

 

Will keep an eye out for one that's in decent condition and pricing!

 

 

Cheers.

[gav240z]

sticky!!!!!!!!!!!!!!

 

Done.

[Millerman]

Sorry guys to bring back a thread from the dead but can anyone confirm if this all still works for 260z gauges? Has anyone tried it? 

thanks

[GregTas]

The 240Z tacho works because the tacho signal picked up by an inductive loop at the back of the tacho. So if +ve supply to new coil loops through the tacho loop it works fine. How I did my RB25. It detects the pulses. No electrical connection required.

I thought the 260Z tacho used a -ve signal from the coil. If this is the case then the above method won't work.

Edited August 6, 2020 by GregTas

[Millerman]

5 hours ago, GregTas said:

The 240Z tacho works because the tacho signal picked up by an inductive loop at the back of the tacho. So if +ve supply to new coil loops through the tacho loop it works fine. How I did my RB25. It detects the pulses. No electrical connection required.

I thought the 260Z tacho used a -ve signal from the coil. If this is the case then the above method won't work.

Time will tell! Ill post up when ive been through it with my conversion and let you guys know the results...

[GregTas]

If it looks like this on the back then you should be able to get it working.