Ignition Switch.....

[britlude]

As we all know, the Ignition switch is problematic on the NSX, often responsible for not turning the engine over ... over the 125k+ miles I've put on mine, its happened a few times over the years, replacing the switch, searching out the hidden relay that the ECU fires when it decides it's OK to start, checking connectors, and such like...

over the last couple of months, it's reared it's head again, the starter not firing as soon as the key is turned, (you'll know the slight pause if you have experienced it, something just not quite right), or refusing to turn over on the key until it's been cycled a few times, or a jump pack connected, despite the battery having sufficient charge....

the car has already had a couple of new ignition switch units, OEM and Rockauto, and I have a shiny new switch spare already, but maybe it's more than just the switch itself, but the design of the system?

so... the power comes from the battery, thru the ignition switch, thru several connectors, thru a relay, more connectors, then to the solenoid on the starter motor... it's the starter solenoid, so a hefty current, going thru a lot of junctions and wires.... any resistance in this line will reduce the current available, and the starter wont play.... (forgot to put the earth on the starter!)
Nsx ignition switch starter mod by JON SUTH, on Flickr

Are the ignition switch contacts underrated for the starter circuit after years of use, wear and corrosion? causing the common ignition switch issues?

now if i use the starter solenoid feed just as a signal wire to fire a local relay, then all the electrical weight is taken off the ignition switch circuit and all the heavy switching is done by the relay....
NSX starter wiring mod by JON SUTH, on Flickr

so i thought i'd give it a go. I have the shiny new switch anyway, but as an experiment i thought i'd see if i have a better solution...

found a relay with a fuse built in to make things easier
Nsx ignition switch starter mod by JON SUTH, on Flickr

and wired it in beside the engine room (!) fuse box....
Nsx ignition switch starter mod by JON SUTH, on Flickr

a short feed wire from the battery terminal to the fused terminal on the relay, the N/O contact to starter motor, a wire added from the old solenoid connection to the coil of the relay, and the other side of the coil earthed to a nearby mounting bolt... simples

Nsx ignition switch starter mod by JON SUTH, on Flickr

neat, tidy, accessible, easily reversible as nothing has been cut... we shall see how it goes....

obviously, as the ignition switch (presumably) had already been playing up, the IGN switch terminals may have already been damaged, but it cold started this morning fine, and yesterday it didn't, and the day before didn't twice, one time at the tyre shop after fitting the new tyres! so some progress in my 3 day test period!

[Kaz-kzukNA1]

Interesting that your modified circuit improved the situation.

From starter point of view, IGSW is only used for the holding & pull-in coils that require just tiny amount of current.

The huge current is provided through the main B and M terminals via the contact face of the solenoid plunger.
You provided us the DIY replacement of that plunger.

I beleive you've already replaced/bypassed the starter cut relay.

The starter armature body mounted on the CL case is the GND.
Thick GND cable at the trans case and another one at the front bank of engine.

Would be interesting to know how it goes over the longer term as there seemed to be several owners with cranking as well as eng starting issue recently.


Kaz

[britlude]

yup, over the years i've replaced the ignition switch (several times), starter solenoid internal switching contacts (several times), and replaced the starter cut relay that's hidden under the computer rack in the passenger foot well

at one point i'd even run a piggy back wire from the starter motor solenoid terminal so i could energise the solenoid from the positive terminal in the fuse box, and start the car without using the ignition key start position. that did the trick, so i was already wondering about voltage drop over the numerous contacts and connections in the circuit

in the past I'd even bought a thermal camera to identify hot spots in the wiring while turning over (or trying to) the engine.... (showed hot-spots on the ignition switch plug!)

after failing to turn over without assistance, either by multiple turns of the key, or a jump pack connected, over the last week or so, I've tried the car at least 8 times now since the mod and its spun over and started straight away every time, so averaging in my favour each time i shall see how it does over the week, with the usual battery drain....

i shall continue to monitor...


edit.... from the limited info out there, and the 'can't be bothered to check on the car as it's dark', the solenoid side of the starter could peak at 40Amps and hold at 10amps, certainly the circuit carries tens of amps.... the relay i substituted takes 150mA, and the relay contacts now take the heavy load, not the ignition switch.... either way, most car firms now use a relay to fire the solenoid! so technically it's modernisation

[Kaz-kzukNA1]

Thank you for sharing the findings, Jonathan.

Learnt something new today!



Kaz

[britlude]

well, 2 weeks later and i've not had a repeat of any ignition start issues, so all good there.. with that i decided to tidy up the relay, as it was looking a bit aftermarket add on

3D printed myself a relay socket....

IMG_20250413_130348 by JON SUTH, on Flickr

IMG_20250413_130407 by JON SUTH, on Flickr

and fitted it beside the fuse box...

before....

Nsx ignition switch starter mod by JON SUTH, on Flickr

and after....

IMG_20250413_132337 by JON SUTH, on Flickr

IMG_20250413_132346 by JON SUTH, on Flickr

a lot neater