NC won't crank and AC controls not working

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: __NC Mk 3.5
  2. I’m based near: __Haslemere
  3. I’m looking for technical help or recommendations on: __electrical issue stopping the car starting

Hi

I was driving home in the rain a couple of weeks ago and all my heating/aircon started working intermittently then stopped completely with no lights on the controls. Everything else on the car was fine - lights, engine, dash, wipers etc.

I read the DTC with Forscan and found the code:

===IC DTC U0028-60===
Code: U0028 - Vehicle Communication Bus A

Status (-60):

At this point the car started fine, so I just thought the heating control module had failed and bought another one from a breakers. When that arrived I tried starting the car again before fitting the new one and despite all the lights on the dash including the immobiliser working as normal it didn’t crank. Suspecting the heating module was stopping things working I removed it and the radio and the car started OK. By then it was getting a bit late and starting to rain so I gave up.

Next day, the car wouldn’t crank at all, either with or without the replacement or original heater module plugged in.

Since then I’ve tried multiple times but haven’t been able to get it to crank. I’ve removed the glovebox and checked in that area for water damage but it all seems fine - it was leaking, but I fixed that last summer.

Does anyone have any idea what could be wrong and where to look next?

Thanks
Matt

2

Suggest you check the fuses, the ones at base of the A pillar passenger side and the main fuses in the engine bay.
Pay particular attention to the room fuse although I can’t remember what this controls, been known to cause this type of failure of the systems if that goes.
Also whilst at the inside fuse board check for water ingress, a possibility even if you’ve check for around the glove box.
Fuses plan below…

3

As MickAP says, also look for evidence of leaking grommets, rust streaks on the metalwork behind the glovebox.

This is a well known weakness on the NCs of all years.

You can see it in the pics I took when I fitted the dashcam on the nine year old 25AE I’d just bought.

So the Scuttle grommets were the next items I sorted.

4

Thanks MickAP

I checked the fuses and it turned out to be fuse 6 - IG Key 2 in the engine bay fuse box. Oddly it hadn’t really failed and when I used a meter across the fuse with the power off it showed OK. It was only when I checked the voltage across it under load it showed about 6v, which obviously shouldn’t be possible.

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