[MX-5 NC] Need help - Intermittent 15A Engine Fuse blowing

Technical Area Engine, Transmission, Exhaust etc
2

Hi everyone,

First, I would like to apologize if the subject was covered in this forum. I could not find it.

As a brief description of the issue I am facing:

I was driving one week ago with my 2005 1.8L NC with a outdoor temperature of 35 deg. C, A/C on when suddenly at a semaphore the engine stopped working and all the lights came on. When I tried to crank the engine again, the immobilizer was signalling that the Keyless fob was not in range.

I tried the emergency key but with no success. Changed the key fob battery, still no success. After some investigations I found that the 15 A Engine Fuse(in the fuse box under the bonnet) was blown. I change it, but it blew as soon as I set the ignition to on. Tried several fuses but no success.

After several days, I have changed again the fuse and it worked.

Today, after 60 KMs of running, it happened again. I waited for a couple of minutes, put a new fuse and it started again.

After I arrived at home, I used VCM II to read the errors and got the following output:

B1600-E0- (IC) - (PATS) Ignition Key Transponder Signal Is Not Received - No Additional Fault Symptom Available for this (DTC), (DTC) present at time of request, (MIL) On for this (DTC), Test Complete

U0121-A0- (IC) - Missing the (CAN) message from the (ABS). - No Additional Fault Symptom Available for this (DTC), Previously Set (DTC) Not Present at Time of Request., (MIL) On for this (DTC), Test Complete

U0100-A0- (IC) - Missing the (CAN) message from the (PCM). - No Additional Fault Symptom Available for this (DTC), Previously Set (DTC) Not Present at Time of Request., (MIL) On for this (DTC), Test Complete

U0100-FF- (RKE) - Lost communication with the (PCM) - No Status Available for this (DTC) P1260-FF- (PCM) - THEFT Detected, Vehicle Immobilized. - No Status Available for this (DTC) 

I tried to start the radiator fans via the VCMII, they worked, tried moving O2 sensors cables, no issue, tried to wiggle relays and fuse cables, no issue.

 

Do you have any idea?

 

Thank you very much,

Rares

 

3

It appears you have a difficult job here. I would get the wiring pdf and the engine control pdf off the mellens site.

Disconnect the battery.

Remove the ECU and clean all connections and make sure all earth leads from the ecu have clean connections where the wires earth to the bodywork.

You may have to replace the ecu but that is after going through all the information from the mellens site.

4

This sounds like the ecu has been getting wet. There is write up on the small plastic cover near the wipers than let’s in water. Easy cheap fix. Then just let the ecu dry off for a couple of days. 

5

the op has a NC, the ecu is normally found under the battery in the engine bay. It’s unlikely the scuttle leak will effect it. 

I also note from the avatar picture that the vehicle is LHD and the op mentions a keyless fob not being in range. I would suggest to the op that if they have not already done so that they should also ask their questions on an American forum. 

Hopefully suggestion on this forum will help the op but we don’t have many 2005 NC’s with keyless systems.

 

Richard. 

 

6

However, the locking system and other cabin control modules and several fuses live just where the scuttle leak drips onto.  It almost looks like a design feature to hasten the end of the car - planned obsolescence. 

That scuttle leak is always the first place to look for electrical troubles in an NC.

7

It is a bit confusing as others have advised. the poster talks about keyless entry, was that available elewhere in Europe on the early NC?

The poster talks about Km, do the use Km in the French parts of Canada?

The car is reported as a 1.8. Unless someone knows better these cars were not imported into the States and therefore probably Canada if the poster is from there. Again unless anyone knows better.

Whatever the answers above, the poster has plenty work to do here tracking down the fault.

8

agreed, I sealed my own before a leak developed. Question, does the scuttle leak problem and in car fuse box position change to the opposite side of the car with a LHD, I’ve no idea! 

 

The op mentions stopping at a semaphore signal, I wonder where in the world!! 

Richard 

 

 

9

Hi guys,

Thank you for the answers.
To answer to Richard, the exact location is Bucharest, Romania.

At this moment I am trying to figure out where the short circuit.
I find that the multimeter is showing continuity between the wire coming out from the fuse (B/W) and ground.
I started removing coil, condenser and O2 sensor harnesses to reduce the possibilities.

Thank you,
Rares

10
  • 7.8V between the B/W wire(coming out of the fuse) and ground.

Rares

11

Are the wires that go to the O2 sensors original or have they been re located or extended.

Richard.

12

They look original in terms of length.
I replaced the front O2 sensor with an OEM one.

Rares

13

I had a quick look on https://forum.miata.net and it seems a regular reason for this fuse to blow is the wires to the O2 sensors.

I was also wondering if a short can happen inside an O2 sensor. Have you disconnected them to see if the short circuit disappears?

 

Did you replace the front sensor as a result of this problem? 

Richard.

14

Hi Richard,

I have checked the from O2 sensor, it’s resistance was in parameters(~ 4Ohms).

At this moment I found another issue:

 

Wire for 1st cilinder coil is ripped from the connector. Pretty sure I wasn’t the cause.

Since this connector also includes the B/W wire from the 15A fuse, it might be the issue.

Wiring diagram:

 

What do you think?

 

Rares

15

That has definitely got to be repaired to rule it out. If it was shorting out I would have though you would have detected the engine dropping on to 3 cylinders occasionally.

Have just read another theory on maita.net, it refers to the ignition relay earth/ground wire over heating on the underside of the engine bay fuse board. This wire had melted onto a live wire and blown the fuse. (A different fuse to your problem on this occasion). A cracked relay was the reason given for this wire overheating. (I suspect the ground wire may have had a poor connection). It had melted through one live wire and damaged two others.

Richard.

16

I have fixed the coil cable issue.

Still I see that the black cable, from the fuse box is heating. Will check if any relay is overheating, maybe I find the culprit there.

Thanks,

Rares

17

Hi all,

After more investigations it looks like the black wire that goes out from the fuse box is heating badly in certain conditions:

  • Car idle and blower+AC off : wire temperature normal, very little heat produced.
  • Car idle and blower on, AC off : wire gets hot, but still doesn’t heat very bad
  • Car idle, AC on auto, blower on max : wire becomes very hot

The wire heats starting from the fuse box harness(near the relays section) and continues towards the wires that are going to the car’s interior.
It heated so bad that it’s insulation is broken in several locations where I also found it to be glued to other wires. Also the wire is not fully connected to the harness, a couple of parts of the wire are tied from the connector.

Any suggestions for this?

Thank you,
Rares

18

Hi all,

After more investigations it looks like the black wire that goes out from the fuse box is heating badly in certain conditions:

  • Car idle and blower+AC off : wire temperature normal, very little heat produced.
  • Car idle and blower on, AC off : wire gets hot, but still doesn’t heat very bad
  • Car idle, AC on auto, blower on max : wire becomes very hot

The wire heats starting from the fuse box harness(near the relays section) and continues towards the wires that are going to the car’s interior.
It heated so bad that it’s insulation is broken in several locations where I also found it to be glued to other wires. Also the wire is not fully connected to the harness, a couple of parts of the wire are tied from the connector.

Any suggestions for this?

Thank you,
Rares

19

This is not good, there is no way that a ground wire from a relay should heat up like this, even if it is a shared ground wire it should not happen.

I suspect from your description that the wiring loom needs to be checked for any more damage. Also the place that the ground wire is connected to the body needs to be found and checked.  I would consider installing a new ground wire.

The thread that I found on miata.net, the poster replaced the relay and the burnt section of wire that was visible. He said that the relay cost $20 (US Dollars). Can you source a replacement relay, maybe try a relay from another Mazda and the check if the ground wire heats again.

Richard.

20

Hi,

After another afternoon under the hood of my Miata I have replaced the burnt ground wire, at least the visible part.

I have also measured the voltage drop from the ground point near the brake cylinder and the battery - terminal with the following results:

 - ignition on, engine off : 0.25 V

 - ignition on, engine on : 0.5 V

 - ignition on, engine on, blower on : 1 V

 - ignition on, engine on, ac + blower on : 1.3 V

 - ignition on, engine on, ac + blower on, headlights on : 2 V 

Tried to remove one fuse at a time and the meter showed 0 V when the ROOM fuse was removed. 

@Richard, can you point me to the thread you found on miata.net?

 

Thank you,

Rares