I'm tracking down a weird electrical issue. When I depress the clutch, my gauge cluster lights get dim and I sometimes pop the meter fuse (15a). I began troubleshooting but am having a hard time finding a short in the circuit. It's a 84 GSL-SE.

To avoid damaging my haltech, I depinned the clutch switch wire into the ECM and patched it straight to a chassis ground for troubleshooting. Looking at wiring diagrams, there is a lot of stuff under this fuse: hazards, reverse lights, cruise control, clock, gauge cluster, and more. I started unplugging things like the cruise control module and transmission sub-harness connectors but still see the dash dim with the clutch switch on.

I haven't been able to disconnect everything because remaining connectors are buried in the dash, and before I undertake removing the dash I wanted to see if anyone here is familiar with this problem or has a better way to narrow it down. The car sat for a few years while undergoing a restoration and in that time a few wires deteriorated and caused shorts, so I expect something similar here, I just need to find it! Any ideas?

 

Long shot, but does the symptom persist when you turn off the Cruise Control Master Switch on the right lower dash? The reason why I ask is that the Clutch Switch has only a few actions, one of which is disconnecting Cruise Control. The Master Switch enables the controller inside the driver's side storage bin, and this is a common area for corrosion and also wire shorts. Inside the metal body control module is a rough circuit board that is mounted low enough that any water intrusion can cause rust and shorts within the module.

It's worth a quick check, as the Master Switch is a 2-second test. Even removing the Cruise Module is only a 30min job. Disconnecting the wiring harness where it connects to the module might also remove any shorts, and the symptoms might stop. There is no Clutch / Transmission / Starter interaction (*i.e., neutral or clutch in start safety) - so I wouldn't waste time on that.

Resist the urge to install a higher rated fuse. This will only cause problems with potential wires melting and then you'll have a much more serious problem. Let us know what you find, and be patient.

 

Thanks, I did unplug the cruise control module (in the bin) but did not mess with the master switch. May as well rule that out.

I did test for continuity to ground on the RY wire from the clutch switch to the ECM and didn't have any, so I think the short is above the clutch switch, below the fuse box, somewhere in the GY wire that goes all over the car.

 

Any chance you have stray wiring that is getting tugged by the pedal, grounding out the power to something in the lighting circuit?

Thinking of cruise control... look at the wiring that runs along the left side of the car for pinched wiring. I don't remember if there is a flat connector in the harness, but if there is, corrosion crusties could be contacting wires that shouldn't. Same for the backside of the cruise connector, it is down where wiring can get damp and corrosion could be jumping two circuits together.

Mazda did not use weatherproof connectors inside the car.

An easy way to find a short is to replace the fuse with a load like a headlight bulb or something noisy like a washer fluid pump. Or a horn if you are a masochist. Ignition on, lights off, block of wood to hold the clutch down, then wiggle wiring in various places, unplug things, etc until the light goes out or the noisy thing shuts up.

 

I think I figured this out, and it wasn't an actual wiring problem so much as a misunderstanding of how to wire the clutch switch into the Haltech ECM. Haltech SPIs are switched to ground, so when I pushed my clutch pedal and closed the clutch switch, I was actually shorting the whole 'meter' fuse circuit through the Haltech. I thought I could simply wire in the existing clutch switch the new ECM and have it work but that's not the case. What I did instead is de-pin the GY +12v wire into the switch and replace it with a wire to ground instead. Then, I turned on the 5v pull-up voltage in the Haltech, so 5v means the clutch is engaged and 0v means it's not.

I did pour through the wiring diagrams and map out everything after the 'meter' fuse (I think) so I'll attach that here for anyone else to reference. It's a lot!

 

See, this is why I don't like computers in my cars (as a retired software engineer).