I have one of those strange electrical glitches. I keep blowing the 10amp heater fuse located in the kick panel. When it blows I lose the HVAC display (and all functions it provides). Symptoms are when I hit the brakes, it blows this fuse. I have been having an issue for quite a while that when I hit the brakes the instrument panel dims. It still does that even after the fuse blows. I have checked grounds BM, BL, IF, and IG which all seem to have connections to all of these functions, but all of those grounds appear to be ok.

Does anyone have any advice on what to check? I am thinking maybe there is something wrong with the light failure sensor, but I don't have any conclusive reason plus I don't know where it's located even if I knew how to check it out.

Edit: Should have stated first this is on a 93 SC300.

 

Ok, I assume no one has encountered this specific issue which doesn't surprise me given the weird nature of the issue. Anyone out there with access to the wiring diagrams and willing to spend a minute or two looking, can you see a connection between the Brake lamp circuit and the 10 amp Heater circuit? I can not other than it feeds from AM1 main power supply.

 

I noticed in the owner's manual that a lot more runs off of that fuse than the name would imply. Have you consulted that list?

 

I have followed each line in the wiring diagram, so I don't see anything else that could be drawing power. I did a little more troubleshooting today, it seems as if the power from the 40 amp Ign Sw fuse to the ignition switch back to the string of fuses (heater, turn, wiper, gauge, engine) is being effected at the ignition switch. I jumped across the power side of the Radio 2 fuse and jumped it to the power side of the heater fuse since the radio 2 is powered through the ACC output of the ignition switch and it seems to be working without the fuse blowing. The ignition switch might be the source of my issue. Unfortunately I traded my ignition switch from my race car for something, wish I had it now.

 

Since you haven't mentioned what year and model is your car (wiring changed in different year / models ) , here are the common circuits that draw power from that 10a Heater fuse

a) 10a Heater Fuse -> Heater Relay Coil -> AC Control Assembly (HVAC Control)
b) 10a Heater Fuse -> Heater Relay Switch -> Heater Blower Motor -> Blower Resistor -> AC Transistor -> AC Control Assembly (HVAC Control)
c) 10a Heater Fuse -> AC Magnetic Clutch Relay Coil -> ECU Magnetic Input Pin
d) 10a Heater Fuse -> AC Magnetic Clutch Relay Switch -> AC Magnetic Clutch & AC Control Assembly (HVAC Control)

The brake lamps has no direct circuit connection from the 10am heater fuse to answer your question in one of your post.

If your ignition switch is bad , then that 10am Heater Fuse won't blow or brake. Or if it is shorted , the 40amp fuse or even the 100amp fuse nearer the battery will blow / brake depending on the amperage of the shorted ciruit.

 

At the bottom of my post I did list it as a 93 SC300.

The connection is all the way back where it splits off coming from AM1. A corroded or bad connection can cause what I am seeing, so it doesn't necessarily have to short, but I don't think at this junction is where the problem is. The other connection between the brake lamp circuit and the heater is the brake lamp failure circuit or bad lamp indicator comes from the Gauge fuse which draws power through the ignition switch just like the 10amp Heater fuse does, When I disconnected the brake lamp failure sensor and just jumped the plug to get brake lights without it it seemed to clear up the dimming of the instrument cluster lights but I still kept blowing the heater fuse which indicates to me a low voltage situation which increases amps and blows the fuse. The light failure sensor locks in the bulb in the cluster and goes straight to ground, so it does draw amps. When I jumped across the input side of the Radio 2 fuse to the input side of the Heater fuse, this jumped the power from the ACC side of the ignition switch to the buss supplying power to all the fuses from Ignition (Heater, turn, wiper, gauge, and engine). When I did this, the Heater fuse quit blowing. It must be getting a full 12v via the jumper across from the ACC. So, all that to say I still think the answer is in the ignition switch.