MX-3.com

Ok. This is the second time I've had the CPU timer go bad that turns on and controls the rear defrost. I know it's the problem because I jumpered the defrost pins on the circuit board and the defroster comes on then. Maybe there's my cure...

Anybody else heard of these going out so much? It was a deceptive problem the first time this happened since everything else associated with the CPU worked fine, same as now. And it ain't listed as a "possible cause" in the electrical troubleshooting book. I mean I know it's a 15 year old unit, but...why they keep bustin??

Aight then. May want to make a note somewhere of what I did for future ref. Unless I'm apparently the only one that ever had these problems.

Evidently there is some little intermittent quirk in the logic chips here I keep running into...so I soldered a couple wires to the pins and ran em outside the little CPU box and taped off the stripped ends....could be run to a separate switch...temporarily shorting these together will bypass the chip when/if the timer "sticks" again and allow the defrost to come on...at least in this case I believe.

Cuz I'm tired of replacin timers! ~Peace

Just found out my rear defrost dosen't work. Could be something else of course but could you post pics of what you did?

I found the reason why it goes bad if anyone is interested in the actual source of the issue as opposed to temporary fixes. The relays under the dash are all mounted on the plate. the one which controls the rear defrost has a heavy blue wire/white tracer and a heavy black wire with yellow tracer and if yours does not work then its likely burnt on the plug. Well being that I hate just replacing a relay without knowing what caused it I dug a little deeper, printed off the wiring schematics and what not looking for the source of the excess ampage causing the burning. Here is what I found, the fused source from the main engine bay fuse box has a 40Amp fuse in it this supplies the constant 12v to the relay (the blue/yellow wire) which is going to be sending it to the rear window defrost element. The relays itself however is only rated at 20 amps. Now do the math here 40 amp source, 20 amp relay. Guess what burns up the relay of course. First rule of running a relay in a circuit is to NEVER fuse it for more than the relay can handle. So using logic here the relay burns up since the contacts are not meant to handle the extra ampage. So the correction? 1 of 2 options use a heavier 40amp relay than the 20amp factory one or option 2 replace the 40amp factory maxifuse with a 20amp one therefore matching the output of the fuse to the relay. You might want to sticky this as a solution to the burnt up rear defrost relay problem guys. As for the cpu timer of course it goes bad since to much juice is running through the circuit.

Could you also just wire in a 20amp fuse in series just before the relay, I'm not sure why I thought of that but doesn't something else in the circuit need 40amps?

Shades wrote:Could you also just wire in a 20amp fuse in series just before the relay, I'm not sure why I thought of that but doesn't something else in the circuit need 40amps?

the cig lighter is also in this series i believe..

What is the part number for the relay?

Are all 40 AMP relays same or is there particular one I need?

I'd imagine a 40A 12v one should do the trick.

yes, and it needs to be 4 post but I can't find one that will plug in, posts are in different locations.

mitmaks wrote:yes, and it needs to be 4 post but I can't find one that will plug in, posts are in different locations.

Wow, that thought didn't even occur to me. I'll be pulling my relay out tomorrow or the day after(I've been meaning to do it for a looooong time), if I beat you to it I'll take it down to a couple of electronics parts stores around me and find some details.

My defrost works but real slow, by the time it finally makes some difference I'm already at work lol. Would putting in 20A fuse make it work any different?

A fuse doesn't affect the power draw, so it won't affect the function of the defroster. It does nothing but blow and break the circuit if the draw is too much. A fuse eitehr lets the power through, or it cuts it off.

The elements just get old and inefficient over time. Nothing different than any heat element. Heated seats go through the same process.

Doesn't make sense. I used to have 88 civic and defroster would literally melt away whatever was on the rear glass within minutes. On mx-3 it takes like 20 minutes to make any difference. Would changing relay help at all?

That's a totally different car, you can't compare, they just designed it to be that much hotter.

A relays is just a (high-current) switch controlled by a (low-current) switch. It will not limit power unless the power your pushing through the relay is more than it's designed to handle (typically 30/40amps), than the relay just fails. You also have the fuse that is your safe guard and is designed to blow before the relay does, or the wiring catches on fire.

Current is an on-demand thing, just the power source has to be able to handle the demmand. If it can't supply the required amount, It'll destroy itself attempting to.