HAILERS

The BTN fuse in the engine compartmet that feeds the ECU. Do you know what it does in life????? Car runs and starts just fine without one. What is its function???

EDIT: Well someone screwed with the subject as I wrote it. The title was *Anybody know what the BTN fuse does?

No7Yet

It's the "Back To Normal" fuse. It's the functional equivalent of pulling the battery cable and tapping the brake in that it resets the ECU.

Brandon

HIMNI Racing

Didn't know that. Cool......

HAILERS

No7Yet......have you seen anything in the fsm (86/87) that mentions this BTN? What section, please.

JerryLH3

iTrader: (11)

This is what I have from my 1991 manual:

BTN 60A - Light switch, heater and air conditioner
Circuit also protected in fuse box (STOP 20A, HAZARD 10A, ROOM 7.5A, ANTENNA 10A, ILLUMI 15A, ABS 15A, SEAT BELT 30A, FOG 15A)

That's the only mention of the BTN fuse I have found. It is in Section T - Body Electrical System.

Samps

I was also wondering about this (which is why I found it searching). I have removed a good bit of stuff from my car and I am now removing all of the associated fuses. I have begun to think that some of those unused wires are still "HOT" for no reason. But the BTN fuse seems to be a mysticle ghost in the manuals and wiring diagrams and like HAILERS said the car runs fine without it. And if removing it resets the ECU then taking it out for good would reset the ECU on every start-up right? Well when I removed mine it did not reset the ECU. Will it only reset if you also remove the battery cable? And to reset the ECU it must have a direct connection to it right? How come no pin on the ECU plugs is marked for this connection?

Icemark

BTN= Body,Tail, iNterior

White/red from the underhood fusebox,

On S4 it Powers:

exterior lights (except headlights)
Stop lights,
horn,
interior lights (except glove box and storage bins),
ECU memory
Idiot light and clock panel
Factory alarm memory
Audio system memory
Power antenna

In the CPU it powers:
seatbelt warning,
ign key warning,
lights buzzer
door and ign key lights

Does not power heater or air or anything to do with climate control on a S4

Samps

Well I guess I better leave it in then huh? Where exactly did you find that at Mark? I'm starting to think the series 4 manual is much more defined than the series 5!

Icemark

You need to disconnect it and drain the capacitive charge from the ECU to reset one.

There is a stored charge in th ECU that last for a couple of minutes. It will only be drained if the battery or BTN fuse is disconnected and there are things on in the car that are connected to the BTN circuit, (like if the brake lights were pressed or the interior lights were turned on).

BTW: the connection is in Pin 3J on the ECU, and yes the ECU will run just fine with it disconnected. It is again only a memory circuit for when the key is off

Icemark

Page 50:11 in the 87 workshop manual suppliment (Turbo manual) (Mazda P/N 9999-95-024b-87)

Page 50:14 in the 88 workshop manual (Mazda P/N 999-95-018S-88)

Scott 89t2

sweet I didn't know about the ecu reseting. that's sweet cause removing / reinstalling the battery term with a 1 farad cap sucks

I knew it powered all the interoir stuff though.

Samps

Hey scott, why are you using a cap? Most modern amps have built in caps that are sufficient enough for most setups. And those things are dangerous. I guess it's easier than upgrading the alternator though.

HAILERS

And I'd like to know what memory is being reset. What particular item? Certainly not codes. Not tps, not clock, not bac, not timing. What item needs a memory on a series four for it to function? Page 50-11 does show where and what the fuse feeds........but where did anyone find where its says its function for the ECU is to retain MEMORY???

Yes, I should have made it explicit in the original question that I was just interested in what function it does in the ECU.

And SAMPS, nope, the 89 up manuals are better than the 87, with the exception of the electrical schematics. They screwed with perfection when they changed from the 87 schematic format.

Icemark

Ahh you want a schematic for the ECU itself.

I am guessing here, but the majority of the time on older Computer controlled circuits (late 70's, early 80's) when there is a memory input circuit, but codes are not retained after switch voltage is used, the memory input is more for a surge circuit, to prevent powering down of parts of the unit, so that it doesn't have to deal with the surge of powering them back up for each use. Being that the S4 cars computers were designed early/mid-eighties when that was still pretty common.

Reted has spent a good amount of time working with the stock ECUs. He might have further ideas on what and why.

HAILERS

Thankyou. Well, I guess I won't cut it and dead end the wire at the ECU, like I was planning to.

Scott 89t2

hmmm connects to ECU on 89