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I went out to reset my ECU. After having done the "battery dance", as Josh calls it, several times already, I decided to try the Fuse 14 version.

I pulled the fuse, and wondered what would happen if I turned the ignition to On. Well, lo and behold, the check engine light came on. I didn't try to start it, but connected the green plugs. All the relays, fans, etc started doing their test mode dance!

So... Is fuse 14 the only power source for the ECU? Or is it just standby power so it doesn't loose its learned settings? I haven't delved into the wiring diagrams to find out for sure, but I thought I'd post it here to get people thinking - Fuse 14 may not be the right way to reset the ECU.

Oh yeah... I ended up just pulling the negative battery cable because I want to be sure it gets reset.

Fuse 14 supplies the ECU with backup power. When running, the ECU gets powered through SBF-2.

I don't think disconnecting fuse 14 is a great way to reset the ECU, since the capacitors in the ECU remain charged.

Warning: Untested idea follows. Maybe you could remove the fuse and then use a short piece of wire with a resistor on it (a small piece of stainless steel would work too) to short the cold side of the fuse's connector to ground.

I've tried the battery dance and the fuse method repeatedly but could never get the ECU to reset. I've even left my battery unhooked for 48 hrs before and that didn't work! The only way that has ever worked for me was doing it with the green D-check connectors. Connect the green connectors, turn the key to on but don't start the engine, hold the accelerator all the way in for 10 sec. then let off the accel. and start the engine. Then drive like you would in D-check. When/if you get the all clear code the ECU memory will be cleared. Like I said, this is the only way I've ever been able to clear codes (except with a scan tool of course).

Vikash, your idea sounds interesting... But I think I'll just stick with disconnecting the negative side of the battery. My radio presets aren't that important to me. I wonder how long the capacitors remain charged. Actually, so then how does depressing the brake pedal affect the caps?

Matt, I'm trying to see if an ECU reset will fix my current problem of the ECU not controlling the boost controller. Unfortunately (I can't believe I said that), I don't have any codes. But yeah, the most reliable way I've seen to clear codes is the method you wrote.

fuse 14 works for me, and that's what the fsm says to do to clear the TCM. I will double check to make sure it does in fact say that.

There also needs to be some clarification between resetting the ecu and clearing the memory. They are two different things.

Doing the connectors does not reset the ECU, it simply clears the codes. The only way to reset its learning maps is to disconnect power from it. At least that's what I've found from my testing.

I have heard of people not being able to clear a code by the battery dance for some reason and the connectors work.....couldn't tell you why though.

verbatium from the 1990 fsm: page 145 sect. 3-2 (says same in the 1992 fsm page 179 instead)

CLEAR MEMORY

Current trouble codes shown on the display are cleared by turning the ignition switch OFF after conducting self-diagnosis operation. (they were referring to hooking the select monitor up to it, but I think the manual way to pull codes would count the same) Previous trouble codes, however, cannot be cleared since they are stored in the ECU memory which is operating on the back-up power supply. These trouble codes can be cleared by removing the specified fuse (located under the right lower portion of the instrument panel), as shown in the following table.

CLEAR MEMORY: Removal of No. 14 fuse (for at least one minute)

- The No. 14 fuse is located in the line to the memory back-up power supply of the TCU and ECU (MPFI). Removal of this fuse clears the previous trouble codes stored in the TCU and ECU (MPFI) memory.
- Be sure to remove the No. 14 fuse for at least the specified length of time. Otherwise, trouble codes may not be cleared.

Weird. I know I've done the negative battery cable removal for over a minute before and not had the codes cleared. Maybe there's something to just removing the fuse? :S

I've tried removing the # 14 fuse multiple times and always for longer then 1 minute and the codes never once cleared that way for me. Like I said before, the only way I've ever had my codes clear (besides using a select monitor) was using the green connectors.

Are you sure that when you clear the memory using the green connectors it doesn't reset the ECU? I swear I could tell a difference in the car every time.

the green connectors only do not clear the ecu's codes. It puts the ecu into D-check mode.

To get in clear mode you need to connect both black and green connectors.

Whoa, what? Really? I've cleared mine before and I could swear it was with just the green ones. Padishar and I actually cleared them on his 93 N/A that way too. I really don't think we had the black ones hooked up. When we got the all clear in D-check, it cleared the stored codes too. I don't know, maybe we were huffin' fumes but damn, I don't remember having the black ones hooked up at all.

Matt - Wait a second. I thought the first time I met you we did reset your ECU with the battery method. That was how we cleared your fuel trims, wasn't it?

Mike - Pressing the brake pedal just powers the brake lights, so any capacitors between the car's +12v rail and ground can discharge through the filaments.

In my experience test mode (green connectors) alone didn't clear the codes. In fact, the only difference between test mode and clear memory mode is whether or not the codes get cleared after you get the "all OK" flash.

It's possible that on a bone stock car there aren't any significant capacitors across the power rails... It seems weird but I guess it could happen. It's also possible that installing an aftermarket accessory (any aftermarket acessory) that uses power could change that. That might explain why the FSM says to do that but why it doesn't work for some people. Or it could just be another error in the FSM.

vrg3 wrote:Matt - Wait a second. I thought the first time I met you we did reset your ECU with the battery method. That was how we cleared your fuel trims, wasn't it?
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Hmmm, I don't think so. I thought it didn't work and we had to use your scan tool. I remember you commenting on how you thought it was strange. We also couldn't get anything to reset or change until we unhooked the O2 sensor. I'm not 100% sure though, I guess I could be getting confused.

I'm starting to get a weird inklin' in my brain about the black connectors too. I kinda remember them both being hooked up now. I know that I followed the directions on your site Josh except that a few seconds on the gas didn't work for me. One of the members across the pond mentioned in a thread of old about holding the accelerator in for 10 seconds instead of 3-4 and that's what finally did it for me. Anyways, if your instructions say to hook up both the green and the black, then that's what I did and my mind is starting to slip.

Hmm. Maybe you're right. I do know the Select Monitor only clears codes, not fuel trims, but maybe we just did both the battery thing and the Select Monitor thing. Two variables in one experiment.

I've never had to do anything with the throttle when clearing codes with the connectors.

Ya, maybe we just cleared the codes w/ the select monitor. I remember we did the battery thing, I was just thinking for some reason that it didn't work but if the select monitor can't reset the ECU then that must be how we did it.

So how does a person know if the ECU definitely reset or not?

Well, if there were codes and now the codes are gone, that's a good indication. Or if you hook up a scantool and find that all the fuel trims/timing corrections are zero...

Ok, I'm totally confused now. Reseting the ECU and clearing the codes are seperate or not. Lets say I do my black connectors only (Read-mode) and I have 3-4 codes, ok. Then I try and reset my ECU by unhooking the battery and depressing the brake for a minute. You're saying that the way I'll know that my ECU reset is by the codes being cleared? Because that does not occure for me. The only way I can clear my codes (except scan tool) is the green and black connector method.

What I am saying is:

Resetting the ECU is more than just clearing codes.

The ECU stores two types of things in its nonvolatile memory. 1) it stores trouble codes, and 2) it stores information it has learned about how to manage the engine well.

Using the red and green connectors should clear the trouble codes but not the learned information.

Removing the ECU's power should clear all of it.

I don't know what to say about the fact that you and several other board members report the battery dance not clearing codes. All I can think of is that possibly the codes are clearing but coming back somehow; some people have observed their ECUs storing codes but never illuminating the CEL.

Ok, I get what you're saying and I did understand that it's more then clearing codes. So short of having a scan tool though, there's really no way to know if the ECU reset after the battery dance (for those of us that don't lose the stored codes, that is) ?

I guess...

But I can't understand how the battery dance could fail to reset the Hitachi ECU. The data is stored in the SRAM built into the HD63140 chip, and unless it's some crazy weird wacky kind of SRAM, it can't retain its contents without power.

Hmm. I should take a closer look at the motherboard. Maybe the 5-volt rail somehow retains power sometimes. And maybe there's a way to drain its caps.

Maybe there's a big smoothing cap that doesn't drain using the brake pedal trick.

I dunno...

While you're in there, see why my stock boost controller doesn't do anything!

Ya, because it sucks.

Remember, we couldn't get the codes to clear that first night. I think you're right that we did the battery dance to reset my fuel trims but we had to use your scan tool to actually clear the codes. Does that ring a bell with you?

Yeah, it does kind of ring a bell. But I thought we never actually got the codes to clear, since we did leave it with the oxygen sensor disconnected at the end.

We did the battery dance twice, right? Once before and once after disconnecting the oxygen sensor. I don't remember at what point during that process we cleared the codes with the Select Monitor.

Were we even trying to clear codes? I thought we were just trying to figure out what was up with the fuel trims.

I remember at one point towards the end, while we were using the select monitor, you looked over and confirmed that I wanted to clear them and I agreed. I remember saying how cool I thought it was and that I wanted a SM for myself for that reason alone. The clearing of the codes wasn't our focus but we did do it.

Do you think you'll be able to get that function going (clearing codes) in your SM sotfware for the laptops?

I might be able to. Or I might be able to just make a simple little separate program to do it. I'll look into it.

Sweet! That'd be cool. Oh, I may be getting a laptop soon too. My partners wifes stepdad works for some huge corporation in the DC area as their PC tech guy and comes across all sorts of stuff. I guess he has already givin' laptops away in the past and is trying to source one for me. I got my fingers crossed.