**I've been searching on the forum extensively, and have read most threads dealing with the backup fuse (which is frequently used for resetting ECU), but couldn't find an answer for my concern. If I've missed a tread that addresses this topic already, please kindly provide a link to the thread.

I've bought a 2002 AP1 recently, and noticed an intermittent idle rpm fluctuation. I searched for the possible cause of the symptom,
and tried cleaning/zip-tying MAP sensor, and IVAC.

After doing the cleaning, I tried to reset the ECU by taking the backup fuse for a minute, but realized that the fuse has been missing the whole time. I'm not sure how it became missing. I'm suspecting the previous owner thought that the fuse was literally a backup, and used it for other blown fuse and didn't bother to replace. The previous owner did a lot of weird stuff, for example making a very good engine oil choice (Amsoil full synthetic) and using wrong weight (0w20). Anyways, realizing the fuse missing, I quickly pulled out a 7.5A fuse for interior light and put it in the spot for the backup fuse. I'm not sure how the missing backup fuse had affected the car.. I'm guessing the car reset the ECU everytime I started the car? My question is,

1. Could it be the missing backup fuse that had been causing the idle RPM fluctuation?

2. Is there any possible problem (on ECU or any other part) that would've been caused by missing backup fuse? What can I do to prevent the problem from exacerbating?

Any input would be greatly appreciated, thank you!

 

I dont believe you have caused any long term problems with the fuse missing.

You are correct in your assumption that without the fuse the ECU reverted back to a blank slate. This could absolutely be the problem with your idle. Anytime the ECU is reset it needs to go through a re learning process so with the fuse missing it hasnt had that chance.

 

On an AP1 the back up fuse To reset the ECU (no.25) is located by the drivers left knee in the under dash relay panel. No. 25 is the 3rd from the top on the right column of fuses ( there is also a left column and a bunch of relays even further left). As this fuse powers the ECM, heater Control Panel, Gage Assembly, and Convertible Top I think you are looking at the wrong fuse. Exactly where is the fuse you are missing?

Utah

 

Thank you for the replies. The idling RPM fluctuation is gone now with the fuse replaced and the MPG is noticeably better, so I think the car was resetting ECU everytime it started.

It was the fuse that is 3rd from the top on the right column. It is the fuse that is marked to be taken out for ECU resetting DIY.

this thread here

and this picture

 

The backup fuse only powers the ECU memory, the gauge lighting and key/light reminder.

 

Interesting. Page 10-9 of the '00-'01 Helm Electrical Troubleshooting Manual is what I am using as a reference? Which schematic are you looking at? Must have changed on the '02(?).

Utah

 

I'm not looking at the manual, I'm quoting from experience of having the fuse removed myself

 

Yeah from my personal experience those were the only components fed by the fuse for an '02 as well.

 

A friend of mine used to do this back in the day when he was running a test pipe to keep the CEL from coming on. It would effectively reset the ecu every time you shut the car off. This def could have been the cause of the poor idle and gas mileage.

But it looks like you've already figured that out now.

 

Agree this is the root of your problem. The ECU will continually reset without it. But I am confused as to why there is no mention your gauges being completely dark (?) in the OP. I was curious about the key light observation so went back to the schematics. As the key light reminder is driven off the gauge assembly (through connector C404) yes that won't work as there is no power to the gauges. Nor will the seat belt or door ajar lights.....also driven by power through the gauge cluster. Pulled F25 in my AP1 and confirm my original post is correct.....as noted in the factory manual.

Utah

P.S. Helm reference page 75 in the Helm AP1 Electrical Troubleshooting Manual