Car electrical question

Factual Questions
1

The other day I brought my 1998 Jeep Cherokee into the county inspection station for my annual emissions test.

I failed, because the attendant said that they were unable to get any signal from my diagnostic system when they plugged in their computer. She said that usually, this is the result of a blown fuse.

At this point, let me say that my car has been running great lately, with the only exception being that my turn signals only work intermittantly.

Anyway, I went home and consulted my manual to determine which fuse was linked to the diagnostic system. I couldn’t find it listed for either fuse box (under the dash or under the hood). So I carefully removed the fuses one at a time and checked them all, replacing them back in their original slots.

Well I turned the car on and – surprise! – my turn signals work great now. However, the “check engine” light is on and, worst of all, my A/C would only blow hot air. Not good since it was 100 degrees in Nashville yesterday.

I thought, “well, these fuses are 11 years old, and seem kind of dirty, maybe I’ll just replace them with new ones.” So I went out, bought some new fuses, replaced them all according to the manual. No difference – still have the ‘check engine’ light and no cool air from the A/C (but the fan works).

Before I take this into the mechanic today to fix whatever I broke, does anyone have any ideas what happened and how I might fix it myself?

ETA: To clarify, I didn’t replace ALL the fuses, just the mini-fuses. There were some bigger ones under the hood that I inspected but didn’t replace.

2

The first thing that I would check is the fusebox itself. I’ve seen these degrade over time, and the fuse connectors get overheated and eventually become unable to make a good connection.

3

Well, in case anyone’s curious, I took it to the shop yesterday. They said they weren’t getting any info from the diagnostic computer and weren’t able to figure out what was wrong. Could be a short; could be a problem with the computer. They suggested another nearby mechanic who’s an electronics expert or, failing that, taking it to the dealership.

We need both cars today so I won’t be able to take it in until tomorrow.

4

Just think how science fiction-y this sentence would have sounded in 1970. This is why I hate computers in cars.

5

Okay, for the benefit of anyone who owns a 1998-era Jeep Cherokee:

It turns out, the fuse box diagram in the Owner’s Manual is incorrect. One of the slots listed as “Open” in the manual is actually a part of a pretty important circuit. I don’t have it with me to tell you which one, but if you care, send me a PM.

Once the techs traced the circuit to this fuse slot, and fitted it with the correct fuse, my A/C came back on and the Check Engine light turned off. They weren’t getting any error messages from the OBD2. So, I’m crossing my fingers and taking it into the inspection station tomorrow.

6

Was there a fuse in there to begin with that you took out?

7

And the rest of us hate inefficient, gas-guzzling, polluting, unreliable dinosaur engines.

8

Yep, apparently. When I was checking the fuses, I pulled them out one at a time to visually inspect them. As I replaced them, I consulted the manual to check the amp rating and location of each fuse.

I’m sure I took out a fuse, looked in the book and said, “Hey! This slot doesn’t take a fuse!” and put it in my little spare fuse container. And forgot about it.