Ok, with the weather turning colder it seems like everyday people are popping up wondering why their jeeps aren't starting like they were in the warm weather. This post is going to try and explain one of the more common problems the YJ is known for and hopefully give you all some way to narrow it down before throwing parts at it that aren't needed. The underlying problem here is that the computer is failing to power up when you turn the key on.
Symptoms:
You guessed it, the jeep doesn't always start right away. Instead it seems to sit there and crank forever. Usually (but not always) this is followed by a rapid clicking sound coming from under the hood and then miraculously the jeep will start. If you're really observant you might have noticed that the check engine light (CEL) doesn't come on at first and usually only comes on when you start to hear the clicking noise. The fuel pump also doesn't prime itself as usual and will also coincide with the clicking. If you've done some digging into the engine bay you might have also found that you have no spark while cranking prior to the CEL coming on. All of these things are connected my friend. So we have 4 symptoms to check off when you're trying to decide if this is your problem or not:
1) No check engine light when you first turn the key to ON, but before START.
2) No fuel pump priming (If you're not sure on this one, check with a gage)
3) No spark prior to the CEL coming on.
4) A clicking noise from the engine bay followed immediately by the jeep starting normally
A check of the trouble codes that the computer stores and returns to you by flashing the CEL with the key trick won't help you with this problem. The whole reason for that is a) symptom number 1; no CEL means no codes either while the problem is on going, and b) even after you get the jeep started, the computer only sees this as a normal start once it gets power, as if you had just turned the key on right then instead of 10 minutes ago. It doesn't realize there's a problem so there's no code stored.
Bottom line is, if you don't have these symptoms (the clicking noise is the ONLY one that is optional) or are dealing with a pre 91 vehicle, then what follows isn't going to apply to you. Sorry but you have other issues that I'm not covering here, like faulty fuel pumps, bad coils, burnt fusible links and whole slew of other possible choices. And for the earlier vehicles, while it certainly could be your computer causing the trouble, all of the tests below with the exception of the ignition switch test, just don't apply to your vehicle. You don't have the same components.
What's REALLY happening:
The whole problem here is that the computer isn't powering up like it should when you first turn the key on. The reason the jeep still cranks is that the starter motor circuit is totally separate from the computer. Without the computer though there's no fuel and no spark, so the engine merely spins without starting. When the key is first turned to the ON position before going to START they computer comes on to prime the fuel pump, send power to the ignition coil for the spark plugs, and turns the CEL light on for the diagnostic check so you know the light is still good (and a few other things that don't apply here). The clicking noise you hear is the auto shutdown (ASD) and the fuel pump relays coming on and switching rapidly as the computer tries to power up. As soon as the clicking stops, it means the computer is now ready to go, so you see the CEL come on and the pump start to prime itself. With the computer now powering the ignition coil you get your spark and your engine will now fire.
Tracing the problem:
So, we know that the computer isn't coming on. What we have to do now is figure out why. With the jeep it's not just a matter of "Oh the computer doesn't come on, it must be bad." There's several tests you want to run before you rush out to drop $200+ on rebuilding your computer only to find out that it wasn't the problem.
You're going to need some form of electrical testing device for this part. Personally I like one of these small multimeters that you can get just about anywhere these days. I'm not talking about a $200 Fluke meter here, just a small analog meter. I mean come on, even Wal-mart has them in the car department. They're not more that $20 and they'll make tracking down electrical gremlins so much easier than a test light. But if you really want to you can make do with a simple 12V test light to test for power, and a battery powered continuity tester with a light or a buzzer.
Since the meter is what I prefer, it's what I'm going to use. And since this article is geared to everyone, non-electrical guru's included, I should probably mention just how to test with a meter. (those of you who already know how to read one or are using a test light, skip these 2 paragraphs). Now if I tell you to test for 12V, set your meter to measure on the DC scale (AC is for household current). Some meters are scaled differently so pick the one that will let you read the smallest scale to still read 12V without being too small. For example, if you have a meter that reads on 10, 50 and 200V scales then you would want the 50V one. 10 is too small, and 200, while still giving you a reading, would only give you a very small change in needle position making it hard to tell if you're getting 7 volts or the full 12. A digital meter will usually still give you small readings even on the larger scales, but use the closest one just to be sure. Once you have your scale, take your black/negative probe and clip/touch it to a good ground. For me, I usually keep a long jumper wire with alligator clips on both ends and use that to jump the lead right back to the negative terminal on the battery. This eliminates any chance of getting a false reading because of a bad ground (ie trying to connect through paint on the body, rust on a bolt, etc). If you don't have a good ground you could see the test as indication no power and thus end up chasing a problem that you don't really have. Doing it this way also means I only have to hold on to one probe, leaving the other hand free to hold whatever it is I'm testing. Once you have your ground hooked to the meter, simply probe the wire in question with the red/positive probe and watch your meter for a signal, usually 12 volts. If you get a reading on the meter that is less than specified, it could point to a bad connection somewhere or another problem (sensor voltage can actually be a range from 1 to 5 or 1 to 8 volts but for the tests that follow 12V will be the norm). Check your probe connections and then repeat the test to confirm the reading and go from there.
Checking for continuity or lack there of is also pretty simple and again you may want/need to make up a jumper wire depending on where you need to test. For example you may need to test between something in the cab and the engine bay. Your probe leads probably won't reach by themselves so you need to jump with a longer wire. The good news is continuity usually works either way when it comes to polarity so you can jump either the positive probe or the negative and still have it work. The only thing you really want to do to be on the safe side is to disconnect the negative battery cable from the battery to shut down all power. Otherwise you could risk sending power through the meter when it doesn't want to see any power, which could damage it. A good example of this would be testing a wire to a light bulb to see if the wire is broken while not realizing it's got power. The light won't work with a broken wire so you don't realize it's powered, but as soon as you put your meter in line you complete the circuit and the power will flow; except it flows through the meter instead of the wire. So be sure to disconnect that cable before testing continuity. Just remember to hook it back up when it comes time to test for 12V power again or you'll be scratching your head. To actually do the test, set your dial to Ohms/resistance/continuity and simply touch one lead from the meter to your starting point, and the other to your ending power and watch you meter for whatever it does when there is continuity. Usually on the analog meters you'll see the needle sweep across to the other side indicating there is continuity but yours might have a buzzer to tell you without having to look. On a digital meter, you're looking for a reading between 0 and 10. Much more than 10 ohms can mean that there is a corroded connection between your probes somewhere. A null reading means no continuity which is a broken/loose wire. Ok, now that we know how to read the meter, on with the testing.
Symptoms:
You guessed it, the jeep doesn't always start right away. Instead it seems to sit there and crank forever. Usually (but not always) this is followed by a rapid clicking sound coming from under the hood and then miraculously the jeep will start. If you're really observant you might have noticed that the check engine light (CEL) doesn't come on at first and usually only comes on when you start to hear the clicking noise. The fuel pump also doesn't prime itself as usual and will also coincide with the clicking. If you've done some digging into the engine bay you might have also found that you have no spark while cranking prior to the CEL coming on. All of these things are connected my friend. So we have 4 symptoms to check off when you're trying to decide if this is your problem or not:
1) No check engine light when you first turn the key to ON, but before START.
2) No fuel pump priming (If you're not sure on this one, check with a gage)
3) No spark prior to the CEL coming on.
4) A clicking noise from the engine bay followed immediately by the jeep starting normally
A check of the trouble codes that the computer stores and returns to you by flashing the CEL with the key trick won't help you with this problem. The whole reason for that is a) symptom number 1; no CEL means no codes either while the problem is on going, and b) even after you get the jeep started, the computer only sees this as a normal start once it gets power, as if you had just turned the key on right then instead of 10 minutes ago. It doesn't realize there's a problem so there's no code stored.
Bottom line is, if you don't have these symptoms (the clicking noise is the ONLY one that is optional) or are dealing with a pre 91 vehicle, then what follows isn't going to apply to you. Sorry but you have other issues that I'm not covering here, like faulty fuel pumps, bad coils, burnt fusible links and whole slew of other possible choices. And for the earlier vehicles, while it certainly could be your computer causing the trouble, all of the tests below with the exception of the ignition switch test, just don't apply to your vehicle. You don't have the same components.
What's REALLY happening:
The whole problem here is that the computer isn't powering up like it should when you first turn the key on. The reason the jeep still cranks is that the starter motor circuit is totally separate from the computer. Without the computer though there's no fuel and no spark, so the engine merely spins without starting. When the key is first turned to the ON position before going to START they computer comes on to prime the fuel pump, send power to the ignition coil for the spark plugs, and turns the CEL light on for the diagnostic check so you know the light is still good (and a few other things that don't apply here). The clicking noise you hear is the auto shutdown (ASD) and the fuel pump relays coming on and switching rapidly as the computer tries to power up. As soon as the clicking stops, it means the computer is now ready to go, so you see the CEL come on and the pump start to prime itself. With the computer now powering the ignition coil you get your spark and your engine will now fire.
Tracing the problem:
So, we know that the computer isn't coming on. What we have to do now is figure out why. With the jeep it's not just a matter of "Oh the computer doesn't come on, it must be bad." There's several tests you want to run before you rush out to drop $200+ on rebuilding your computer only to find out that it wasn't the problem.
You're going to need some form of electrical testing device for this part. Personally I like one of these small multimeters that you can get just about anywhere these days. I'm not talking about a $200 Fluke meter here, just a small analog meter. I mean come on, even Wal-mart has them in the car department. They're not more that $20 and they'll make tracking down electrical gremlins so much easier than a test light. But if you really want to you can make do with a simple 12V test light to test for power, and a battery powered continuity tester with a light or a buzzer.
Since the meter is what I prefer, it's what I'm going to use. And since this article is geared to everyone, non-electrical guru's included, I should probably mention just how to test with a meter. (those of you who already know how to read one or are using a test light, skip these 2 paragraphs). Now if I tell you to test for 12V, set your meter to measure on the DC scale (AC is for household current). Some meters are scaled differently so pick the one that will let you read the smallest scale to still read 12V without being too small. For example, if you have a meter that reads on 10, 50 and 200V scales then you would want the 50V one. 10 is too small, and 200, while still giving you a reading, would only give you a very small change in needle position making it hard to tell if you're getting 7 volts or the full 12. A digital meter will usually still give you small readings even on the larger scales, but use the closest one just to be sure. Once you have your scale, take your black/negative probe and clip/touch it to a good ground. For me, I usually keep a long jumper wire with alligator clips on both ends and use that to jump the lead right back to the negative terminal on the battery. This eliminates any chance of getting a false reading because of a bad ground (ie trying to connect through paint on the body, rust on a bolt, etc). If you don't have a good ground you could see the test as indication no power and thus end up chasing a problem that you don't really have. Doing it this way also means I only have to hold on to one probe, leaving the other hand free to hold whatever it is I'm testing. Once you have your ground hooked to the meter, simply probe the wire in question with the red/positive probe and watch your meter for a signal, usually 12 volts. If you get a reading on the meter that is less than specified, it could point to a bad connection somewhere or another problem (sensor voltage can actually be a range from 1 to 5 or 1 to 8 volts but for the tests that follow 12V will be the norm). Check your probe connections and then repeat the test to confirm the reading and go from there.
Checking for continuity or lack there of is also pretty simple and again you may want/need to make up a jumper wire depending on where you need to test. For example you may need to test between something in the cab and the engine bay. Your probe leads probably won't reach by themselves so you need to jump with a longer wire. The good news is continuity usually works either way when it comes to polarity so you can jump either the positive probe or the negative and still have it work. The only thing you really want to do to be on the safe side is to disconnect the negative battery cable from the battery to shut down all power. Otherwise you could risk sending power through the meter when it doesn't want to see any power, which could damage it. A good example of this would be testing a wire to a light bulb to see if the wire is broken while not realizing it's got power. The light won't work with a broken wire so you don't realize it's powered, but as soon as you put your meter in line you complete the circuit and the power will flow; except it flows through the meter instead of the wire. So be sure to disconnect that cable before testing continuity. Just remember to hook it back up when it comes time to test for 12V power again or you'll be scratching your head. To actually do the test, set your dial to Ohms/resistance/continuity and simply touch one lead from the meter to your starting point, and the other to your ending power and watch you meter for whatever it does when there is continuity. Usually on the analog meters you'll see the needle sweep across to the other side indicating there is continuity but yours might have a buzzer to tell you without having to look. On a digital meter, you're looking for a reading between 0 and 10. Much more than 10 ohms can mean that there is a corroded connection between your probes somewhere. A null reading means no continuity which is a broken/loose wire. Ok, now that we know how to read the meter, on with the testing.