Please fill out your profile. All I know from your post is you have a 99 TJ. If your profile was filled out, I could quickly determine which engine and which transmission you have, and it would be easier to help. Fortunately, I recall your previous posts about a manual transmission/clutch problem. Here's the starting system wiring diagram:
View attachment 535371
If you look at that, the relay is the key component. A relay is a way to switch a high-current system with a low-current switch (so the switch doesn't melt). The solenoid on the starter draws enough current to require a relay. The relay is a mechanical device, and the schematic above helps understand how it works. On the left side of the schematic is the coil of the relay, part of an electro-magnet. It's shown by the zig-zaggy line and the rectangle with a slash in it. On a manual transmission TJ, the BR/LB wire (
BRown with a
Light
Blue stripe) grounds one side of the relay's coil all the time. Energization of the relay is controlled by the positive side only.
When you turn the key to the start position, battery positive is connected through contact #1 in the ignition switch schematic (upper left corner above) via the YL wire (
Ye
Llow), through either Fuse 20 (the "clutch bypass fuse", which is not installed by the factory) or through the clutch pedal switch once you depress the clutch. Notice that Fuse 20 and the clutch pedal switch are in parallel circuits, both using YL/RD wires (
Ye
Llow with a
Re
D stripe). That means that the starter relay coil will be energized anytime the ignition switch is rotated to the start position, and either or both the clutch pedal switch is engaged by pressing the clutch pedal or Fuse 20 is inserted. With Fuse 20 in place, you can start the TJ in gear, which is helpful if you are off-road on a hill. It's not installed by default for safety reasons.
Now, once the coil on the relay engages, it's magnetic field moves the switching mechanism. That's the dashed line running left-right inside the relay schematic above. When the mechanism moves, the switch on the right side of the relay schematic changes position, closing the circuit that supplies 12V to the starter solenoid. The starter solenoid performs two actions. First, it moves the drive gear on the starter outwards to engage the ring gear on the flywheel. Second, it connects the starter to battery 12V so that it starts spinning.
How the switch on the right side of the solenoid works is pretty straight-forward, if you study the schematic some more. Fuse 2 supplies 12V to the stationary side of the relay's switch via the PK/BK wire (Pink with a Black stripe). When the relay's switch is closed by the relay's coil energizing, that 12V goes through the switch to the BR wire (Brown) on the other side. That Brown wire goes through harness connector #104 ("C104") and on to the starter solenoid, which fires up your TJ.
To check your starter system, I'd suggest checking for all these conditions when the key is in the start position (you'll probably need a helper) and with the starter relay installed (you may have to get creative to get readings on your meter with the relay in place - perhaps remove the PDC and turn it upside down to touch the terminals):
- 12V on the YL/RD wire's terminal at the starter relay (if no, check the ignition switch and harness)
- Continuity with Batt - at the BR/LB wire's terminal at the starter relay (if no, check the harness)
- 12V on the PK/BK wire's terminal at the starter relay (if no, check Fuse 2 and the harness)
- 12V on the BR wire's terminal at the starter relay (if no, check the relay)
- 12V on the BR wire's terminal at the starter solenoid terminal on the starter (if no, check the harness)
You should also have 12V at the starter's largest terminal at all times (it's connected to the battery). If all of this checks out, it's likely a battery connection or ground issue. Never assume that those are good from just looking at them. It takes a near-perfect connection to transfer a couple-hundred amps when the starter motor is running. That's a lot of energy!
The manual has a helpful section on how relay terminals are numbered, but it's not exactly easy to understand. First, rather than saying what type of relay was used, they just show examples of two different configurations. Secondly, on one of them, they show the terminal labels from the perspective of looking at the relay socket in the PDC, but on the other it's shown from the perspective of looking at the relay, with the terminals pointing up. Therefore, one of them is a mirror image of the other. Here are both of them, and I've put the wire colors on it, so that you can relate it to the schematic. They are both edited so that the terminal arrangement is from the perspective of looking down at the relay socket in the PDC, as though the relay is not there. The one on the right is a mirror-image, so I've re-labelled terminals 85 and 86. Also, remember that if you turn the PDC upside down to probe the terminals, they will then be a mirror image of what's shown below. Note that the normally-closed terminal (87A) is not used for a starter relay, so that terminal slot is likely empty in the PDC.
View attachment 535378
Also, you mentioned that you couldn't figure out how to jumper the starter relay. It's pretty easy, and it might be the first thing to try. Remove the starter relay, and use a paper clip to carefully connect terminals 87 (BR) and 30 (PK/BK). What I mean by carefully is that the paper clip will become energized with 12V as soon as you touch terminal 30 (PK/BK). If it touches any metal, you'll get fireworks. Jumpering the relay terminals will simulate the starter relay energizing and closing the switch. If that starts the TJ, your relay is bad - either the coil is burned out, or the switch mechanism is "finky". Good luck. Post back with your discoveries.
Also, there don't appear to be any computers involved in the starter circuit, so I don't think your "NO BUS" problem is causing the starter problem. I am also unfamiliar with that error. I'm assuming it's what is displayed on the dash, where the odometer normally is?
