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On your Jeep, I don't have specifics on the Pioneer, but I feel relatively confident that the information I am sending you will track down your Starter that is not spinning the motor over so it will actually run.
Here is the schematic for a typical Jeep Cherokee with the 4.0 engine.
Your Pioneer should have one large wire to the starter solenoid and one small wire to the small terminal on the solenoid. Pick up a 12V test light and the information I send you should find your problem.
See if your test light illuminates when you ground the clamp and test the large terminal of the starter solenoid. It should. Now while a friend turns the key to try to engage the starter, see if the small terminal has power and illuminates the test lamp.
If not, you need to go back to your starter relay. The Starter Relay should have 4 wires going to it. I'll try to isloate it location for you. When you turn the ignition key to start, the starter relay should click. If it does not click, You either have a bad starter relay, A bad Park Neutral Switch, bad wiring either power or ground circuits or a faulty ignition switch.
I didn't want to get too deep into the diagnosis of all circuits until you do the basic electrical checks.
There is also a Fusible Link which feeds the Ignition Switch, and if this is bad your symtoms could be very much the same. The Starter would not make a sound when you try to engage it.
Starter Relay Location on 88 Jeep Cherokee
These relays are all mounted together on the front right wheelwell in the engine compartment.
I sincerely XXXXX XXXXX this information will help to find the starting problem on your vehicle.
Thanks...Appreciate the Reply
I'm reading your reply and the first check is at the starter solenoid with the test lamp.
If the light lights on the large terminal...And then you have someone attempt to start by the ignition key with the test lamp on the small terminal at the solenoid, and the light lights there too, you can assume that the system is operational from there.
The test lamp should illuminate when the key is cycled to the start position and Stay Lighted. Does it? And is the light on the small terminal of the solenoid as bright as it is on the large terminal? A dim light could mean it's not enough to engage the solenoid and starter.
This test tells us whether we are getting the proper voltage signal to the starter in order to engage the starter.
Another Test that can help determine if there is a unwanted voltage drop is to place the test lamp from ground to the large terminal while someone is attempting to start the engine and make sure that light stays bright during attempt to start. The lamp should stay bright or the starter should engage and begin to crank the engine.
If you are getting a good strong 12volts to both terminals of the solenoid...Full time on the large and solid 12volts on the small wihile key is moved to start, then there is one of two problems.
The starter does not have a good ground or the starter/solenoid is bad.
If you have had the starter tested, and they tell you it is OK, my next question would be, did they test the solenoid and starter as a unit?
If so, then you need to be sure the Engine has a good ground cable between the engine and frame or negative terminal of the battery.
Back to Basics:
Make sure the battery cables and thier terminals are clean and making a good connection. Make sure the Cables are in good condition. Make sure the Ground connection to the Engine Block is good and clean and tight.
By the Schmetic, you would not get a 12volt signal to the small solenoid terminal if the Fuse Link was bad. Additionally, if the fuse link were bad, the starter relay would not click.
I hope you are able to isolate the problem from here.
With 12 volts to the large terminal at the starter.
Run a wire from the positive battery terminal and touch it to the small terminal at the solenoid.
Or you can carefully short the large terminal to the small terminal at the solenoid. This is the same as running a test wire down from the battery in order to touch it to the small terminal.
If the starter and solenoid are good and the starter is grounded properly to the engine block and chassis, the starter will crank the engine.
If the starter cranks when you do this, then your signal from the relay is not sufficiant to energize the solenoid using the key. Then you need to backtrace the reason for a weak voltage signal to the starter solenoid small terminal.
I'm beginning to feel we may want to step back and take a deep breath.
We must be sure we are talking the same language so that we don't make this more difficult than it should be.
I just want to make sure we are on the same page.
The starter and starter circuitry is what You and I have been tracing. The starter's job is to cause the engine to begin to rotate, so that other systems can then cause the engine to run. Your initial description had us focus on the starter system which only rotates the engine.
So that we are not sidetracked by un-necessary steps, please verify the we are after a problem where the engine is not rotating when the key is turned to the start position.
The large terminal on the starter has 12volts going to it even when you have the key in your pocket. Let's pretend that the green wires aren't even there.
If you take a screwdriver or short piece of wire and touch the large and small terminals together, the Starter should engage and begin to rotate the Engine.
If the starter has been removed from the vehicle and has been check and said to be OK, then you should be able to cause the starter to engage simply by connecting the Positive Battery cable to the large terminal and shorting over to the small terminal.
IF the starter still does not rotate, then you need to run a set of jumper cables from the Battery Negative Terminal and clamp it to the Starter Housing and then re-check for starter operation while the Positive battery cable is still connected to the large terminal of the solenoid at the starter.
If you do what I just explained, and the starter does indeed engage and begin to rotate the engine, then you have a bad ground between the Engine/Starter Housing and the negative battery connection.
Now remember, this test can be performed with the ignition key in your pocket.
If you verify a good connection with your jumper cable between the starter housing and the negative battery cable, and the Positive Battery Cable is Good that goes to the Large Post at the Starter is making a good connection at the positive battery post.
Then your Starter and Starter Solenoid Needs to be replaced.
Make absolutely certain that your Battery Cables and Connections are Clean and Solid Both at the battery and where they connect to the frame/engine block and Starter. Make sure you have a good ground between the engine block and Frame of the vehicle.
This is all there is. You have already done all of the other checks that are required to be sure the system is going to be operational.
You should have it under control from here.