ZJ Intermittent Ignition Problem
 

Hi Guys

Would anyone have an idea why I'm having these ignition problems on my 1996 ZJ Cherokee LTD 4.0 Petrol.

It will start normal most of time, then the next time it's dead won't turn over at all.
But once I pop the bonnet and hotwire from the starter motor it starts, then its ok for a while.

I've jiggled the T-Bar, put in neutral, but still doesn't start.

Any suggestions?

Thanks....Nick

Welcome to the forum, Nick.

First of all, let's clear up some terminology so we can help you. When you say, "it won't turn over at all", do you mean the starter motor does not make the engine spin (which is the correct use of that term)? OR do you mean that the engine won't start (which is an all-too-common misuse of that term)?

Hi yes, the starter motor doesnt make the engine spin, only when I use a screwdriver to jump it from the starter motor then it starts

My guess is that it is the starter solenoid going bad.

You are getting a good supply of feed power from the battery to the starter. If you were not getting a good supply of feed power to the starter, jumping the starter terminals would not work to engage the starter motor. I believe this means that the other possible reasons for your problem of it not starting could basically be ruled out... Such as the neutral safety switch, security system, ignition switch, fuses, battery, battery terminals, all of the wiring leading up to the starter, the grounds, and starter relay. I believe all of those other things that I just mentioned would prevent power from being supplied to the starter in the first place right? Which makes me think it is the starter solenoid. We know the starter motor itself works fine when you bypass the solenoid to engage it.

The starter and solenoid is a unit. You have to replace the whole thing if the selenoid has gone bad.

You could test it with a digital voltmeter on the terminals of the starter.. I don't think that's necessary though, since it doesn't seem very possible for anything else to be causing the problem you are having?

Neutral Safety Switch is bad.

Doesn't the neutral safety switch allow the electrical current to cycle from the ignition switch to the engine starter? If so, it would not be possible for him to have been able to jump the starter solenoid with a screwdriver. There would be no electrical current present at the starter to make the jump, and activate the starter motor.

I think I understand now? The main terminal of the starter where the positive battery cable connects to recieves a constant 12V. That makes sense. There is the little wire that connects to that tab. That plays a part with the ignition switch, NSS, relay etc..? It is kind of a confusing circuit that I should learn more about for in case my vehicle(s) don't engage the starter some day down the line.

Yeah, by jumping that small terminal directly to the battery supply, you're bypassing the ignition switch circuit, which feeds thru the NSS. If the solenoid is bad, you can jump the battery cable connection to the heavy terminal after the solenoid, but watch out for lots of sparks and your jumper wire getting HOT!

Ha ha! You are dang right! When trying to figure out my non cranking WJ one day, that was what I did.. jump the two big terminals, and it was a firworks display of sparks! I thought I knew what all I was doing when troubleshooting it back then. Apparently, all I was really doing was verifying that the positve battery cable to the starter connection was indeed intact, and feeding the starter.

I am soo glad this post came up! I spent some time last night researching all about what takes place when we turn the key to start our vehicles, and I have a much better understanding. I thought about posting here in this thread about the different ways to test out these system components, because that helps me to know it better, and the information could be useful maybe for other people who may stumble up on this thread too. It actually is not too difficult to narrow things down when trying to figure out what the issue is when the starter does not engage.

A basic quick test:
If you turn on all of the interior lights and turn the key to crank it, that can help determine wether the problem is with the battery/terminals -vs- the other components. If it does not crank and the lights greatly dim, it is probably from inadequate power supply related to the battery condition or the terminals. If it does not crank and the lights do not dim, it is most likely one of the other components causing the starter not to crank.

For the sake of simplicity. Lets say that you know the starter and the solenoid is okay. You have tested it already and it checks out.

You can go pull the starter relay. If you are getting power at terminal 86 when you turn the key to crank, it is either the NSS or the relay itself being the issue. The NSS prevents this circuit from completing its run, to finish and provide power to the starter (I believe that's right, isn't it?). It is my understanding that the relay is hardly ever the problem, they are not really known to go bad. You can still test it by swapping in one of the known good relays, and if the starter still does not crank, it is likely to be the NSS causing it at this point of the troubleshooting. The ignition switch supplies power to terminal 86. So, if not getting a voltage reading at terminal 86 when turning the key to crank, then you know it is the ignition switch causing the problem.

(edited to add):
Other considerations would be a security system if equipped, grounds, and the condition of some of the wiring.

https://cimg7.ibsrv.net/gimg/www.che...94e5dcfe83.jpg

(terminal 86)

I am still a little confused on how the NSS factors in... Mainly just for with the troubleshooting aspect. Am I correct about what I stated above for when testing at terminal 86? The NSS doesn't prevent the ignition switch from providing power to terminal 86 does it? If the starter is not cranking when you turn the key, and you have power to that terminal when testing.. Is it safe to say that the problem therein lies with either the relay, OR THE NEUTRAL SAFETY SWITCH (in the above scenario knowing that the starter and/or solenoid is not the problem)?

Relay logic - the ignition key switch applies 12V and the NSS switches the low side to ground. Together, they allow the current to flow. By this, the NSS does not prevent the ignition switch from supplying power to terminal 86 does it?

It's still confusing!https://cimg4.ibsrv.net/gimg/www.che...3e0d83779d.jpg

No, I see it. By that diagram specific to these Jeeps, it appears that I am correct. I see the path.. Ignition switch is point of origin for activation of course. It allows a pull of power from the battery to first go through the fuse. From there it travels into the relay at terminal 86. This allows the relay to connect up and send this power down to the starter solenoid. The solenoid connects up, and this allows the power to now be drawn from the battery via the wire that is connected up to its (the starter assembly) large terminal .. it crosses the contacts in the solenoid to activate and engage the starter motor to crank the engine. The NSS does not look like it ties into all of this until the process has gone all of the way down the line, definitely past the relays terminal 86 being lit up. Its simple!

Hats off to electricians and engineers!

Why thank you, thank you very much.