when it rains outside or it's humid, the car has a tendency to have a starting proble - JeepForum.com
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post #1 of 9 Old 07-16-2020, 01:56 AM Thread Starter
jordanmarkos
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when it rains outside or it's humid, the car has a tendency to have a starting proble

Hey guys,

I'm currently looking at a car (Jeep Grand Cherokee year 2000 4.7 petrol/gas) to buy.

The only thing that pushes me back is that it has a little problem, well if the owner is right.
He said when it rains outside or it's humid, the car has a tendency to have a starting problem.
  • He said after 15x times of starting it could start. The rest of the time (when there is no rain) it starts fine and it also runs fine.
  • He said that he once removed cables from the battery and it fixed the problem, but then the problem came back.
  • He has replaced a lot of things including (battery, starter, generator, camshaft sensor etc.)

What electric problem could be that?

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post #2 of 9 Old 07-16-2020, 08:17 AM
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It could be corroded power and/or ground wires. Fairly common issue depending where you live; road salt is a big contributor. It could need nothing more than cleaning the grounds or replacing the cables, jeepcables.com sells a set for $100 that comes with everything you need except the elbow grease. Not saying this is a surefire cure but it could be.

It could also be the Neutral Safety Switch, which provides a crucial ground in the starting circuit. Bad NSS is a common issue. If it doesn't start, try wiggling the shifter or starting in N instead of P.


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ok mostly just repairs, but we are closing in on a lift
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post #3 of 9 Old 07-16-2020, 08:25 AM
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The pcm is defective. Anytime removing the battery cables and forcing the pcm to lose it's adaptive memory and revert back to base programming yields a positive result that's a clear indication it's failing.

The 99 & 2000 model year pcm's are especially prone to malfunction in temperature and humidity swings.
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post #4 of 9 Old 07-16-2020, 04:27 PM
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Quote:
Originally Posted by Waterluvr View Post
The pcm is defective. Anytime removing the battery cables and forcing the pcm to lose it's adaptive memory and revert back to base programming yields a positive result that's a clear indication it's failing.

The 99 & 2000 model year pcm's are especially prone to malfunction in temperature and humidity swings.
Where does the PCM enter into the starting circuit? Honest question, I don't know how a bad PCM could cause a no-start situation.


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ok mostly just repairs, but we are closing in on a lift
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post #5 of 9 Old 07-17-2020, 07:11 AM
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Quote:
Originally Posted by GmanWJ View Post
Where does the PCM enter into the starting circuit? Honest question, I don't know how a bad PCM could cause a no-start situation.
A bad pcm can fail and cause several different issues in the starting sequence, everything from a no crank situation to a crank no start caused by defective drivers causing malfunction of the ASD relay or failure of the ignition and fuel injection drivers.

This is why live data from a shop level scanner is so useful and needed for diagnosing these issues without playing the guessing game, you can see what's going on with those individual circuits in real time.

With humidity related failure of the early WJ's what I see most often in the shop is failure of the pcm ignition driver circuits failing to ground the individual coil's on the 4.7 or one or more of the 3 rail pack coils on the 4.0

For those that aren't familiar with how the ignition works on the WJ Jeep, the pcm calls for the ASD relay to power up when cranked or in the on/run ignition position. When powered up, the ASD relay supplies constant power to both the fuel pump and the ignition coils and the pcm controls ignition spark and fuel delivery by switching ground on and off as needed.
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post #6 of 9 Old 07-17-2020, 05:12 PM Thread Starter
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Quote:
Originally Posted by Waterluvr View Post
A bad pcm can fail and cause several different issues in the starting sequence, everything from a no crank situation to a crank no start caused by defective drivers causing malfunction of the ASD relay or failure of the ignition and fuel injection drivers.

This is why live data from a shop level scanner is so useful and needed for diagnosing these issues without playing the guessing game, you can see what's going on with those individual circuits in real time.

With humidity related failure of the early WJ's what I see most often in the shop is failure of the pcm ignition driver circuits failing to ground the individual coil's on the 4.7 or one or more of the 3 rail pack coils on the 4.0

For those that aren't familiar with how the ignition works on the WJ Jeep, the pcm calls for the ASD relay to power up when cranked or in the on/run ignition position. When powered up, the ASD relay supplies constant power to both the fuel pump and the ignition coils and the pcm controls ignition spark and fuel delivery by switching ground on and off as needed.
Okay, apparently the problem isn't when it's only raining or humid.

It also happens when it's dry. Basically, it happens sometimes in both occasions.

So a simple cable unplug from the battery and then plugging back works. My question is, how to figure out what is the exact problem and how to fix it? Would that be to check for errors by plugging a pc to the jeep? If yes, what software and tools would you suggest to use it?
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post #7 of 9 Old 07-17-2020, 06:06 PM
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You don't plug PC's into anything, real shop grade scanners capable of doing the job are purpose build heavy duty tablets that can take the daily hard use in a shop setting.

The two real OE level tools on the market are the Snap-On Versus Pro or the Autel MaxiSys Elite with the Autel being the better of the two it runs on a Kit Kat OS and is much faster and easier to use.
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post #8 of 9 Old 07-18-2020, 01:03 AM Thread Starter
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okay, I will do that and hope it can tell me errors.

Also, this car is based in the Europe, so the car runs both petrol and LPG.

Perhaps something is about the LPG or so..

If you could tell me what should I do, I will do it to fix this issue. But I believe I will start to scan car to find any errors.
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post #9 of 9 Old 07-18-2020, 09:40 AM
Waterluvr
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I would suggest you find a reasonable service provider for the diagnostics in this case or refer back to post #3 in this thread, the cost of buying a real scanner for DIY use is generally prohibitive versus the costs of a diagnostic session at an independent shop by a qualified tech accompanied with printed results for reference.
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