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Old 05-09-2004, 05:28 AM   #1
jam851
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Unhappy Grand Cherokee, Tachometer to ZERO!

Ah I have put my car in the shop about 10 times for this one problem and it wont be fixed! I am looking to see if anyone has heard of such a situation and could give me some advice.

I have a '94 Jeep Grand Cherokee, Limited. The car is nice but the engine is driving me insane.

The problem: *Occasionally* while driving, the tachometer will simply drop to zero instantly, and I'm talking about while i'm on the interstate, but in other situations as well. While it is at zero, the engine does not make any "reving" noise either, so its not just the gauge going crazy. Despite me giving it gas, the car just slows down more and more. After this, either the engine will turn off completely, or it will pick back up again and will be fine. Lately I have tried pumping it gas when it drops to zero, and I think this improves the chances of it picking back up. I recently had the fuel injector replaced, but the problem still occurs.

Thoughts?
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Old 05-09-2004, 05:31 AM   #2
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It's always nice to introduce yourself before expecting help...there's a section dedicated to it here too!
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Old 05-09-2004, 01:32 PM   #3
DodgeRida67
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Sounds like a bad ignition coil.
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Old 05-11-2004, 09:00 PM   #4
jam851
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Quote:
Originally Posted by DodgeRida67
Sounds like a bad ignition coil.

Sorry about the lack of an introduction. I dont know too much about cars, so I wouldn't have much to contribute. Its just I have had this problem with my car for a while now and I cant get any help with it, and I've tried to look online but thats no help raelly either. I decided I needed to talk to some people and I came across this board.


But about the ignition coil, obviously I do not know much about this, but would that affect the performance of the car while its already running?
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Old 05-12-2004, 02:05 PM   #5
HighWayMan
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Quote:
Originally Posted by jam851
Sorry about the lack of an introduction. I dont know too much about cars, so I wouldn't have much to contribute. Its just I have had this problem with my car for a while now and I cant get any help with it, and I've tried to look online but thats no help raelly either. I decided I needed to talk to some people and I came across this board.


But about the ignition coil, obviously I do not know much about this, but would that affect the performance of the car while its already running?

Ignition coil would ONLY affect it while starting and running. It don't do much when the car's switched off.

Problem is when you buy electronic parts they are non returnable. Take the coil off and to Autozone or Advance and have them test it. Chances are good that if it's the problem it'll show up off spec in a diagnostic.

I don't see the link with the coil being faulty and the tach going to 0. You may have a loss of electical power somewhere. Peek around for a loose or corroded electrical connection.
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Last edited by HighWayMan : 05-12-2004 at 02:08 PM.
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Old 05-12-2004, 05:00 PM   #6
sidecar_jon
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Quote:
Originally Posted by HighWayMan
I don't see the link with the coil being faulty and the tach going to 0. You may have a loss of electical power somewhere. Peek around for a loose or corroded electrical connection.


the tacho is probably takeing its signal from the spark plug lead> The sparks go AWOL so the tach goes off too... i say Ignition problem. Coil/or the whole electronics.. maybe a Computer diagnostic would pick it up but sometimes these intemitant faults are just a question of subsitution till its fixed.
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Old 05-12-2004, 06:33 PM   #7
HighWayMan
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Quote:
Originally Posted by sidecar_jon
the tacho is probably takeing its signal from the spark plug lead> .

You mean they don't use spinning magnets on the end of cables any more??

Note to self: "Please engage brain before operating keyboard!"
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Old 05-12-2004, 07:35 PM   #8
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not in the 21st century!...its all flying cars and silver foil suit now you know..
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Old 05-15-2004, 11:58 PM   #9
Donk
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Alright,

I'm brand new to this forum. Actually registered to answer this post, but I believe I may have this one pegged. I have a 94 Grand Cherokee Laredo, which had very, VERY similar problems. It would stall out at highway speeds, it would stall out at stopsigns/lights, it would stall out when I was on a date. Basically, the car had a deep seeded hate for me, and went out of its' way to inconvenience me. Sometimes it would crank right back up, sometimes it wouldn't start again for 20 minutes. Often times, it would stall on the highway, I could just throw it in neutral and crank it right back up. And oh yes, the mechanics were all at a total loss. And THIS is why I take my car to no one other than myself these days.
Crankshaft Position Sensor, my friend. Your problem seems very similar to the one I had. Essentially, when the CPS begins to act up, this happens:
The CPS monitors RPM's from the engine, sending this to the computer and to your tachometer. When the CPS fails, it sends no signal to the computer, telling the car that the engine is off (your tachometer as well). Well, at this point, the engine will shut itself off, since it believes it is not supposed to be on. Yes, sometimes you can catch it by rev'ing the engine, but this is just throwing the car into high enough rpm's that it is still cycling when the CPS comes back around. It is a problem that will haunt you for months before it absolutely screws you at the worst time possible. Anyway, It's a very simple fix. Go to your local parts store, pick up a CPS for around $40. The CPS is mounted on the bellhousing of the transmission, right at the back of the engine. Crawl under your car, simply unscrew the existing CPS, pull it out, drop the new one in and fasten it. Now, follow the wire of the old CPS up through the back of the engine along the firewall, and plug the new connector to where the old CPS was connected. I do hope this helps, and I feel fairly certain that it will. Enjoy,
-Donk
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Old 05-16-2004, 03:17 AM   #10
tbaxleyjr
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Quote:
Originally Posted by tbaxleyjr
I don't know of any car which gets a tach signal for anything off of the #1 plug wire.

Depending on the make of the car and the year the car was build, the tach on the dash will pick up its signal from the ignition coil or from a tach terminal on the ignition module.

The ECM usually gets its engine speed signal from the crankshaft position sensor and will interface directly with the ECM or interface with the ECM through an ignition module

Pull the trouble codes. this can be done without a scantool


This procedure may be used on 1991-96 models.

1. Start the engine, if possible, cycle the transmission selector and the A/C switch if applicable. Shut off the engine.
2. Turn the ignition switch ON-OFF, ON-OFF, ON-OFF, ON within 5 seconds.
3. Observe the MIL on the instrument panel.
4. Just after the last ON cycle, MIL will begin flashing the stored codes.
5. The codes are transmitted as two digit flashes. For example Code 21 will be displayed as a FLASH FLASH pause FLASH.
6. Be ready to write down the codes as they appear; the only way to repeat the codes is to start over at the beginning.

let us know what you find
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Old 05-16-2004, 06:15 AM   #11
Donk
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Yeah, I didn't word that correctly. The tach on those cars gets the rpm reading through the computer, which is why it drops so abruptly when the CPS acts up. Obviously the engine still is cycling, but it gets a signal, or really the lack of any signal to give the 0 readout.
-Donk
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Old 05-16-2004, 11:13 AM   #12
tbaxleyjr
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lets see the rouble codes
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Old 05-16-2004, 07:44 PM   #13
Donk
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When I had the codes pulled from my car, it didn't actually show the CPS as a problem. However, it did have a record of how many times the car had just shut down (like 64 times), and it informed me that my #4 injector was having problems. Fortunately, a friend of mine knew the CPS units were notoriously problematic on those cars, and the symptoms matched. It fixed it right up for me.
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