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98 Grand Cherokee eats coils
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albersondh
New User
Nov 9, 2012, 7:40 PM
Post #1 of 8
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98 Grand Cherokee eats coils
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98 Grand Cherokee
5.2L
130k miles
OE coil failed (cracked potting material) at 120k miles. Replaced with Borg Warner, lasted 6-months, new Borg coil, 3-months, another Borg coil 1-week FAIL. Running an OE coil now but affraid to drive it anymore...... Ohm checked OE specs on primary and secondary sides of Borg coil and OE coil, both within specs. First two coil fails I got a coil primary CEL, after that, just a coil fail and no CEL. Dies on the freeway, or at a light, crank wont start, replace coil, it fires up. Also, after heat soak, starting is labored, kind of misses a bit then gets its act together and finaly starts.
Replaced:
Cap, rotor, plugs, plug wires, crank pos sensor, coolant temp sensor, cam angle sensor, fuel pump/sender assy, PCM.....
Showing continuity (a few ohms) between coil harness hot side and ground. Checked all terminal points on the coil hot circuit, no component shorts (injectors, O2-sensors, ect). Visual on the entire harness shows no burnt spots or rubbed areas. With DVOM set to audible, lead at the harness coil hot, other at grnd, shake the hell out of harness all over the place, no change in buzzzzzz, or ohm reading. All tests done with all harnesses to PCM and all injectors disconnected.
Im lost..... Fairly certain a short is causing too much current flow through the coil and this is killing them. Why would I show any resistance between the coil harness hot and ground with the PCM disconnected unless I have a short someplace??
(This post was edited by albersondh on Nov 9, 2012, 7:47 PM)
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Discretesignals
Ultimate Carjunky
/ Moderator
Nov 10, 2012, 8:47 AM
Post #2 of 8
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Re: 98 Grand Cherokee eats coils
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Kind of hard to check what is going on with just an ohm meter. A tool that would really help to see what is going on with the primary and secondary sides of the ignition coil would be a lab scope. Unfortunately, not many DIYs have one laying around in the garage.
The scope would allow you to view what is going on inside the coil as it is operating. You'll be able to monitor the secondary KV output to see if the coil is being overloaded on the secondary side. You can also current ramp the primary coil to see how much current the coil is actually using. The primary coil voltage can also be viewed to see how the driver in the engine control module is acting. With all that information you can see what is going on inside the coil and determine if there is anything that could cause shortened coil life span.
Most coil failures are due to secondary problems causing the coil to be overloaded or operating at near full output all the time. Coils can also be overheated on the primary side if the dwell time is really excessive. Dwell time is the amount of time the coil is turned on.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Nov 10, 2012, 8:50 AM)
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Discretesignals
Ultimate Carjunky
/ Moderator
Nov 10, 2012, 9:19 AM
Post #4 of 8
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Re: 98 Grand Cherokee eats coils
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You have to be careful with low resistance wires such as solid core wires. The electromagnetic interference can cause problems with solid state components that aren't shielded well. The main reason for suppression is to protect solid state components.
I'm not sure what you mean by primary harness and ground. If you could give wiring colors where you are doing resistance checks, that would be better.
The amount of current in the circuit on the primary side is determined by the coil itself. Just like any electrical circuit the amount of current in the circuit is determined by how much the load requires. If the primary side was shorted to ground, the engine wouldn't run. If it is shorted on the primary power feed side (DK GRN/ORNG), the fuse would blow and if it was shorted on the control side (GRY/WHT) that goes to the PCM, the coil would be on all the time and wouldn't produce any high voltage on the secondary.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Nov 10, 2012, 9:23 AM)
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albersondh
New User
Nov 10, 2012, 9:54 AM
Post #5 of 8
(7195 views)
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Re: 98 Grand Cherokee eats coils
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You have to be careful with low resistance wires such as solid core wires. The electromagnetic interference can cause problems with solid state components that aren't shielded well. The main reason for suppression is to protect solid state components.
I'm not sure what you mean by primary harness and ground. If you could give wiring colors where you are doing resistance checks, that would be better.
The amount of current in the circuit on the primary side is determined by the coil itself. Just like any electrical circuit the amount of current in the circuit is determined by how much the load requires. If the primary side was shorted to ground, the engine wouldn't run. If it is shorted on the primary power feed side (DK GRN/ORNG), the fuse would blow and if it was shorted on the control side (GRY/WHT) that goes to the PCM, the coil would be on all the time and wouldn't produce any high voltage on the secondary. The connector from harness to coil was suspect, so I clipped it off, got a new OE clip/pins, but that is where I am probing the primary harness side (grn/orng). Probing the harness right where I clipped off the old coil connector.
I agree that there should be NO continuity between the grn/orng and ground, at any point in the circuit. To eleminate the possibility of a component internal short I am probing with all PCM harness connectors, disconnected, all injectors disconnected, the ASD relay removed. Still I show about 2-ohms between the grn/orng and ground. Moving the probe from the coil harness primary grn/orng, to grn/orng at an injector clip or O2 sensor, I get the same result. I would think this condition would blow the 20A fuse before the ASD relay, but it does not, just eats coils, and does not like to hot restart.....
Thinking about EMI damage to solid state PCM components. It ran fine for a year with these wires, then all this coil drama, replaced the PCM, and the symptoms are exactly the same. Has not ate a coil but thats because I wont drive it more than 15min. The hot restart is still there, so I assume the coil issue is as well. I suspect that heat soak decreases the resistance in what I believe is a partial short to ground on the primary harness, and this causes the hot restart stumbles. Starts great from dead cold.....
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Discretesignals
Ultimate Carjunky
/ Moderator
Nov 10, 2012, 10:04 AM
Post #6 of 8
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Re: 98 Grand Cherokee eats coils
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If that primary circuit dk grn orange had a direct short to ground, it would blow the 20 amp fuse. You might be measuring resistance through some other load in the circuit that isn't disconnected. Short to ground in the primary feed isn't going to cause coil problems other than popping the fuse and not supplying the coil with any primary current. I think you are chasing a ghost on that circuit.
You really need to be looking at the circuit when it is operational. Ohm checking is great for finding direct shorts and checking resistances of coil windings, but everything changes when the circuit has current flowing in it.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Nov 10, 2012, 10:04 AM)
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Discretesignals
Ultimate Carjunky
/ Moderator
Nov 10, 2012, 10:40 AM
Post #8 of 8
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Re: 98 Grand Cherokee eats coils
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Your right. Lower resistance plug wires will make the coil work less harder and allow more voltage to jump the plug gap, but at the sacrifice of the suppression of EMI. The coil is designed to operate with the OE spec resistances of plug wires, so that isn't a concern for the coil until the resistances gets too excessive.
Like what was stated above, the amount of current used is determined by the coil itself. If you have shorted windings in the primary, the coil will use more current and output less voltage on the secondary usually resulting in misfires when the coil is under demand. The things that take out a coil are excessive resistances in the secondary side and excessive primary coil on time causing the coil to overheat. If the circuit checks out good and your seeing acceptable output readings, you may just be running into a bad batch of coils. Some of those China coils are junk.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Nov 10, 2012, 10:51 AM)
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