XJ607

Hey everybody, so Im working on a 2000 jeep Cherokee 4.0 with a coil pack motor, just swapped a different motor in (0331 was bad) when done it started up great and I ran it in total a half hour or so, went around the block once to test it out everything seemed good, then when I was going to bring it back to my girlfriend it stalled like someone shut the key off, so I find out the im getting no spark and when I go to crank the motor over my coil pack plug voltage starts at 10 volts and then goes down to 2 or 3 and jumps around to no higher than 5 or so volts, also when trying to start it and testing volts ect, I noticed my grounding strap is getting really to the point its burning/smoking the plastic casing/electrical tape on the harness, ive checked my grounds and all seem to be good , replaced the cam and crank postion sensors before I noticed the strap smoking but yet im still not getting the correct volts to my coil pack plug therefore no spark/start. starting to really **** me off so any help would be greatly appreciated, im going to check the ground/signal wires from the coil pack plug to the pcm and see if the is any resistance there but im really thinking it a bad block/chassis ground but like I said they all seem good, I cleaned them all up and everything, THANKS AGAIN Brian

'90Cherokee

Is the measured low voltage with the coil pack connected? If so I'd say it's shorted internally drawing too much power, would explain the hot ground and low voltage, I'd assume the incoming power wire would also be hot. If it were a ground connection problem the offending terminal would get hot passing some of that heat to the wire, the whole wire being hot sounds more like it's over-loaded.

XJ607

no that's with the coil pack off, straight from the plug, ive tried a couple different coil packs before I realized the low voltage

XJ607

double checked all my grounds and ran new grounds from block to battery, battery to fender, head to firewall, head to fender/battery, and still no change!!! the new "bonding strap" doesn't seem to be getting hot like before but im still getting voltage jumps to my coil pack, pissing me off. any body have an ideas?

'90Cherokee

Voltage jumps?

CCKen

OP, can you get rid of that silly signature. It's very distracting.


I had to watch several 3 stooges movies, back to back, to shake it out of my head.

XJ607

I should of said dips, my voltage to coil pack starts out a ten volts then after the first crank it dips down to like 2-6 volts and just jumps between that voltage until u stop turning the motor over

BlueRidgeMark

Most likely explanation is either have bad grounds, or a poor connection upstream of the point where you are measuring. Corrosion can do that. Could be in a connector or in a wire itself.


Start taking measurements at different places along the path. Go back from where you are measuring now, toward the source of the voltage.

Try it with a test light, too, instead of a jolts-meter.

CCKen

OP,


You never did follow up on this post:


http://www.jeepforum.com/forum/f11/v...r-hot-3450714/

XJ607

I have power to the coil pack just not enough power to get a spark, it decreases as you try to start the car so a test light I feel wouldn't do any good since im getting power all the way to the pack

BlueRidgeMark

Yes, you have "power" there. (Well, not really. What you are measuring is voltage, not power.) But you don't have enough. Therefore, you need to find out why it's not getting where you need it.

The way you do that is to start at a point and work backwards in the circuit toward the battery, testing at different points. You will probably find a point where you have insufficient voltage, and then adequate voltage farther back. Your problem is in between those two test points.

Why use a test light?

When a circuit is loaded, it behaves differently than when it is NOT loaded. You need to put some load on the circuit to see how it behaves.

A test light is one way to do that. It puts a small load on a circuit. A DMM puts a very, very tiny load on a circuit; almost zero. For automotive circuits, it's close enough to zero so that we can just call it zero.

You need to use BOTH. The DMM for accurate measurements, and a test light to see what happens when you load the circuit.

An example from my own experience:

I installed a bunch of business radios for a tow truck company. A dozen or so, IIRC. One truck kept having problems with the radio not turning on. I went to the customer's location and verified the problem. Hmm. My voltmeter showed me that the radio had the correct voltage at the plug, but as soon as I plugged it in and turned on the radio, the voltage went to zero. Unplug the radio, and I measured 12v at the plug. Then I tried the test light on the plug. It would not light. Hmmm. Voltmeter shows 12v. Test light won't light. How is that possible?

Simple. The wires for the radio went out of the cab to the battery. The ground was attached at the fender where the battery was grounded to the fender. That's a very good ground point. The 12v lead was attached to the main battery cable via a tap. (I didn't like those, but my boss insisted on using them.)

Turns out there was a bunch of corrosion in the tap. Corrosion causes high resistance. This can allow a DMM to read the correct voltage, but it won't pass enough current to carry the load. In this case, a radio. In your case, a coil pack. When there is no load (like with the radio turned off), the voltage can be measured by a DMM, but if you put a load on it, you'll see the voltage drop, sometimes to zero or very close to it.



By the way, it doesn't matter how you feel about it. You can feel hot or cold, or sad or happy, but you can't feel about a test light.

You can think about a test light, but your thinking may not be accurate if you don't have an understanding of the electrical principles involved.

Now you've been given some accurate information which may improve the accuracy of your thinking (if you so choose), which will help you find and fix your problem, and you'll probably feel good about that.

XJ607

fixed it, swapped in a new harness I had laying around, thanks guys