Ignition system test readings!

Greetings,

I have a no start on a 90 XJ 4.0 litre. It ran fine Wed night, Thurs morning...no start. It turns over like a champ but it wont catch and run.

I have no fire from the coil. The coil tests good. So in diagnosing the problem, I believe I need to check the following:

The crank position sensor.
The pick up coil
The ignition module under the coil
The ignition run circuit

What I dont know is what to test for (ohms, voltage, ect.) and where to access the test point. I have a circuit checker and a multimeter.

Thanks in advance...this is the first time my xj has me stumped and I caint find my Gol Danged Service Manual!

The gizmo in the distributor (which looks like a ignition pick-up) has a murky function (I´ve run with it unhooked), it´s not a pick-up in the classic sense, it´s a cam possition sensor, tells the ECU when number six has fired, indexer. The test for the cam position sensor, is (needle type volt meter), blue wire (+) to black wire with the plug connected (through the back of the connector or through the wire) near 5 volts, crank the motor with the high voltage wire (coil wire) removed, the needle on the meter should pulse.

The CPS, provides the pulse to trip the ignition module. If the CPS doesn´t pulse or the pulse is weak, no spark. There is an ohm test for the CPS, 200 ohms (+/- 75) with motor at orperating temp, between the A and B terminal at the connector.

Ignition control Module, power in, is the large yellow wire, the small yellow goes to the ECU and is the trigger wire. The green wire goes to the tach. and if your tach. is working, the chances of a good pulse/trigger are good, the pulse from the ECU to trigger the ignition, also operates the tach.

I´ve found most of the problems in the ignition circuit to be in the connectors, the coil to cap wire, the cap or someplace in the wiring.

Thanks for the info!

Between the time I first posted and now, I changed out the CPS. Before I changed it, the engine would turn over as long as the key was held in the start position. Now it turns over for a second or two then it drags a bit, turns over for a second or two and drags a second again.

I don't understand the "drag" thing. It didn't do it before. I have an Optima battery so I don't think it's power related. Any ideas?

That was my first CPS install...and I hope my last! What a beatch!

I'm gonna try the test in the distributor next. I'll let you know what I find out.

99%of the time it appears to be a bad crank sensor, especially on the earlier xj's. the test for the crank sensor is to unplug it and hook your multimeter to it on the sensor side. place your meter on the ac voltage, it should read as close to .500 as possible. if it is significantly lower or not reading it is bad

Re: Ignition system test readings! HELP!!!

Thanks. I put a brand new crank sensor on it today. No help. I am begining to believe it is the computer.

I get no spark from the coil. I get current through the big yellow wire to the ecm module under the coil, but the other two wires on the plug aren't passing current. The tach moves when I crank the engine so I think the module is good.

As far as I can figure..(without a schematic)..the next thing upstream in the system is the computer itself.

I'm about lost as far as diagnosing the trouble. Any help is appreciated...the rig is my daily driver and I gotta woik Monday!

are you sure the fuel pump runs

I have lots of pressure at the fuel rail check valve. :dunno:

try swapping the ASD relay with one of the relays that dont affect starting, like the ac compressor relay or fog lamp relay.

That's worth a try. Thanks! Hadn't thunk of it. First thing in the morning.... :sunshine:

Re: Ignition system test readings! HELP!!!

pcsutton said:

I get no spark from the coil. I get current through the big yellow wire to the ecm module under the coil, but the other two wires on the plug aren't passing current. The tach moves when I crank the engine so I think the module is good.

Click to expand...

The module under the coil is the ignition module, not the ECM. The ECM is the engine control module and on a '90 it's located under the dashboard on the driver's side.

The way the coil works is, the coil is loaded with current and then the ECM triggers, and through the ignition control module, collapses the field to ground. Creating the 20-30,000 volts that comes out of the other end. If your tach is functioning, the trigger from the ECU is working somewhat, wether it is triggering the collapse of the field in the coil, is a guestion. If the coil has an open circuit, it´s not gonna fire (should be very little resistnace). The coil and the ignition control unit, are two different peices, they can be seperated.
Try another coil (high voltage) wire, hook it up to the coil and hold it a quarter of an inch from a good ground, when the engine is cranking. Rubber gloves and care, are recommended.
The reason for the ignition control module is, that the ECU uses a very small voltage as a trigger and the igniton control module, then moves the major current to ground. Small current and dirty connectors, equals poor function. I´ve had moisture and corrosion, cause ignition trouble, much more often than failed componenets.

OK...re-cap. No-start on a 90 XJ 4.0. Ran Wed night, no start thurs morning.

The engine spins freely but won't catch and run. I replaced the CPS yesterday. Now the starter drags intermittantly when it turns over.

I am getting fuel to the test port on the fuel rail.

The coil tests good, but there is no spark from the coil wire terminal.

I switched positions of the relays. No joy.

Interestingly, when I went out to switch the relays, I forgot to re-install the coil and ICM. The engine again turned over freely with no dragging.

Upon re-installation of the coil and ICM it again drags and still has no spark.

I get current to the large yellow wire on the 3 pin plug going to the ICM, but can't see any current from any of the other wires attached to the ICM.

So...I know the problem isn't:
1. The CPS.
2. The coil.
3. The distributor or cap or wires.
4. The fuel system.
5. The battery.

What I don't know about is the Ignition Control Module under the coil and the ECM. Is there anyway to test them to eliminate them from the problem diagnosis? I really hate to shell out a couple hundred bucks to replace them both.

Thanks

Started a new thread. "I'm Desperate"

With the connector connected (through the back of the connector), I tested for voltage on the small yellow wire on mine (with the ignition on), it tested to around 3 volts DC (to battery ground) and around 6 volts AC voltage. Wasn´t sure which kind of voltage is in the wire, tested for both.
Bad news is, I have two different meters one analoge and one electronic and they showed different values. Think my electronic is a bit flacky (battery?) and my analog doesn´t have a fine enough scale to be near accurate.
But unless both had a meltdown, there is some kind of current, on the small yellow wire, with the ignition on.
Might want to follow the small yellow wire, back to the next connector and check for voltage on the ECU/ECM side of the harness connector.
I have a coil and ignition control module sitting right in front of me, was gonna take some continuity and resitance numbers, so you could compare yours to mine. But not being able to trust my meter, it would be a waste of time.
Try a IM, maybe we can work something out together, or someone else can jump in and give us, some imput. Eagle, Jneary and RichP usually have, some answers. A link to a 89 FSM, is available on the the modified board, a good wiring diagram is usefull.
My best guess on a probability scale is, connector/wire to ICM, some sensor (maybe other than the CPS)(unplugged my MAP sensor with the motor running, stopped quick) interrupting the information ECM needs for ignition control, coil/ICM, power/connectors/relay for ECM and then bad ECM.
If I remember correctly, the ECM, gets it´s juice from the battery through a fusible link at the starter relay, through a five pin connector near the relay rack. And grounds to the dipstick holder. The ignition switch just turns it on, it´s always under power. The latch relay also supplies power to some parts of the ECM.