4.0L won't start. No Spark. ECM won't produce 8v signal. Why? - JeepForum.com
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Unread 02-03-2012, 10:50 PM   #1
Barrishautomoti
Registered User
1992 XJ Cherokee 
 
Join Date: Feb 2010
Location: Boise, ID
Posts: 101
4.0L won't start. No Spark. ECM won't produce 8v signal. Why?

Sorry to cross post here guys, but I was having minimal luck in the XJ specific forum. Since this is really more of a 4.0L issue, I figured I'd post here.

Ok so I have a new problem with my XJ. 95 4.0 Auto. The dang thing wonít start. For the full backstory check out the following thread:

http://www.jeepforum.com/forum/f11/h...pping-1292546/

So Iíve just gone through and done a ton of work to this Jeep including: plugs, wires, cap, rotor, air filter, fuel filter, valve cover gasket, rear main seal, oil pan gasket, oil filter adapter seal, power steering pump, front and rear shocks, front sway bar bushings, inner and outer tie rods, transmission mount, transfer case chain, all new transfer case seals, transmission output seal, transmission solenoids, front brake pads and rotors, rear brake shoes, heater blower motor, blower motor resistor, blower motor switch, and serviced both differentials.

Now the Jeep wonít start. I drove it into the garage 2 months ago and slowly went through all of this in my spare time and after everything, it wonít start. I did a quick spark check and it has no spark at the coil. A visual inspection of the coil showed major cracks in the coil housing so I immediately bought a replacement. It still wouldnít start. So I checked for DTC and pulled code 11, crankshaft position sensor. No problem. I can do that. So I buy a new Borg Warner CKP and install it. No luck.

So at this point I rescan the DTCs and Iím still getting code 11. So I break out the DVOM and check to make sure Iím getting 8 volts in to the sensor.

Nope. So I yank the plastic cover off the back of the ECM harness and measure pin 7 (the orange 8v output) at the ECM. No output. So I measure input voltage. 12.2v in at batt in and key-switch in. Grounds show minimal resistance. The ECM is powering up just fine since I can retrieve DTCs. So itís getting correct voltage in but itís not producing any voltage out on the 8v output. I checked for shorts to ground in the output wire and didnít detect anything. According to the FSM and to my logic, this all would indicate a bad ECM. So I went and got a junk yard ECM from a wrecked 95 and tried that.

Exact same problem. With both ECMs, Iím getting no output voltage on the 8v orange output wire. Well, technically Iím getting around 14 millivolts on both. Again, the FSM says that this is a symptom of another bad ECM but Iím not convinced. I pulled the 2nd ECM from a 95 which had been t-boned. Logically, youíd assume that in order to be in a wreck, it must have been running. Additionally, why would both ECMs fail in exactly the same way? My understanding is that there is a voltage reducer built into the ECM which reduces voltage from 12 to 8 volts for the sensors. I guess if the voltage reducer was a particularly fragile part I could see it going bad on multiple units, but I still have a hard time believing that I just happened to have 2 ECMs go bad in the same specific way. Also, strangely, both ECMs are still producing their 5v outputs. Iím guessing thereís another problem. Unfortunately, the FSM is no help at this point since it just tells me to replace the ECM again.

If anybody has faced anything like this before Iíd appreciate some help. I just donít want to waste money on another replacement ECM if the problem lies somewhere else.

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Unread 07-06-2012, 06:24 PM   #2
thegreenbastard
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Join Date: Jun 2012
Posts: 3
Man I have the exact same problem I don't know what to do either other than junk the thing! Someone help save jeeps from the junk pile!!
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