No start, no CEL, and No communication with ECU

[Ben824]

1997 XJ auto 4x4. I was doing some probing and testing of circuits in the PDC under the hood trying to find a switched source to trigger a relay for when the Jeep was on only. I got to the relay for the factory cooling fan and found one terminal only gave power when the Jeep was on. I put a test lead in it to see if it would power the relay I was trying to hook up and the lead wire accidentally grounded. The wire in the factory harness is a blue wire with a white stripe.

Now there is no communication to the computer to see if there is a code. The CEL came on but when I turned the key to off and then back on the CEL did not come back on. The terminal I was testing originally gave me 11.9ish volts on my voltmeter. Now it only gives me 2.3ish volts. The Jeep will turn over but not fire. None of the gauges seem to respond now either. The battery has over 12 volts but the gauge does not register a reading. The only gauge that moves is the oil pressure but only a slight bit and not even enough to get to the first notch. All the dash lights light up as normal when the key is turned to the ON position.

I replaced the computer with a used one from another 97 and nothing changed. I now put in a brand new computer and still NOTHING changed.

I am at a total loss as to what to try next. Any and all suggestions will be entertained!

 

[Ecomike]

Following that path, the next logical step is to replace the jeep.

But the way to save money, if you are smart, and lucky, is to find the fuse(s) you blew and replace it for about $1 ea. It is probably a PCM fuse in the fuse-relay box. May be more than one fuse.

 

[RenegadeStang]

Did you get the computer flashed when you put it in? That's what I had to do on my '98 when I installed a junkyard computer.

 

[sidewaysstarion]

On page 14-24 of the factory FSM they layout the Jtec's role in your jeep.

NOTE: PCM Inputs:
* A/C request (if equipped with factory A/C)
* A/C select (if equipped with factory A/C)
* Auto shutdown (ASD) sense
* Battery temperature
* Battery voltage
* Brake switch
* CCD bus (+) circuits
* CCD bus (-) circuits
* Camshaft position sensor signal
* Crankshaft position sensor
* Data link connection for DRB scan tool
* Engine coolant temperature sensor
* Extended idle switch (4.0L engine with police
package)
* Fuel level
* Generator (battery voltage) output
* Ignition circuit sense (ignition switch in on/off/
crank/run position)
* Intake manifold air temperature sensor
* Leak detection pump (switch) sense (if equipped)
* Manifold absolute pressure (MAP) sensor
* Oil pressure
* Oxygen sensors
* Park/neutral switch (auto. trans. only)
* Power ground
* Power steering pressure switch (2.5L engine
only)
* Sensor return
* Signal ground
* Speed control multiplexed single wire input
* Throttle position sensor
* Vehicle speed sensor

NOTE: PCM Outputs:
* A/C clutch relay
* Auto shutdown (ASD) relay
* CCD bus (+/-) circuits for: speedometer, voltmeter,
fuel gauge, oil pressure gauge/lamp, engine temp.
gauge and speed control warn. lamp
* Data link connection for DRB scan tool
EGR valve control solenoid (if equipped)
* EVAP canister purge solenoid
* Five volt sensor supply (primary)
* Five volt sensor supply (secondary)
* Fuel injectors
* Fuel pump relay
* Generator field driver (-)
* Generator field driver (+)
* Idle air control (IAC) motor
* Ignition coil
* Leak detection pump (if equipped)
* Malfunction indicator lamp (Check engine lamp).
Driven through CCD circuits.
* Radiator cooling fan relay
* Speed control vacuum solenoid
* Speed control vent solenoid
* Tachometer (if equipped). Driven through CCD
circuits.
* Transmission convertor clutch circuit

So based on that we know the PCM is communicating to the cluster but its not reading the battery voltage and is not running the motor, so lets dig deeper into the FSM.aperwork

So here we can see that the ecu gets key power from fuse 11 in the Junction block which connects to pin A2 on the ECU and constant 12v from fuse 21 in the PDC which connects to pint A22 on the ecu. It also has ground on pins A32 and A32 which go to G101.

Here is the pinout of the A connector on the Jtec so you can check your power and grounds.

Lets check the basics first and work from there.:wave1:

 

[Ecomike]

Add fuse 18, 20 and 23 to your list (11, 20 and 21) to check.

My daughter ignored my instructions recently, jump starting a discharged battery (head lights left on), and took out three fuses. Had a no start with a new battery next, and she had blown the PCM-B fuse on here Saturn, and an AC fan fuse, radio fuse.....etc. I did an OBD-II scan, and freaked out, as there was no PCM life!!!! Turned out the fuse did its job, sacrificed itself, selflessly, LOL, to save the PCM!!!!!:clap:

 

[sidewaysstarion]

So rereading your post I decided to dig deeper into the power distribution to figure out which fuse you popped when you shorted out that wire.

So going through the wiring diagrams I found the pinout for the cooling fan relay.

and for what you describe it sounds like you shorted out C11 (Dark Blue w/ White), so lets see where that goes.

so that ties into the power being sent to ECU pin A2, I think we found our issue.

Here is the junction block in the passenger footwell, I bet if you replace fuse 11 with another 20 amp fuse the jeep will spring to life.:gee:

 

[GrimmJeeper]

Mark, I'm calling you next time I have an electrical issue, haha. nice detective work.

 

[Ben824]

So rereading your post I decided to dig deeper into the power distribution to figure out which fuse you popped when you shorted out that wire.

So going through the wiring diagrams I found the pinout for the cooling fan relay.

and for what you describe it sounds like you shorted out C11 (Dark Blue w/ White), so lets see where that goes.

so that ties into the power being sent to ECU pin A2, I think we found our issue.

Here is the junction block in the passenger footwell, I bet if you replace fuse 11 with another 20 amp fuse the jeep will spring to life.:gee:

Fuse 11 was blown and replacing it fixed it!!!!!! Didn't have a spare so had to borrow a 20 amp from the rear wiper until I can get back to autozone to return that new computer.

THANK YOU SO SO SO MUCH!!!! I though I had really done something wrong but thankfully it was only a tiny fuse.

 

[sidewaysstarion]

A $.50 fuse is cheaper then a few hundred dollar ecu, I'm glad that fixed it.

 

[GrimmJeeper]

just saved you a couple hundred bucks, time to become a member of the club

 

[Ben824]

just saved you a couple hundred bucks, time to become a member of the club

I know! I joined years ago when I was still a teenager and had only had my Jeep a couple of years. This site and the whole community have been such a huge help to me when it came time to modifying, fixing, or maintaining this awesome vehicle. I just recently got offered a real job like a big boy job haha so as soon as my first pay check comes in I will be going to red letters!

Thanks again everyone for the amazing help!