TJ Cranks but doesn't Start

[nich]

Good day all,

Context:
My 1997 Jeep Wrangler TJ (4.0L 5spd manual) decided on Monday (23.02.2026) to not start. On Sunday, the day before, using the Jeep, I did jump start an old truck of mine (1970 F700), as I occasionally do to keep it turning over. However, this time, once the truck was running, I left the jumper leads connected for longer than usual (about 4 minutes instead of 10 or so seconds). The jumper leads have no surge protection built in. After I disconnected the jumper leads, I turned off my Jeep and was fiddling with the truck for a bit. Then I started my Jeep up and parked it at home just round the corner. Then come Monday (the next day), it would crank but not start. I provide this context as I suspect that this combination of no surge protection on the jumper leads, and prolonged connection time, may have damaged some electrical componentry in my Jeep.

Testing:
These are the tests I have conducted thus far:

• Start-ya-Bastard (ether) into air intake. No start.
• Check sparkplug lead for spark. No spark.
• Checked ignition coil lead for spark. No spark.
• Removed cap of bicycle-styled check valve on fuel rail. Pressed in valve. No fuel ejected from check valve.
• Removed ignition coil, tested primary and secondary resistance. Primary: 1–2 Ω (about 1.6 Ω), Secondary: 12 – 14 kΩ (about 13 kΩ). From my research online, these values are within the normal range.
• Tested ASD (Automatic Shut Down) relay. It works normally.
• Swapped horn relay and ASD relay. I know the horn relay works as the horn produces sound. Still cranks but doesn't start.
• Tested CPS loom voltages with CPS connected, and key in ON position. Brown/Yellow stripe wire has continuity to earth. Orange is 300 mV. Grey/Black stripe is 300mV. I read online that the Orange wire should be at 5 V.
• Tried a couple OBDII scanner tools. They couldn't connect. I read in one forum that Jeep requires a special (and expensive) scanner tool.
• Performed dash self-test as one forum suggested. All dials and displays are functioning.
• Tried turning the key between ON and OFF positions thrice within 5 seconds, according to the suggestion in the two previously-linked forums. No codes show on the odometer. No "donE", no "55". Nothing, except the current mileage and trip meter.

Thoughts:
I've read in a forum that the ASD (which I assume is controlled by the PCM), briefly provides power to the fuel pump and ignition coil when the key is turned to ON. I suppose this is to prime the system with fuel. ASD then stays off until the PCM sees a 5V square wave signal from the CPS, when ASD turns on again. This allows fuel and spark while the engine is cranking.

Based on that reading, I suspect there is something wrong with the PCM, because the CPS isn't being powered properly (300 mV instead of 5 V). Who knows what other sensors aren't being powered properly. With incorrect supply voltages, no signals are being sent, so the PCM doesn't power the ASD, so the engine doesn't have fuel and spark, therefore it can only crank and not start. This leads me back to my context section — it is interesting that I could start it within 10 minutes of jumping the truck, but not the next day.

Any input would be greatly appreciated. Is there something I'm missing? Would jumping my truck have caused this, or was it coincidence? Does anyone have experience with testing the PCM at home? I have already removed it and plan to take a look inside.

I'm moving house soon, so most of my electrical tools are packed up. I also work full time now. Testing and responses will be delayed. Sorry in advance.


Thanks,
Nicholas.

 

[JKP]

Pull up the FSM for your 97 under Resources. I don't remember if the key trick works for 97s to get codes.

Does your check engine light come on briefly when key is turned on, then go off?

Turning the key to on should prime the fuel system for a couple of seconds.

Spark requires crank sensor, PCM, coil.

 

[macleanflood]

After checking fuses, relays wiring and grounds you might want to send your PCM to @Wranglerfix for testing. Wranglerfix.com.

-Mac

 

[nich]

I checked the 1997 Service Manual, available from this forum.

Page 272/1152 suggests using the key trick to get the odometer to show diagnostic codes. So it should work.

No, "CHECK ENGINE" light does not turn on at all. Fuel and "CHECK GAUGES" lights stay on for ~3 s, seatbelt and "AIRBAG" lights stay on for ~8 s.

Regarding priming, when I have my father turn the key while I'm at the fuse box, I don't hear or feel the ASD relay click.

All fuses have continuity. I'll have to go over the wiring and grounds.

Thanks for the suggestion, but the shipping to the USA, testing, and return shipping to Australia would probably cost more than buying a guaranteed second-hand working unit. I would like to do everything in my capability before getting a replacement PCM.

 

[JKP]

Pretty sure no check engine light indicates the PCM is dead.

No OBDII communication supports this.

Check the PCM grounds and power.

 

[nich]

You are right. The "CHECK ENGINE" lamp should illuminate for a couple seconds.

Well, what I thought was the PCM was actually a computer module for the airbag, so I reinstalled that.

I found the PCM on the left hand side of the vehicle next to the battery (I have a RHD Jeep here in Aus).

I removed it and opened it up. I tore some componentry off the board in the process of opening the aluminium housing, so I definitely need a new PCM now... One is supposed to bend the aluminium housing open, as if one was opening a book. I tried to slide it off, which sheared a few components off.

There is a patch of discolouration on the board, perhaps from an arc event or a leaking capacitor. I have attached photos. In fact, in a post by @Wranglerfix himself, he showed a photo of a PCM board damaged by a leaking capacitor.

Thanks, Mac, for that info. I bought a new unit from Wranglerfix. I don't want to take any chances on buying a second hand unit whose capacitors are also on the way out...

Will provide an update once I install the new PCM.

Nicholas.

 

[Wranglerfix]

There is a key ingredient missing that keeps the caps from exploding. Save a nickel and give Mark Cantrell a business was their 97 motto.

 

[nich]

I installed the new PCM and the Keep runs fine now. The computer re-taught itself the trims and all that within 10 minutes of first start.

Thanks all for helping me, and thanks to Wranglerfix for the PCM. Shipping to to regional Australia from the US took less than a week.

To those who are experiencing the same issue of cranking but no start, consider the following potential problem makers (ideas collated from my scouring of the internet):

• Spark plugs and ignition coil
• Fuel pump and injectors
• PCM (the "brains" of the Jeep)
• CPS / CKS / CKP (Crankshaft Position Sensor)
• Camshaft Position Sensor
• Relays/Fuses (ASD, Fuel, Ign)

Remember that the Jeep's engine relies on the PCM and the proper voltage being supplied to various sensors, and for these sensors to be working properly. All of this on top of properly functioning mechanical components. Good luck.


Thanks,
Nicholas.