Diagnosing bad ECU/PCM/computer OBD-1

[Phil]

Looking for some help/fresh perspective.

1994 Cherokee 4.0L/auto

So it’s in my buggy #StellaFTH4 so not exactly OEM and there is no key to turn to run five times and count the blinks.

It was running fine on Rubicon, made it back to camp, shut it off to hang out. Subsequently would not re-start. Cranks well but is not priming the fuel pump.

I pulled it apart and confirmed that the fuel pump will prime if I apply voltage to the output of the relay. Switched relays with no luck.

It suggests to me that the relay is not getting triggered by the computer.

Any other thoughts on diagnosing the computer or just throw a part at it?

Thanks for the help.

(Yes we made it home)

 

[RCP Phx]

I would start by checking for fuel pump voltage at the ECU.

 

[RCP Phx]

I would start by checking for fuel pump voltage at the ECU.

That's with the ecu connector dis-connented from the ecu. Also try grounding that pin while you have it un-plugged.

 

[Phil]

Thanks for your input. I’m not sure I follow on checking the ECU with the ECU disconnected as there is only one cable. With it disconnected the ECU should be inert? This is a 1994 OBD-1.

 

[RCP Phx]

Thanks for your input. I’m not sure I follow on checking the ECU with the ECU disconnected as there is only one cable. With it disconnected the ECU should be inert? This is a 1994 OBD-1.

Sorry, I mixed you up with OBD II

 

[Phil]

Last night, the check engine light came on and stayed on. Tonight, it is not coming on at all. Decent voltage at the battery and PDC still. I ordered a replacement ECU from a yard off Car-part. If that fixes it, I’ll order another as a spare and/or open this up and see if there is an obvious and fixable failure.

Still open to comments though.

 

[blondejoncherokee]

Im sure you have already checked all fuses and relays?
so no voltage at the fuel pump relay? Does that voltage come directly from the ECU ?

I always want to say swap in a fresh CPS when there is a random no start.

 

[75SV1]

I had issues on my 2000 XJ. Turning over etc. Checked fuses etc. Replaced fuel pump. Checked injectors with a Noid light. A friend mentioned the key tumbler. Mine would flash the Noid light, when turning over then releasing the key. Changed tumbler. Worked then.

 

[Phil]

Im sure you have already checked all fuses and relays?
so no voltage at the fuel pump relay? Does that voltage come directly from the ECU ?

I always want to say swap in a fresh CPS when there is a random no start.

With the doghouse off it is super easy to change the CPS. I’ll try putting a fresh one in but as noted, the fuel pump is not getting primed when ignition turns on. The trigger for the relay traces back to ECU.

I had issues on my 2000 XJ. Turning over etc. Checked fuses etc. Replaced fuel pump. Checked injectors with a Noid light. A friend mentioned the key tumbler. Mine would flash the Noid light, when turning over then releasing the key. Changed tumbler. Worked then.

Funny. Maybe that will help someone - I have already removed that variable on mine (no key).

 

[Phil]

Mine was the computer. $125 later she started on the first push of the button.

 

[kastein]

If you hear a quiet high pitched noise from the ECU and/or the CEL does not come on at key-on (or ignition-on I guess in your case), it is highly likely that the ECU internal voltage regulator capacitors have finally failed badly enough that the regulator cannot maintain VCC rail stability well enough for the ECU to boot reliably. There are 3 or 4 little electrolytic capacitors in these 91-95 SBEC ECUs and they started going bad approximately 20 years after manufacture and have continued since then, it's a fairly common ECU failure at this point. Luckily there are howtos on other forums (google "1995 Jeep ECU capacitor replacement" or similar) and you'll find them. Same ECU was used on the YJ in the same year range just with slightly different firmware. I would expect the repair to take about an hour if you can solder PCBs fairly well and it shouldn't cost more than 30 bucks in materials between the handful of capacitors and the tube of non acid cure silicone RTV (I would recommend MG Chemicals #1035, 85ml tube is more than enough, it's about 20 bucks on Amazon, but you can probably get away with something like permatex ultra black since it is also non acid cure).