YJ computer question

Heh, no idea but if you don't live in the high desert like I do all the ground connections are rusted by now...

Cheers,

Earle

Does anyone know if the ´95 YJ PCM waits for a signal from any sensor before providing ground for turning on the check engine light, ASD relay and fuel pump relay?

Thanks

Thanks Earl. This problem has been going on for a while now and is mostly documented in the "Delayed start problem" thread. I've tried a lot of things including cleaning the ground connections and just can't figure it out. I'm considering taking it to the dealership at this point, regardless of the potential cost.

Thanks Earl. This problem has been going on for a while now and is mostly documented in the "Delayed start problem" thread. I've tried a lot of things including cleaning the ground connections and just can't figure it out. I'm considering taking it to the dealership at this point, regardless of the potential cost.

--- --- --- --- --- --- --- --- --- --- While I am generally in favor of the scientific approach, have you priced or located a junk yard PCM? That will isolate whether the problem is interior to the PCM or not, and may be cheaper than a couple of ASE mechanics scratching their heads for two hours on your dollar.

My '95 YJ has always taken one or two seconds of cranking before it will fire cold. It did this new, and I always figured it was part of the nature of the beast. Has it thrown any codes? They say you can read them by switching the key on and off a few times and watching the check engine light flash.

Cheers,

Earle

My YJ also takes second or two of cranking to start but this is different, the fuel pump does not get activated unless the check engine comes light comes on for a short while, after that it is completely normal. No relavant codes, just battery disconnected and AC clutch (because it does not have AC). I think it is worth checking on used PCMs, thanks for the idea.

Can't say for the 95, but the on the 93 if it fails to get sensor input from the CPS, the ignition shuts down (ASD) after about two cranks.

I've started the engine without the coolant, O2, and MAT sensors. Haven't tried disconnecting the MAP though, but it should run at least in limp-home mode.

The last time I had a problem with nostart it was due to a coil going bad. One of the issues (if you have a return style fuel rail) is the pressure regulator leaking. This makes the jeep take more time to pressurize the rail and is usually the cause. Easy way to tell is to turn the key on (let the pump buzz) then turn the key off, then start the engine. That primes the rail. In some real bad cases air can get into the rail and make for a real interesting start. (think bucking horse)

Wow, so it can run with some sensors disabled. This makes it even less likely that it is a sensor problem. I'm thinking PCM much more seriously now. The problem I'm having is that whan I turn the key to run to let the pump buzz (this also usually lights the check engine light at the same time) it does nothing for a while (16 seconds this morning), then it suddenly comes on and I am able to start the jeep. If I don't confirm that the pump has run, it just won't start when cranking (well, maybe it would if I let it crank for 16 seconds!)

Just in case it is the PCM and I decide to get a used one, how can I make sure I'm getting the right one for my YJ?

Have you ruled out your ignition switch yet?

nrs wrote:

I am thinking grease congealed in the mechanism, preventing the contacts from hitting for a few seconds, eh?

Cheers,

Earle

It doesn't wait for anything. If you've verified the grounds, battery feed and and power from the ignition switch, the PCM is toast.

Yup, I verified all these things and they are just about perfect. I was pretty sure and hoping one of these would be bad. I took it to the dealership today for diagnostic check just to make sure but at this point I'll most likely have to buy a computer.

If I get a used computer, what do I need to look out for to make sure it is the right one?

Yup, I verified all these things and they are just about perfect. I was pretty sure and hoping one of these would be bad. I took it to the dealership today for diagnostic check just to make sure but at this point I'll most likely have to buy a computer.

If I get a used computer, what do I need to look out for to make sure it is the right one?

Make sure the part # on the new one matches the part # on the old one.

For a 1995 YJ four cylinder I think the only two PCM part numbers are likely to be California and 49 State. Any large recycler in the U.S. will have a parts interchange catalog that will tell you which vehicles are valid donors. They get this information better than the dealers. The possibility exists that earlier vehicles and the four cylinder Cherokee are a match too. The dealer can tell you the current part number for your PCM and whether it has been superseded in service parts. This can be helpful if you find a new replacement at a reasonable price.

Cheers,

Earle

Thanks for all the help. I'll post the final outcome to let you all know how it turns out.

You can always get a used one to. But if you do get a used one make sure it works first. Ebay has them cheap 95% of the time. Thanks Jamie

Some people have an addiction to drink... Some people have an addiction to drugs.... I just have an addiction to Jeeps and their accessories....

Here are a few pictures of my rig

The dealership confirmed it is the PCM. I'll be looking for a cheap used PCM now. Thanks for all the help.

The main connection to the computer is known to get corroded pins and cause a 'failure'. You look for green stains.

Mike

Thanks Mike. You mentioned that before and I checked it but it looked pretty clean. I cleaned it with contact cleaner and scraped each cavity in the connector a bit but it still had the problem. I'm pretty convinced its the PCM at this point.