P0505 & P0500, no gauges

[StG58]

Well, the TJ seems to be mad that there is a new JK in the family.

Went to the store to pick up some stuff and the gauges stopped working,the check engine light came on with a P0505 & P0500 code, the idle jumped to about 1,400 RPM. Checked the fuses and all seems well behind the glove box, and under the hood.

Did I loose a ground connection or a connector?

Thanks in advance for any tips on chasing this down...

 

[StG58]

Checked the grounds and all's good there. I've got 12 volts across fuse 10. Every thing in the PDC checks out. The connectors on the ECM are clean and dry. Reset the codes and they came right back. Did the little Jeep just eat an ECM?

 

[StG58]

Alrighty then. Where's the best place to get an ECM?

 

[StG58]

Anybody?

 

[Chris]

Not sure how I missed this!

P0505: IAC (Idle Air Control) Malfunction.

Causes
Potential causes of an P0505 code include:

• IAC motor connector is damaged (check connector and pins for damage, and moisture)
• One or more IAC circuits open or shorted to ground
• One or more IAC circuits are shorted to voltage
• IAC has failed
• PCM has failed

Possible Solutions
This should be a fairly straightforward diagnosis. Start by clearing the fault codes. Then unplug the IAC and start the engine (This may cause other IAC codes to set. Just ignore them for now). If the P0505 code doesn't reset, replace the IAC. It is internally shorting one of the IAC circuits. If the code DOES return after unplugging the IAC and starting the engine, then you know it isn't the IAC motor. Visually check the wiring harness all the way back to the PCM. You may need to visually inspect the harness by removing all the insulation off the wires. It is possible that the wires could be shorting to eachother or to other wires in the harness.

If there are no wiring problems, and the code keeps returning with the IAC unplugged, you can check the PCM by removing the IAC wires from the PCM connector (or clipping the wires in a location where they can be repaired easily) and then start the engine (If you're not sure, don't do it. Take it to a qualified professional). Now if the code doesn't reset there is a problem with the wiring. Check for continuity on all the IAC wires. Any resistance points to a problem. Check for voltage on any(At this point there shouldn't be any voltages on the wires because they're clipped at one end and unplugged at the IAC). Repair as necessary. But if the P0505 code DOES reset after removing the IAC wires from the PCM connector (or clipping them), replace the PCM, there's a problem internally on the IAC control circuit.


P0500: Vehicle Speed Sensor Malfunction

Causes
A code P0500 may mean that one or more of the following has happened:

• The Vehicle Speed Sensor (VSS) is not reading (functioning) properly
• There is a broken/frayed wire leading to the vehicle speed sensor
• The vehicle's ECM is not correctly configured for the actual tire size on
  the vehicle
  

Possible Solutions
The simplest thing to do is to reset the code and see if it comes back.

Next, check all wiring and connectors that lead to the speed sensor. The location
of the sensor depends on your vehicle. The sensor could be on the rear axle,
transmission, or perhaps the wheel hub (brake) assembly.

If the wiring and connectors are OK, then check the voltage at the speed sensor.
Again, the exact procedure wil depend on your make and model of vehicle.

If everything looks OK, replace the sensor.



The first thing I would do since it's cheap and free, is to clean the hell out of your throttle body and IAC. If those are both clean, then I would replace the IAC (with an OE Mopar unit of course).

The last thing I would suspect is the ECM. However, if it is the ECM is bad, get one from here:
http://rover.ebay.com/rover/1/711-5...0001&campid=5337789113&icep_item=351891042606

That's the cheapest one on eBay, but it's guaranteed to work or your money back. In addition to that, you can make them an offer on that since it's a "Best Offer" auction.

The key here is that it's an ECM from a 2.5 cylinder with a manual transmission, which is a must in your case.

 

[Chris]

As for the P0500 code, have you recently installed a new speedometer gear?

If so, check to make sure you aligned the raised dot with the correct numbers for whatever gear you have.

I don't think you've changed your speedometer gear (I'm sure you would have told us), but just incase you've been messing around down there by the transfer case, be sure to check that just to rule it out.

 

[StG58]

Thanks Chris. The defrost was running and I turned it off. The PCM threw codes and the gauges stopped working. The pcm on the 2.5's kick the idle up when the a/c is on (defrost) so I really don't know what to think. Lost all the gauges, though the oil pressure is back but is maxed out. Gas guage reads zero, but the low level light isn't on. Temp gauge seems to work. The idle is 1400 rpm plus or minus.

Haven't changed the spedo gear yet, waiting to plug the limited slips and gears in first.

 

[Chris]

So this all happened right after you turned off the defrost?

Have you tried clearing the codes to see if they come back? And better yet, if you clear the codes do the gauges or anything else work again?

Those are really odd symptoms that sure make it sound like it could be the PCM. It does run and drive though, right?

 

[StG58]

That's the crazy thing. Runs like it always has. Starts right up, motors right on down the road. I was looking at the readouts on the scan tool and it seems that everything is running the way it's supposed to.

 

[StG58]

Ouch. Cleared the codes, unplugged the IAC, and the P0505 came back. So, it seems that the IAC is reasonably good then. I'll get the Fluke meter out and check for continuity to see what I shall see. Still no gauges.

General question though. How often do TJ's display seemingly unrelated multiple failures? I get that the speed sensor, and IAC are related through the ECM, and that the ECM feeds information to the gauge panel, but what are the chances that these issues are unrelated and that it is just coincidence that all three things happened at once?

Oh, I'm doing this out in the wet sloppy snow, and not getting cranky. That must mean that I love my TJ, right?

 

[Stinger]

Steve I can't help but think it's a ground problem

 

[StG58]

Steve I can't help but think it's a ground problem

That makes good sense, Frank. I did check the instrument panel ground yesterday evening and it was fine. Good continuity across the ground. I'll chase more grounding points down to see it it's an engine ground or something else.

 

[Stinger]

You have two totally unrelated codes. So what is common to both? That is what points me to think ground.

 

[StG58]

You have two totally unrelated codes. So what is common to both? That is what points me to think ground.

Yup, that's my thinking too, but I immediately thought that the ECM was going wacky. I have three faults, the gauges (speedometer, gas and oil pressure), a speed sensor and the IAC code. The TJ runs fine, other than the high idle. You would think that if the ECM was cacking a hair ball, the engine would run crappy. It's not. Once it's off idle, it runs just like it always does. Even with the high idle it's still smooth and even.

So, (thinking out loud here) What do the IAC / high idle, speedometer and gas gauge have in common? The ECM and grounding. That is if all three are truly related. If they are just coincidental, then that's another thing. The engine runs fine, so it's probably not the ECM, right? Hmmm... Well, off to NAPA for some throttle body cleaner then. I'll try addressing each issue one at a time and see what happens.

 

[StG58]

OK, working on a TJ in the snow is not fun. Must be penance for past sins or something.

 

[Chris]

I agree with Frank, it sounds like a ground to me as well.

While it could be your ECM, I just highly doubt that's what this is. Seems like a bad connection to me.

If you're absolutely, positively certain it's not a ground issue, then it could end up being the ECM. Of course, we always want to rule out the simplest things first.

 

[StG58]

Not absolutely, positively sure it's not a bad ground. I'll start chasing the rest of those tomorrow though, as I've got to get ready for work and my fingers are cold and stiff at the moment.

Thanks for the help here guys, I do appreciate it. I hate chasing electrical stuff.

 

[Chris]

It helps to have a garage, with a heater... Do you not have one?

I had something similar to this happen with my old XJ, but it was trying to determine why the blower motor for the fan wouldn't work.

I tested everything possible. It wasn't the blower motor, wasn't the resistor, wasn't the switches, fuse, etc. I was confused as all hell.

I finally had to break out the wiring diagrams and trace all the wires with a test light. I finally found the culprit to be a bad connection of one of the wires where it meets the fusebox under the dash. If it wasn't for the test light, I never would have been able to find it.

And talk about random too. Usually the wiring harness itself is the last thing you need to check.

 

[StG58]

Yes, yes it does help to have a garage and a heater. That is best left to a discussion off-line over three fingers of reasonably good bourbon and a campfire though.

As for the wiring diagram. I have a digital copy of the FSM for my specific TJ, but haven't looked for the wiring diagrams yet. It's time to do a little rooting around in there, no?

 

[Chris]

I'm thinking it might be time for the wiring diagrams. However, you could pay $100 get and a used ECM just to rule it out. Just buy one with a return policy incase it doesn't end up being the ECM.