How do I turn off my CEL?

adman02

Well-Known Member
Joined
May 15, 2009
Location
Durham, NC
79 CJ with 93 4.0L swapped in.

I am running a very basic system - just the computer and a couple of relays (no PDC). Problem is that the CEL is always on when the engine is running - dim but on. The only code I get is 33 (A/C circuit open/shorted). I have no A/C compressor or components.

Seems like I should be able to short a couple of wires in the harness and get the light to go out, but I can't seem to figure it out. I tried grounding these pinouts (27, 28 and 34) but no dice.

Any thoughts or advice?
 
I'm betting its looking for a reference voltage. So a resistor could solve your problem.
But what that voltage is I dont know with the info given.

The best chance at answering this is @Jody Treadway but I can about guarantee you are going to need to give him a bunch more info...
 
@adman02 this should be everything you need to figure out how to fix it...i am assuming it's a 4.0 from a wrangler..
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One side of the clutch relay is connected to power, and the PCM switches the other side to ground to engage the relay. The is a signal line inside the PCM that monitors the voltage on that PCM output, and it's presently not detecting the 12V through the relay coil on that PCM output pin. The detection of an open or a short both show as the same low/zero volts detected at that PCM output pin.

You should be able to tie that PCM output directly to 12V, but it's better and safer to put a resistor between 12V and the PCM pin, because that will provide protection for the PCM output pin under some situations that I won't bore you with. The amount of current needed to detect the voltage is very low, so really any resistor will likely work the same as any other, but I'd start somewhere between the range of say 300 to 1000 ohms, whatever common value you can find that's around that range somewhere. The relay coil itself is likely equivalent to 50-120 ohms (somewhere in there likely) but there's no reason to mimic the coil resistance for what you're doing. You need to mimic what the PCM is looking for, but there is no specific need to closely mimic the relay itself, if that makes any sense...
 
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I belive you can put a resistor across c21 and c91,possible at that plug that says "left of steering column "......
 
WOW! Thanks for all of the information, fellas. Being that I'm a complete dim wit when it comes to electrical things, I'm going to have to think through all of this and ask lots of questions. So thanks in advance for being patient.

@krehel24 - thanks for the diagrams and info. I've kept the bulkhead connector for the 79 CJ, so I'm not sure if the bulkhead diagram still applies (?). I did "five wires to fire" method to graft the YJ engine harness into my existing 79 CJ harness. And, I don't have a PDC or a PCM (unless we're using PCM and ECM interchangeably) in the system as far as I know. It's literally the computer (what I'm calling the ECM), a relay to power the ECM, a relay to power the fuel pump and the wiring harness with just about everything but the basic FI components taken out. So the A/C pinouts/wires leave the ECM connector plug and simply dead end - they are not connected to anything.

@Fabrik8 - if I'm following you correctly I need to connect at 12v source to one of the A/C pins leading into the ECM with a resistor (300-1000 Ohms) in between. If that's correct, any idea which of the three A/C pins should be mated with 12v power/resistor? I'm thinking it's 27 or 28 from the attached pinout diagram.

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I'm trying to remember how this all works. I believe the A/C Select input is just a sense for when the A/C is actually operating, to correct engine mapping, etc. Someone please chime in if I'm wrong about that.

I believe you need the resistor on the relay output pin (34?) if I'm thinking about this correctly. I think it's not sensing the presence of the relay coil, which would be sensed on pin 34. It will never get the A/C Request signal (because there is no A/C system to turn on, so the PCM should never try to engage the relay.
 
Splice to whatever the power source for the relay coil ordinarily would be, so either battery or ignition switched power. I can't tell from a quick glance at the schematics.
 
That code will not turn on the cel. If the lamp is dimly lit, look for a corroded connection or shorted wire. Electrical tape will cover it up as well. ;-)
 
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