• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Did I fry my ECM?

fyrfytr1717

NAXJA Forum User
Location
Turlock, CA
So I was finishing up wiring my engine fan override switch... The override worked, but I wanted to double check that I had tapped the right wires. I pulled Connector #1 from the ECM and checked continuity to make sure that I had tapped the ST-RUN wire and not the A/C Sense wire as they are both the same color. I confirmed I had the right one and went to plug the connector back in. I heard relays fluttering open and shut and realized that I had left the ignition in the "on" position.:doh:

I turned the ignition off, plugged the connector in, and turned the ignition back on. There was the dreaded CEL...

Code reader showed:

P1491 - ECM has detected an open or shorted condition in the radiator fan relay circuit.

P1694 - ECM has not received any CCD messages from the Aisin trans module within the ECM.

I had the codes erased to see if maybe something had just been momentarily shorted, triggering the CEL. As I pulled away from Autozone, the light came back on. It happened right when I turned my A/C on but that may have just been coincidence. Haven't had a chance to get the codes read again so I don't know if one or both codes came back.

The P1491 is not supposed to activate the CEL so I'm guessing I caused that one while I was doing my fan override wiring. Hopefully it is still gone.

The 1694 activates the CEL so I'm pretty sure that one came back. Now here's the strange thing, everything seems to be functioning just fine. Drives fine, shifts fine, no issues whatsoever that I detected driving it around today. I checked all the fuses that are part of either the radiator fan relay circuit or the transmission circuit and they're all fine. I know the fan relay is good as it still activates my fan with the manual switch. The radiator fan does not however seem to cycle on and off like it should when running the A/C.

So did I fry it? Is there an easy fix I'm overlooking?
 
Either try running codes/ clearing them, or just drive as usual and see if the CEL goes off after however many drive cycles.
 
Try unplugging and reconnecting the connector again, this time with the ignition off. Maybe you caused an intermittent connection with the CCD bus pins - since the TCU does its own thing and (afaik) only uses the CCD bus for error/condition reporting, this would cause the code. If not, try replacing the TCU before you try replacing the ECU since it seems to run fine, TCUs are a lot cheaper at the junkyard (like ten bucks usually around here, if they have one.)

Also definitely try just driving it for a while to see if it goes away like Johnnie said.
 
What about removing both battery cables and touching them together which would clear everything then seeing what happens. The jeep would run rough until it re-learned everything but might clear all codes. Besides what were you doing plugging and unplugging and splicing into wires while the battery was connected anyways. If the battery had been d/c like it should have then there wouldn't have been any problem.
 
P1491 - ECM has detected an open or shorted condition in the radiator fan relay circuit.
===========================================================
Could you do a quick wiring die of you mod?


================================================
P1694 - ECM has not received any CCD messages from the Aisin trans module within the ECM.
=================================================
Check all connection/grounds for fit and corrosion
 
Last edited:
I've been living with the 1491 CEL from my manual override for over a year. When it warms up, I'll switch to 2awg mains, 4awg accessory, lose the adj temp controller for the 2nd fan, and wire a 10awg circuit for both fans.
 
P1491 throws a CEL on mine as well- hence why I put it back to stock while someone sharper sorted it out.

My setups would operate the fan fine, although the first attempt threw a CEL every time. The second attempt would only throw a CEL when the compressor ran, or when the PCM tried to turn the aux fan on at temp. The fan wouldn't run when the CEL was tripped- I wondered if the PCM was programmed to keep the fan off if it detects a short in the control circuit. Can anyone confirm/deny that?

Doesn't sound too bad to me (I left the battery connected when I was working on mine the first time, and got additional codes for the PCM or TCM, in addition to the 1491). I borrowed a ScanGauge so I could pull/clear the codes without wondering if I fried something every time the light came on.
 
Oh, the 1491 is triggered by open or short? If it's something like the TCU solenoid outputs, set your override switch up so that when the fan is in override, the wire from the ECU is connected to say a 330 ohm 1 watt resistor to +12V instead of being left open. The ECU (at least on a 97, don't have my 99 FSM handy so I'm looking at the 97 which should be pretty similar) switches the fan on by pulling the negative end of the relay coil to ground. It's not smart enough to monitor actual current flow, just whether there is any current flowing.

As a side note, if you want to avoid this issue entirely, you should be able to install the override downstream of the relay (instead of between the relay and ECU), because the ECU can only determine the current draw of the relay coil, it can't tell if the fan is actually drawing current through the relay.

EDIT: hubs, I'm WAGing now, but looking at the schematic (8W-42-7 if you have a 97 FSM) the AC compressor clutch relay and fan control relay are powered by the same positive line, and individually controlled by the ECU via the negatives. Perhaps the ECU is seeing too little/too much current draw when it turns on the compressor for some reason, though I can't really explain it.
 
Thanks for all the replies folks. I started her up to drive to work this morning and low and behold the CEL was gone! I haven't had a chance to mess around with it anymore yet (working a 48 hour shift) but it seems to have cleared on it's own. The real test will be whether or not I can turn on the A/C without triggering it again. Let's see if I can answer a few more questions...

Try unplugging and reconnecting the connector again, this time with the ignition off. Maybe you caused an intermittent connection with the CCD bus pins - since the TCU does its own thing and (afaik) only uses the CCD bus for error/condition reporting, this would cause the code.

Unplugging and replugging the connector with the ignition off is exactly what I did after realizing my mistake. The thing that confuses me is why the CEL came back on after being erased but has now disappeared on it's own. I know it can reset itself after 3 consecutive "trips". My guess is that it's because I haven't turned on my A/C for the last 3 trips since getting the codes read/cleared.

Besides what were you doing plugging and unplugging and splicing into wires while the battery was connected anyways. If the battery had been d/c like it should have then there wouldn't have been any problem.

I know, I know... The rest of the electrical mods I've been working on are run through an add-on fuse box that I have protected by a resettable circuit breaker. I had it in the tripped postion but just wasn't thinking when I did this one.

P1491 will throw a check engine light.

Pull the codes again.

The listings for the codes showed an (M) after the 1694 code but not after the 1491 code. This is where I got the impression that 1491 would not activate the light.

I've been living with the 1491 CEL from my manual override for over a year. When it warms up, I'll switch to 2awg mains, 4awg accessory, lose the adj temp controller for the 2nd fan, and wire a 10awg circuit for both fans.

I've upgraded all my mains to 1awg and my accessory fuse box is on 4awg. I was just hoping to piggyback onto the factory system so I would not have to run additional power wires to the fan. Partly due to space, partly due to my fuse box only being rated at 30amps per circuit / 120amps total, and partly because I still believe I can outsmart the system...

P1491 throws a CEL on mine as well- hence why I put it back to stock while someone sharper sorted it out.

My setups would operate the fan fine, although the first attempt threw a CEL every time. The second attempt would only throw a CEL when the compressor ran, or when the PCM tried to turn the aux fan on at temp. The fan wouldn't run when the CEL was tripped- I wondered if the PCM was programmed to keep the fan off if it detects a short in the control circuit. Can anyone confirm/deny that?

Doesn't sound too bad to me (I left the battery connected when I was working on mine the first time, and got additional codes for the PCM or TCM, in addition to the 1491). I borrowed a ScanGauge so I could pull/clear the codes without wondering if I fried something every time the light came on.

I'm still in the troubleshooting phase here myself, but I have confirmation that the wiring set-up I'm using worked without throwing codes. I won't be convinced until I see whether or not my fan will activate with the A/C as it is supposed to and that it won't throw any codes when it does. And yes, I definately need to get an OBDII scanner. I didn't realize that CA had outlawed parts stores renting them out. I'm just trying to decide which one to buy.

Oh, the 1491 is triggered by open or short? If it's something like the TCU solenoid outputs, set your override switch up so that when the fan is in override, the wire from the ECU is connected to say a 330 ohm 1 watt resistor to +12V instead of being left open. The ECU (at least on a 97, don't have my 99 FSM handy so I'm looking at the 97 which should be pretty similar) switches the fan on by pulling the negative end of the relay coil to ground. It's not smart enough to monitor actual current flow, just whether there is any current flowing.

As a side note, if you want to avoid this issue entirely, you should be able to install the override downstream of the relay (instead of between the relay and ECU), because the ECU can only determine the current draw of the relay coil, it can't tell if the fan is actually drawing current through the relay.

The description for 1491 says "open or short". No help there. 1999 is the same as the 1997 in that the ECM provides ground to the relay coil.

EDIT: hubs, I'm WAGing now, but looking at the schematic (8W-42-7 if you have a 97 FSM) the AC compressor clutch relay and fan control relay are powered by the same positive line, and individually controlled by the ECU via the negatives. Perhaps the ECU is seeing too little/too much current draw when it turns on the compressor for some reason, though I can't really explain it.

Yep, they are both powered by the ST-RUN F20 power wire which I believe is an internal circuit within the the PDC. I'm tapped into ST-RUN A21 (Dark Blue / White) for the positive side of the relay coil.

Here's a schematic of my wiring set-up. I've been working on this in a thread in Mod Tech and I drew this up based upon a description I was given by the guy I mentioned above as having run this mod without a CEL.



AuxFanWiring2.jpg


This seems like it should work perfectly. The ECM still activates a relay. The override switch is isolated by the relay.

Possible problems...

1. The relay the ECM is grounding is recieving it's power from ST-RUN A21 instead of ST-RUN F20. Wouldn't think this should matter as I believe it's just looking for 12V+. However, the ECM is also supplied it's ST-RUN power by circuit ST-RUN A21. Maybe it doesn't like the fact that it is activating the relay coil with the same power source it draws it's power from?

2. The new relay I installed has an internal diode. Maybe the voltage drop caused by the diode is upsetting the ECM? Possible change in current draw? Or is the ECM even that smart?

3. My override switch's ground is internally shared by the circuit for the indicator light within the switch. Wouldn't think this should matter as both circuits are isolated from the ECM.

I plan on posting up pics and more details once (if) I get this to work properly. Any insight on something I may be overlooking?

Regardless... pretty darn happy my ECM doesn't appear to be fried. :roll:
 
Interesting... yeah, I would tend to agree. Try swapping the wires to the coil on the new relay. The diode is there to shunt the back EMF created by the collapsing magnetic field in the relay coil when the relay turns off, which protects the ECU relay driver circuit from the several thousand volt spike it would otherwise see. If you connected the wrong side of the coil (and thus the wrong side of the diode) to +12V, the ECU will see nearly a dead short because the diode will be conducting when the ECU attempts to turn the relay on, instead of only when the field collapses.

My personal leaning on setting something like this up would be to scavenge the terminals required from a PDC at the junkyard and install the entire setup in one of the spare relay positions in the OEM PDC, but it'd take an hour or two extra and all you'd gain is an absolutely factory look under the hood (I really hate extra wires hanging around, and it's tough to find good spots to put them.) Not all that important to most people.
 
Looks like my ECM is in good shape! I started up my XJ again tonight and still no CEL. I even shifted it through all the gears and turned the A/C on and off repeatedly. I'll have to hook up an OBDII scanner just to make sure there's no remaining codes logged in the sytem, but at this point things are looking good. I'll be going on a nice long snow wheeling trip tomorrow so it ought to get run through the paces then.

Turns out my problem was that I had the coil in my didoe protected relay wired backwards. I'm still new to using them, but hopefully I've got it figured out now. The ECM is once again kicking on the fan when the A/C compressor cycles as it should. I haven't ran it hard enough to see if it kicks on due to engine temp, but I don't anticipate there being any problems there.

Thanks all for your help and suggestions! If you're curious about the auxillary fan wiring setup I'm using. Check out this thread:

http://www.naxja.org/forum/showthread.php?t=1022169

I plan on doing a short write up on it to hopefully help others avoid the problems I ran into, but if you're interested in trying it out, the above thread should get you going in the right direction for now.

Time to finish wiring up my auxillary reverse lights...
 
Back
Top