Electrical box smoking

[scprandy]

I have a 91 four door, Laredo, 4X4 XJ that has had the dash lights and radio out for quite some time ( keeps blowing the fuse). I keep replacing the dash light 10 amp fuse under the dash and it keeps blowing every time I turn the headlights on. I took all the dash components out looking for the short, found nothing, I put a 15amp in and stared the jeep. I heard the 15amp fuse blow and then saw the box on the left fender in the engine bay start to smoke !!
I cant find a name for this box and don't know what it is. I pulled the box out and opened it up to find a very thick jelly like substance that appears to be encasing some electronics under there ( I guess to keep it weather resistant) there are also what appear to be rectifiers or some type of heat sinks in the box as well. I'm going to see if I can find one at our pull a part and hope that I found the offensive source of my shorting, however, it would be nice to know what the heck this thing is called. Can anyone give any input? Thank you in advance.

 

[old_man]

From your description, it sounds like the computer aka ECU.

 

[scprandy]

Thanks for your reply. It has one marking on it: 56005 707 07/16/90
I guess that's the build date and model #. Do you know if this is a common source of shorts?
I would hate to get a new one and put it in only to have it happen again. My first concern was that by putting in a bigger fuse may have overloaded that circuit and damaged the ECU if that's what it is. Seems kind of odd that just by going from a 10amp fuse to a 15amp that it would blow the ECU....

 

[old_man]

Well the size of the fuse was selected to protect the ECU. Apparently you thought you knew more than the engineer. :dunce:

The short most likely was not in the ECU, so just replacing the ECU may yield another blown part. If you are not competent at troubleshooting electrical circuits, I would most likely be trying to find a good tech.

 

[KD3NE_]

Not sure what it is, but I can say this.

If you replaces a 10 AMP fuse with a 15 AMP fuse you are allowing more current on the circuit that it was designed for. Do that long enough, and something will probably burn up before the fuse blows.

You really should NOT over fuse a circuit unless you are prepared to standby and wait for something to begin smoking.

As to your problem, you say the 10 AMP fuse blew every time you turned on the headlights?

Prior to using the 15 Amp fuse did you check the wiring harness in the front panel to the headlights and such???????

I remember reading a comment somewhere that the wiring harness in for the headlights was one of the engineering weaknesses in the XJ. At the time I was researching problems related to another circuit, the heater/ac fan control circuit. Seems that one is another one that needs a little care.

 

[scprandy]

Thanks for your input. I realize now that it was a pretty stupid thing to do putting a larger fuse in there.

I did replace the headlight switch a few months back and the problem never changed. It wasn't a daily driver at the time so I never pursued the problem any further.

To answer your question KD3NE, yes, every time I pull the headlight switch to the running lights position the fuse would blow. My running lights, turn signals, brake lights or reverse lights have never been affected. The only problem is no instrument lights or radio power.

I did a pretty thorough search of the wiring harness behind/under the dash to see if there were any worn wires or signs of shorting without any luck.

I will do some looking in the front along the headlight circuit, thanks again for the guidance.

 

[joe_peters]

Yeah, you have some work set out for you, but with determination and luck you may get there.

Popping fuses gets a little old after a few boxes. Get yourself an audible circuit breaker:

http://www.amazon.com/Tool-Aid-SGT2..._1?ie=UTF8&s=automotive&qid=1297561011&sr=1-1

Or you can get a bit more sophisticated (read as more costly):

http://www.amazon.com/Power-Probe-E..._5?ie=UTF8&s=automotive&qid=1297561011&sr=1-5

And having the FSM for wiring diagrams helps.

Good luck.

 

[KD3NE_]

To answer your question KD3NE, yes, every time I pull the headlight switch to the running lights position the fuse would blow. My running lights, turn signals, brake lights or reverse lights have never been affected. The only problem is no instrument lights or radio power.

Well let me ask this.

After you pull the headlight switch to the running lights position and the fuse blew killing instrument lights and radio power, did the running lights remain on?

Somehow the added current of that circuit contributed to the overload condition that blew the fuse.

I don't have a schematic so I can only 'wing it'.

There has to be some relationship between the headlight/running light circuit and the instrument lights and radio power.

As mentioned one common point is the headlight switch.

Pulled out it turns on the Running Lights and Instrument Panel, but is it also supposed to control power to the radio?

in my mind the radio is on another circuit, or should be.

The headlight switch should not affect the radio as it should get power when the ignition is turn forward to ON or back to the Accessory position.

You truly need a schematic/wiring diagram and trace out all circuits applicable to the fuse that blew.

Once you have identified all the circuits and devices removed them from the circuit as you can and test for shorts.

With all of the circuits/devices disconnected from the fuse, install a new fuse.

Connect one circuit and power up the vehicle. Power down and add another circuit and power up. If the fuse does not blow repeat this process. If there is a short somewhere, the fuse will blow and the last connected circuit will be suspect.

One thing you could try at this point .....

Disconnect the box that burned.
Install a 10 Amp Fuse
Pull out the headlight switch and see if the 10 Ampl blows.
With the burned box removed, if it was the problem the fuse should not blow.
If the fuse blows, then look to the running light/headlight circuit.

It's almost completely impossible to troubleshoot this without a schematic or FSM for the electrical circuit otherwise.

 

[lawsoncl]

The part number he quoted is a 1991 ECM. I'm not sure why it smoked, unless maybe there's something shorted to ground and poor grounding elsewhere caused it to backfeed through the ground in the computer somehow. Strange.

The dash lites are often wired to the radio to control the display backlight.

Does your Jeep have daytime running lights?

 

[scprandy]

No daytime running lights. I will try putting that 10amp fuse in and power up without the ECM installed, I think that would eliminate it as a source of the short. Great idea, thanks.

I don't have a FSM but will try to locate one and start the process of isolating and testing circuits.

I can tell this will be quite a time consuming process, hopefully I can get a replacement ECM. Are they typically hard to find?

 

[KD3NE_]

I can tell this will be quite a time consuming process, hopefully I can get a replacement ECM. Are they typically hard to find?

Found 2 in a matter of seconds..... ( these match your part number - there are many more if you remove your part number from the search string )

http://cgi.ebay.com/ebaymotors/Jeep...rQ5fTruckQ5fPartsQ5fAccessories#ht_1598wt_941

http://cgi.ebay.com/ebaymotors/5600...Q5fTruckQ5fPartsQ5fAccessories#ht_1751wt_1167

http://desc.shop.ebay.com/i.html?_n...sid=p3286.c0.m270.l1313&LH_TitleDesc=1&_rdc=1

The dash lites are often wired to the radio to control the display backlight.

That might explain him losing the light in the radio, but the entire radio?

 

[scprandy]

Thanks KD3NE, i was really hopeful to find one at my pull a part and not spend $100. I guess if you do stupid things you must pay the price....

Yea I don't get the weird loss of the radio, I checked the harness going to it and found no issues. Will be checking all major grounds next, also found no issues with headlight harness.

I'm not an electrician (as you can tell) so this is a learning curve for me.