Topic: 95 yj no spark headaches

[tmmsec]

Hi All,

I have a 95 yj that would not deliver any spark.

The coil was replaced.
The camshaft position sensor checked ok
All the fuses, both inside and under the hood were checked.  All good.

I towed it to a repair shop, they said the jeep's computer was bad and needed to be replaced.
The computer was replaced, and still no spark.

A friend of mine is an instructor at the local Community College Auto Shop.  He let his students try to figure it out.

They found the CPU has two power inputs, one comes from the 40 amp fuse in the main block next to the battery.  It has a second power wire for the computer that comes from the "dome light" fuse on the inside fuse block.  They replaced a corroded fuse, and it solved the problem.

I can't imagine why Jeep would put a power wire to the computer and not label it in any way, and then disguise it by attaching it to the dome light circuit.  The fuse that was replaced was a good fuse, it just was not passing enough current through it to power the computer.

Just in case you have a "no spark" problem that you can't figure out, make sure your computer is getting power from both the large fuse in the engine compartment and the small dome light fuse in the inside fuse block.

[sharpxmen]

Hi All,

I have a 95 yj that would not deliver any spark.

The coil was replaced.
The camshaft position sensor checked ok
All the fuses, both inside and under the hood were checked.  All good.

I towed it to a repair shop, they said the jeep's computer was bad and needed to be replaced.
The computer was replaced, and still no spark.

A friend of mine is an instructor at the local Community College Auto Shop.  He let his students try to figure it out.

They found the CPU has two power inputs, one comes from the 40 amp fuse in the main block next to the battery.  It has a second power wire for the computer that comes from the "dome light" fuse on the inside fuse block.  They replaced a corroded fuse, and it solved the problem.

I can't imagine why Jeep would put a power wire to the computer and not label it in any way, and then disguise it by attaching it to the dome light circuit.  The fuse that was replaced was a good fuse, it just was not passing enough current through it to power the computer.

Just in case you have a "no spark" problem that you can't figure out, make sure your computer is getting power from both the large fuse in the engine compartment and the small dome light fuse in the inside fuse block.

this is interesting

i didn't think the YJs came with a dome light from factory - is it the light under the dashboard that turns on when you open the doors the one you mention?

if that was fuse #5 (red color, 10Amps) that is for ASD and fuel pump and in that case you will not have the any spark or gas, not because of the PCM but b/c your coil has no power.

Which fuse was it?

EDIT: there is an ASD sense wire that goes to the computer, it is as the name says meant to determine if the Injectors and Coil receive power, it does not provide power to the computer. The computer (PCM) is powered by fuse 1 from power distribution center under the hood which is 30 Amp and it goes to pin 3 on the PCM connector (that receives power at all times, even when the engine is not running and keys out), the 40Amp fuse powers your ignition switch and from there it has an input to the PCM on pin 9 so it knows that the contact is in the run/start position, it does not provide power to the PCM just info (same as the ASD sense).

[tmmsec]

My YJ has no dome light.  One of the two hot wires that go to the Computer is plumbed into the "dome light" circutit.  Given there is no dome light maybe somebody in the factory thought it would be a good place to put one of the cpu power wires.

[sharpxmen]

My YJ has no dome light.  One of the two hot wires that go to the Computer is plumbed into the "dome light" circutit.  Given there is no dome light maybe somebody in the factory thought it would be a good place to put one of the cpu power wires.

you lost me there, is the fuse labelled "dome light"?

[chardrc]

im pretty sure my 1990yj has a fuel labeled "dome" (no dome light as well)... ill have to confirm tonight (although my computer system is total different than the mpi one.)

[sharpxmen]

I was wrong actually - the factory hardtops have a dome light wired to the same circuit as the courtesy lamps under the dash (right and left side) and the light inside the engine compartment under the hood - although those are apparently wired on a fuse in the PDC by the battery a 10 Amp fuse (fuse 12)
the inside fuseblock has no dome light fuse, that's why i was asking which one was it - here's the diagram of the fuse block:



@tmmsec: which one of these fuses was the one that gave you the trouble?

[Torch_Ind]

ya the hard tops have the dome.

very interested in this also just for future reference. Haven't had that prob yet lol

Hope you got you cash back from the shop that said you needed a pcm.