Check engine light not working

[Wirenut]

A while ago, the engine light in my '94 4.0 stopped illuminating when I started the truck so I swapped in a known good bulb and also tested the CEL bulb and the issue seems to be that the CEL simply isn't turning on. It stays off even when I unplug the MAF sensor and start the engine. I don't have any issues with the truck and it runs really well but it's old and high mileage (350k or so) and I'd like to have the light working so I can catch future issues. If it matters, I swapped the old incandescent bulb with an LED back when I got the truck but I don't think that's the problem since it worked fine for several years and still lights up when installed in other sockets.
Has anyone dealt with this or have any ideas where I should start looking?

 

[franklin2]

I assume your temp, fuel guages work? The check engine light is fed voltage from fuse #7, 15 amp which also feeds those gauges. That is where the "D" in the diagram below comes from.

Also in the diagram below, you can see the wire for the light also goes to the data link connector, which since this is a 1994 I am going to assume is still under the hood. This truck is not OBDII yet correct? You could turn the key on, then ground the pink/lightgreen wire at the data link connector and see if the light lights up.

If it doesn't light up, you might have to get into the cluster again and make sure you have 12v on one side of the bulb with the key on.

 

[Wirenut]

You're right that it's pre-OBD II. All the other gauges and lights work and grounding the wire at the connector did not cause the light to come on so I'll check for voltage at the bulb tomorrow.

 

[Wirenut]

I actually pulled the cluster this time instead of just prying it out enough to get at the bulbs and while I was getting the speedometer cable off, the CEL started to flicker. Once it came out, I took a look at all of the connections and while they didn't look corroded or dirty, there was a slightly sticky film on them, So I cleaned all the leads on the cluster and the harnesses and the light now works like it should. Never thought I'd be relieved to see a check engine light turn on.

 

[Kira]

What did you use to clean all the contacts?
What did you use to get into the harness females?
Alcohol? Electric Parts Cleaner?

 

[Wirenut]

Contact cleaner and a fine abrasive pad for the traces on the back of the cluster since they're so thin and flexible. One was even peeling a little. The harnesses are the male side and very sturdy so I hit them with just a brass brush.

 

[franklin2]

If they are stuck down good, I have used a pencil eraser before to shine the copper pads up. If the copper is lifting off the clear plastic, a small dab of superglue under the copper trace works great for sticking it back down quickly so you can continue putting it back together. The pads tend to really come up where the large wiring plugs plug in on some models. The superglue trick works good there too.