My 91 v6 had the "check engne light" come on this morning. I've got a newer O2 sensor with about ~100,000km on it. Also a newly oiled K&N filter. I haven't checked any codes. Does anyone know why the light comes on?

Easy enough to pull the code(s) and find out:
- http://www.4crawler.com/4x4/CheapTricks/index.shtml#TimingCheckConnector

Thanks 4Crawler,
I got a code 4 - 2 which I guess is 42 and which from the chart on your site gives:
VEHICLE SPEED SENSOR SIGNAL CIRCUIT--NO SPD. SIGNAL FOR SEVERAL SECONDS

WHILE VEHICLE IS OPERATED UNDER HEAVY LOAD (TPS/MAP/AIR FLOW INPUTS)

--SPEED SENSOR/CIRCUIT

--ECU
I didn't know there was a speed sensor unless its picked up by the rear axle anti lock brake system?
I was going about 125km/hr.
Thx

The speed sensor is in the speedometer... it outputs to the ECU.

I believe you but, my 91 has the standard mechanical speedo which unlike the newer 93? and up which have the electronic pick up at the rear trany output shaft. If my speedo has a bit of bind up (where the speedometer wobles at low speeds) do you guys think that could be the problem for this code?
I also have a lot of kilometers on this truck; now just reaching ~600,000km. The drive shaft has recently gotten some vibration in it and that is very likely from the hanger bearing. Rubber dampner pops out of the holder and has to be stuffed back in and then it's quiet. Is this code just from pushing it or something else? Thanks again!

The VSS is on the back of the speedo as noted if you have a mechanical unit. The ECU gets that signal as does the cruise control module, if you have that. Also for the A/T ECU if you have an automatic. Since the VSS is driven off the speedo cable, the binding you see might be related to that. Maybe try lubing tbe cable or replace it if needed. Center bearing can be replaced or you can eliminate it with a one-piece drive shaft.

Hi 4Crawler, The instrument pannel is "upgraded" over my old base package. There is no cruise and I have a 5 speed.
I did lubricate the cable a while back with some very low viscosity synthetic oil as I was trying to reduce the speedometer needle vibration. It took care if most of it but not all of the vibration. I'll look for a newer speedo cable and get a new hanger bearing and see what happens.
Thanks