SO I know that there is an issue with some of the FCM and PDC out there, but i did confirm that i do not have an open circuit in the PDC. by traceing and checking resistance on Pin 15 and 86 to the relay for the parking light relay.

What i do not have is the output from the FCM. i am not sure what drives the output. I see that the wiring diagrams say the signal comes from the Multi function switch on the Column. But what should the signal be coming into the relay on terminal 86.
SO the second part of the question is could I put a jumper on #86 of the parking relay and connect it to the relay for the Fog lamps. #86. SO that when I pull out on the switch the fog lamps can power the parking lamp relay . Will this cause a problem for the FCM?
thanks,

Karl.

 

#85 is 12v+ from the fuse
#86 is ground from the FCM

in order to trigger a relay coil you need (+) on one side and (-) on the other

to test this and the lights, take a wire or a meter lead and connect one end to the ground and the other to #86

if you wanted to control this from another circuit like the fogs, you would need a switched ground from that circuit

a lot of the circuit in the truck are hot all the time. The FCM and PCM use floating grounds to turn on and off different circuits as needed by the operator

 

First off, what year truck are we dealing with? 02-05? Have you checked for corrosion under the fuse/relay panel?

just curious... All of this elaborate stuff... Have you at least tried to replace the Relay? Obvious question I know but you didnt actually say so.

There are a couple of diagrams you are missing here to fully disclose the circuit.

Ground pin #86 and see if the lights come on and/or you here the relay clicking.
You would want to cut that wire to prevent feedback into the FCM or you could drain your battery.

 

it is a 2004 and yes I checked the relay. I appreciate the comments everyone.

 

well the Park Lamp relay control is just the headlight switch sending X voltage to the FCM/IPM and tell it to allow current to pass through the 85 and 86 PIN on your relay. (it does this by allowing PIN 86 to ground, via a floating ground as mentioned by weedahoe I believe.)

Then your relay will energize and now Current from the Battery is flowing from PIN 30 to PIN 87 which powers up your parking lamps.

Your parking lamps should be DUAL FILAMENT, and if something shorted you may have simply burnt out those filaments. Have you tried replacing the bulbs with new dual filaments already?

from your post I assume you've gotten through the bulb and made sure it was good.

Next thing to check is Voltage across your headlight switch... you need to see if there is any voltage or amperage across the switch.

Next step is to Cut (physically cut) the link between PIN 15 on the Park Lamp Relay Control and ground PIN 86...(this may be impossible to do if it's on the circuit board, which I 99% guarantee it is) if your parking lamps come on then you know there is a problem with the FCM/IPM OR there is a problem with the signal from the switch.

Either way you'll likely have to really dig into the FCM to fix it.

I personally wouldn't want to drive the relay through the fog lights, as it may cause issues if the FCM ever decides it wants to work correctly again...

You may also want to check for any C-Codes that may be present, as this may help you narrow the problem down... The method for pulling these codes is in the link in my signature.

Integrated Power Modules of any kind are nearly impossible to repair unless it's an issue with a physical wire outside of the module, which is generally why the whole module gets replaced when it's found to be at fault.