Expert: Roland Finston - 1/24/2012

Question
The light comes on the defoger switch, ebl 40a fuse is ok, ebl relay energizes when checked out of socket but does not energize when in the socket, also the 20a fuse is ok that feeds the defoger control switch.

The defroster heats up when the relay is shunted out.
It looks like the control switch is not grounding the coil on the ebl relay.

Thanks, Charles

Answer
Hi Charles,
Your analysis seems to be correct except for the fact that the light goes 'on' which to me says the switch is ok. You could check out the failure to ground the actuation coil of the relay by measuring for voltage change on the grounding pin of the coil at the coil socket proper. If you see no voltage change, or you fail to see a shorting of the socket pin to ground when you request the defogger then an issue between the switch and the relay would be the likely cause of the problem. The actual request from the request switch goes to the front control module of the power box and is handled digitally (on the PCI bus) so unless you have a sophisticated diagnostic box you can't test if there is a failure of the digital communication.
The connection to the relay coil grounding pin is from pin 31 of the 49-pin plug on the front side of the 'front control module'. You can release the integrated power module (the main power box) from its clips and rotate it so that you can then remove the screws that hold the very front-most part of the box which is the 'front control module' plug. Then gently pry it apart to view the 49-pin interface between the two parts and then locate pin 31 in the 20-pin back row (numbered 30 to 49) of the IPM plug. Verify that it is connected to the relay coil grounding pin.
Sorry to say that this does appear to require some digging into these components. Because the light goes 'on' when you activate the switch then the switch is likely not be the problem. That points to the connection between pin 31 and the relay socket or the connection between the socket itself and the relay grounding pin may be corroded or dirty. It seems improbable that the digital communication aspect would be 'bad' on this single circuit of so many.
Roland
PS Sorry for the delay in responding to your question but I just found it in the 'pool' to which it was referred by Kevin. If you would be so kind as to 'rate' my answer and also consider clicking on 'yes' where you see the question about a nomination of me to be 'volunteer of the month' that would be most appreciated.