He Couldn't See At Night

While we were out of state, a fellow called to see if I could look at the headlights on his 97 Eldorado.  We always forward calls from the shop to my cell, that way I’m available to my customers anytime they need me. Someday I’ll let you know what that get’s to be like. He explained that he and several others have already looked at this trying to figure it out and they haven’t had any success at all. He bought and replaced all of the bulbs and that didn’t help and trying to follow the wiring schematic in his Haynes book he couldn’t make any sense of the circuit at all.
We do our diagnostics by selling blocks of time and I do any and all testing required in that period of time. In some cases we sell an hour, and if the car is diagnosed quickly we reduce the fee appropriately. In others we sell the hour and I occasionally go over that time if I am just about to arrive at the answer. We don’t charge extra when we go over like that, because we stick to the agreed price. There are times that we have to get more time if a problem turns out to be really complicated, you never really know what you are up against until you complete the diagnostics. The customer always has the choice about whether to stop or continue. The biggest mistake anyone ever makes is to assume they have seen a particular problem before, or assume that the problem someone else’s car had is the same since the symptom was similar, as in this case no headlights.
He didn’t want to agree to an hours’ worth of time, so we set the limit at half an hour and he wanted to wait for the car. Our shop doesn’t have a fancy waiting room, every dime is spent on the things we need for the cars. That means he can watch every step that is taken and in this case he got to see some things that he never knew ever went on.
The first step of any diagnosis is to confirm the symptom and I found the parking lights and the daytime running lights worked, but the headlights were inoperative, even with the flash to pass position of the dimmer switch. Had the high beams worked in that position, it would have helped me narrow down some of the testing. The next steps take place simultaneously, between attaching the TechII scan tool and printing out the schematic the real work was about to begin.
From the schematic;
There is a parking light relay that is controlled by the headlight switch, it sends power to the parking lights of course, and it also powers up the three relays in the under hood power distribution center.
The high beam, low beam relays are ground controlled by the body computer, the main headlight relay is ground controlled by the headlight switch, or the body computer or the instrument cluster, all three components can command that relay on. With the scan tool, confirming operation of the three relays was easy using the bi-directional commands. The high beam and low beam relays were being commanded as confirmed by the audible click they would make. The headlight relay was not turning on/off with the headlight switch or through the BCM with the scan tool.
At that point a little more explanation of the wiring is in order. All three relays get the same power to allow them to be turned on, but the ground circuits for them are all different. Since the headlight relay itself had three possible ways to be turned on, and it shared the same power to it that the other two did, that meant the problem was as easy as just a failed relay, or the wire from the under hood power center to the interior of the car had failed. This is a yellow wire and it goes to pin C of the headlight switch, but again there is a splice in the harness and it also goes to the instrument cluster as well as the BCM. So the next step of the diagnostics is to pull the relay and re-install it on top of a pedestal that allows testing connections. In doing that both powers to it were easily confirmed. The next step was to check the ground control circuit and that is easily done with an ordinary test light. When I touched the ground control circuit, the relay clicked and the headlights came on.
By this time the customer was standing right beside the car. I demonstrated  that with the test light commanding the headlight realy on, I could then control the high beam, and the low beam from both inside the car, and with the scan tool.  I explained that there was still more work to be done in the way of possibly locating the actual location of the damaged wire, or it could simply be bypassed.  However his agreed half an hour’s time was up and we knew that the yellow wire in the harness was the problem. 
The only thing left now was for him to agree to have me actually fix the problem or not.