This 2001 Chevrolet Impala came in with the complaint that the low beam headlights would not work. I went to the headlight bulb and found power on both terminals of the low beam bulb. This indicates that a ground is missing. After looking at a wiring diagram I determined that there were several possibilities: broken wire, loose ground connection at left side of dash, blown fuse,faulty headlamp relay, faulty dimmer switch and faulty body control computer. The dimmer switch however was the one point were most things could be checked. I removed the lower dash cover and the metal bracket behind it to access the wiring connector.
The yellow connector is for the airbag system and the connector that I was looking for is just below it.
Working room was pretty tight and the plastic edges are very sharp so I pulled the connector down to gain better access.
The white lever has to be flipped over to separate the two connector halves.
With the connector separated I decided to reattach it to it’s mounting clip. You can see the mounting clip for the air bag connector just in front of the connector.
From looking at diagrams earlier I had determined that I needed to check terminal C4, Dark Blue for bleed through power from the low beam headlight bulbs. It is the second terminal from the right in the second row down. Terminal C3, Light Blue is the ground supply wire that is sent to the dimmer switch from the headlamp relay. I do not know why the color codes change from the main harness to the combination switch harness but on the main harness side the C3 wire is Light Blue and on the combination switch harness the wire is yellow. On the main harness side the C4 wire is Dark Blue and on the combination switch side it is Brown.
Anyhow, I had bleed through power on the C4 terminal and I had ground on C3. I could touch the two wires together and the low beams would come on. Diagnosis of a faulty combination switch. Now that I had it diagnosed, all I had left to do was to change the switch. I had to remove the steering column cover. There are two torx screws attaching the lower cover and the tilt lever had to be pulled out of the column. There are two inverted torx screws holding the upper cover from the inside that have to be removed. Once the covers are removed the two attaching screws can be removed.
I use a torx bit and 1/4″ ratcheting wrench because it allows me to change the switch without removing the steering wheel and air bag. Takes a little coordination and effort but it saves a lot of time.
There are a couple of wire ties that have to be removed and one harness support that needs to be unclipped. Then the cruise control subharness can be detached.
The wire connector also has to be removed from the shifter interlock solenoid.
Out with the old and in with the new and all is well. I took a picture of the headlight relay. It is the upper, larger rectangular box that cannot be fully seen in the picture. This view is of the backside of the fuse box that is at the left end of the instrument panel.
Are the terminals of the relay labeled 30, 85,86 and 87?
Yes they are. If the labeling on the side matches the view from above then 87 is top right. 30 lower right.
With 85 and 86 top and bottom left respectively.
BTW I double checked and top right only has 9V. Bleed through I presume. But lower right has full 12V
Yes, the relay could be faulty. Since it clicks when the headlight switch is turned on we can determine that the relay coil and coil control are okay. You would need to check the contact circuits. Terminal “E10” at the relay harness connector should have continuity to terminal “C3” of the dimmer switch harness connector (vehicle side). Terminal “E11” of the relay harness connector should have continuity to chassis ground. If you remove the headlight switch relay you will likely find the terminal designations of “30” and “87” for the contact terminals.
While waiting for the reply I swapped the relay with no joy. So the relay was good. I’m following your new advice and am puzzled. Either I’m not understanding or they don’t apply to my 2000.
The top right relay socket (87?) has continuity with C3. The lower right does not have continuity with the chassis.
I thought I could supply my own to that blade on the relay as a test. I inserted the relay leaving a little exposed of the blade then stuck a wire against the blade and touched it to the chassis. But it drew an arc! I then checked the voltage and that lower right socket has 12V on it. Even when the connector is unPlugged. When the connector is plugged, both those sockets have 12V (relay removed)
If there is no continuity between that lower right socket and chassis, shouldn’t I be able to supply my own and have it work?
Side note: I tested continuity between the switch side of C3 and C4 and there is continuity when switched to low beam but continuity drops when I switch the switch to high beam. I presume this means the switch is good. Correct?
Thank you again.
Another tip for emergency situation. My daughter called me around 10:00 at night with no lights. I went to where she was and sure enough no lights. In order for her to get home safely with the car I removed the wiring from the fog lights and routed up to the headlights and plugged the fog light circuit into the headlights. Then you just use the foglight button on the dash to turn the headlights on and off. She actually drove the car like this for a couple of weeks before I got a chance to tear into the dash.
It makes me happy that you really read the posted information and thought about what could be done as a limited or temporary repair. I have done that several times for customers that needed to have at least low beams and did not have the funds for this repair.
Just a suggestion. I followed this guide and did have a multiswitch problem, no ground being supplied. I did not have a new multiswitch and had not ordered one so as a temp fix it took C3 and C4 wires on the multiswitch side of the connector and stripped back the insulation, jumpered the wires together and then soldered and taped the connection. This allows everything to work as normal except the high beams will not stay on by themselves, you must hold the stick back. Thanks Sparky for the guide it was very useful.