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.
Glad you have that solved. I have dealt with a lot of rodent damage over the years. Normally with a flashlight and a keen eye you can spot some sort of trail. Bits of colored plastic, tape, split loom, acorns, dog food and of course waste. Once you find the signs, it is usually pretty easy to find the wire damage and fix it. I do recommend finding it as rarely is there only one circuit damaged.
If you supply an external ground to the C4,dark blue wire connection under the dash (vehicle side) do the headlights work? If they do not you are correct that you have damaged wires (probably under the hood in the air filter area). If they do the problem is in the interior of the vehicle.
They do not light if I apply a ground to C4.
I skinned dark blue at both ends (under dash connector) and at driver side light. I twisted the ends of a long scrap wire I had onto those skinned portions effectively bypassing the blue wire. The lights worked normally using the switch and all.
Thanx
You do not need to go to such extremes. If you just need to get the lights functional, all of the wiring can be done under the dash. Myself I would just fix it properly, as you only appear to be missing the ground signal from the headlight relay.
Take a look at your relay terminal designations again. 87 and 30 should be in diagonally opposite corners. Also remember that there is a mirror effect when looking at the relay and back at the socket. I always hold the two terminals (87 & 30) with one finger and my thumb then rotate the relay around as if I were going to install it. Then I look at the corners where my finger and thumb are located. I am making the assumption that all four terminals are the same size.
Ok. I was thinking that the ground from the Connector to the light was bad. But then I measured 7V feedback from the bulb. Zero with bulbs out.I don’t know what it’s supposed to be. But that confirns continuity. But I hunched that maybe the ground to the light was just damaged and not broken. So I took a length of wire and skinned the dark blue at the light and at the terminal and wrapped each end of the wire at those locations effectively jumping out that dark blue.
Voila!! Success.
I think I’ll get proper gauge wire, route it properly and solder in the ends. Better than tearing apart the whole car to find the damage.
Thank you for the guidance.
There should be numbers molded into the plastic beside each terminal as well. This is where you should identify terminal location.
I disconnected the connector again and checked continuity between C3 (vehicle side) and chassis. When I pull out the dash switch for the lights, there IS continuity. So I guess it’s all good past the relay. Shorting C3 and C4 (vehicle side) won’t turn on lights. And, as I said, there is C3/C4 continuity switch side of connector when in low beam position.