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.
I have a similar headlight issue with my 2004 Impala. However, both my low and high beams do not work. The only time the lights come on is when you pull on the dimmer switch lever. However, as soon as you let go the lights go off. The dash lights also do not work but all other interior lights do work. I don’t have power at C4 and when I connect C4 to C3 the lights do not come on. I cannot find the fuse for the headlamps. What is it labeled? How do you test the relay? Any other ideas?
Thanks
The power on the C4 terminal is a bleed through current. It flows from the fuse to the low beam headlight bulbs and back to the switch. Check your fuse and bulbs first.
i have no power at the c4 terminal
Apparently you did not read and follow this part of the post and test properly or you found the post after you had already bought a switch.
“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.”
If there is no ground present the relay will need to be tested.
I had the exact same symptom (headlights – low beams – were not turning on and the flashers – high beams – worked.) I purchased a new multi switch and tried to replace it as described in the steps above and it did not work for me. As it stands now – all fuses/relays seem to be working fine. the new multi-switch and the old multi-switch seem to have the same problem – so i dont think its the switch in my case.
To diagnose this a bit further. At the headlamp – both the terminals are high! i dont know if this is normal. its the same for the flashers (both terminals are normally high) and when you turn the flasher on – one of the terminals/wires goes to low/ground and that turns on the flashers. I just cant get either of the low beam terminals go to low/ground