Headlight problem (Fixed)
Low beams don't work, but high beam indicator is on.
My headlights stopped working recently on my 1990 pickup and having since fixed the problem, I thought I'd leave a post on here to maybe help someone else. This problem stumped me for a while, and I was worried I'd have to go digging around my truck for a loose ground wire! Ugh, I hate electrical problems like that :-( I had a search on here, but I actually got the tip on where to look from these two pages:
Low beam headlights stopped working, but strangely the high beam indicator is lit on the dash.
High beam headlights also don't work, but if you pull the stalk forward (to flash) they will come on.
What this means:
Power is going to the headlights, through the low beam filament, back to the combo stalk, and instead of going through the combo switch to ground, it's being grounded via the high beam indicator.
Before you pull apart your steering column, it's much quicker to first check the usual things:
- The two headlight fuses
- Check the headlight relay clicks in when you turn on the low beams
- Un-plug the headlights and run some “jumper” wires just to check that both your low beam globes/filaments haven't blown (apparently can happen with alternator problems).
- Turn on the “parker” lights and check you get a +12v on one terminal of the headlight plug
- Turn on the low beams, then high beams, and check you're getting a 0v ground on the other 2 pins of the headlight plug.
Note, it's a bit mis-leading that the low beam wire to the headlight plug will always show 0v ground. This is because it's permanently connected to ground via the high beam indicator light.
Finding the real problem:
Remove the trim from around your steering column and under the dash. You need access to the wires from the combo switch and the wide plug (actually it's 2 plugs) where it connects into the main wiring loom.
Disconnect these two plugs (they're screwed onto the underside of the steering column) and have a close look for any corrosion, melting or burn marks.
(See the brown heat marks on this photo & below)
To test if there's a problem here, re-connect these plugs and turn on your low beam headlights. (They'll be off, but the high beam indicator will be on)
Get a piece of wire, strip the ends, look for the low beam headlight wire (red w/ green trace) and push in your wire to the back of the plug to it, and the other end goes to ground.
See wiring diagram here, courtesy ncttora.com
(This headlight wiring diagram has more detail than the usual FSM)
This will ground one side of your low beams and (hopefully) the low beam headlights will turn on, and the high beam indicator will be off (how it's meant to work)
If this happens, we're getting close to finding the problem...
Doing that ground wire test bypassed the combo switch. Now, look for the big ground wire (white w/ black trace) on the same plug. Push your spare wire into the back of the plug to touch it, and again the other end goes to ground somewhere.
Did your low beams come on now?
If no, this means there may be a problem with your combo stalk. We grounded the headlight side of the switch first, and then the other side which doesn't work, so it's probably the switch that's broken.
If yes, this means the combo stalk is also fine, but you've got a grounding problem. In my case, where the big white ground wire went through the plug was a bad connection (but it wasn't easily visible). This wire has to take the current load of two headlights, and the contacts in the plug are the weak point. Over time, some residue built up, the contacts were 'arcing' and got hot, it melted a bit, and finally stopped making a good connection. This meant the only wire path for the headlight to ground was via the high beam indicator, and that won't allow enough current throughto turn on the headlights.
On my truck, this earth wire is actually a different guage on each side of the connector, so when enough current is flowing, I think this would make the connector a (weak) point of high resistance.
In my case, the earth wire inside the plug wasn't making a good connection. I tried to pull it out (you have to poke in a tiny tool to force the holding tab back) but it was too melted in place.
So I just jumpered a new, thick wired across the plug in its place. If you want to still be able to unplug the combo switch, just solder on a single-pin plug (just make sure it's a heavy duty one!)
Strip the big white-black ground wire on one side of the plug (I stripped the insulation without cutting through)
...and the other side of the plug. It's interesting to note that the white-black ground wire is smaller gauge on this side.
The finished jumper wire (thick red wire - a bad choice of color yes
I think wiring problems like this are always the worst, because something was just designed poorly and you have to trace an intermittent problem.
Another good idea is to install an aftermarket headlight loom, so your combo switch is just turning on a relay, rather than handling all the power to the headlights. Something I haven't gotten around to yet....
Hope this helps someone else...