I just purchased a 1998 Chevy Blazer the first time I drove it in the dark I noticed.
With the headlights switch onthe LF & RH LOW beams work. When i switch on the HI beams the I noticed there was no light coming from the LF headlight only the right.so I got a new bulb and replaced it. still no flight from the left headlamp.upon further inspection I noticed with the high beams switched on The RH HI & LOW are are both on and the LF HI & LOW are both off.If I turn the headlight switch off I assume I get daytime running lights I have no dash lights or parking lights but the LF HEADLAMP HI & LOW are both on and the RH HEADLAMP HI & LOW are noth off.

 

Do you have and know how to use a digital test meter and are you willing to do some diagnostics?


George

 

Yes I have a meter and a test light. If someone tells me what to test I can do that.

 

Let’s start simple. If you put the headlights in manual on mode do you get both low beams and multifunction switch to high beams?


George

 

Yes low beams work and when I switch the high beams on the left is not working only the right.

I know the left hi beam bulb is good because my DRL use the hi beams and they work.

 

All of the high and low beam bulbs get their power from the "headlamp relay" except for the left high beam. That bulb gets its power from the DRL relay after the headlamp relay which is switched on and off by the BCM. So the failure can be the relay, BCM or the wiring. I would start with the relay and if thats not it then we test for the signal to that relay from the BCM.

George

 

I swapped the relay with another and nothing's changedStill no Left Hi beam

 

OK, buckle up as we dive into the relay rabbit hole. The left HB is treated differently than the other 3 headlamps. Power is applied directly to the headlamps by the headlamp relay for the other three but before power gets to the LHB it also has to go subsequently through the DRL relay. The DRL relay is a 5 pin relay. There are 2 types, type A and type B, I don't know which you have yet but we will figure it out. Either way the pins are:

85 - coil
86 - coil
30 - contact common
87 - NO contact
87a - NC contact

Most full size automotive relays follow the Bosch Type A or Type B layout. The pinouts are:



So first we have to figure out what's happening at the socket plugs with the relay removed. This is what needs to be verified:

1. There is continuity and low ohm value between DRL relay socket pin 30 and the positive left headlamp connector (dk green/wht) with headlamp connector unplugged. This is also how you will determine if this is a type A or type B pinout
2. During manual call for headlamps there is 12v power on 87a or 87. Check them both and tell me which one it is.
3. During manual call for power there is 12V on 86

Report back.George

 

Yes, there's continuity between pin 30 and the green headlight wire 000.3 ohms resistance 12Volts at 85 and 87a No power on 86

 

Just to be clear, there are three relays that control the headlights:

Power relay
Ground relay
DRL relay

We need to be working on the DRL relay. Is that the one that you replaced with a known good relay and is that the one that we are testing now?

George