1988RedT2
1988RedT2 MegaDork
1/6/22 4:03 p.m.

So, another issue with my Dodge Ram 1500:  The RH headlight works on high beam only.  No low beam.  The LH light works fine, both high and low.  I changed the bulb on the RH.  No difference. 

The connector is a 3-wire plug with a black center wire. I checked for voltage on each of the two outer wires on the bad side.  Voltage on one side, but not the other.  Okay, Dodge trucks have issues with the TIPM, so I'll just run a wire from the good low beam to a relay, and use that to put 12-volts on the low beam on each side.

Not so fast!  In checking the good side before I cut a wire to run to the relay holding coil, I find to my surprise that BOTH wires have voltage with the light switch on either high or low beam.  What the heck? 

Can someone explain to me how a light switch works?  I thought it was simple.  12-volts to low beam wire, current flows to ground and low beam light comes on.  12-volts to high beam wire and high beam light comes on.  But that's not how it works.

Help me understand! 

HotNotch
HotNotch New Reader
1/6/22 4:05 p.m.

Sounds like a switched ground setup.

APEowner
APEowner SuperDork
1/6/22 4:06 p.m.

You're using a voltmeter right?  Try it with an old fashioned 12v incandescent test light.  Voltmeters draw almost no current and bad connections look the same as good ones.

1988RedT2
1988RedT2 MegaDork
1/6/22 4:19 p.m.
APEowner said:

You're using a voltmeter right?  Try it with an old fashioned 12v incandescent test light.  Voltmeters draw almost no current and bad connections look the same as good ones.

Good call.  Yes, using a digital VOM.  I will try a light.

 

daeman
daeman Dork
1/6/22 4:28 p.m.

Sorry if im stating the obvious here, but pulling a wiring diagram would be a big help.

 

1988RedT2
1988RedT2 MegaDork
1/6/22 5:19 p.m.
daeman said:

Sorry if im stating the obvious here, but pulling a wiring diagram would be a big help.

 

Yeah, but the diagrams in the Haynes manual cover multiple years and are very vague, and everything comes out of the mysterious black box, aka totally integrated power module (TIPM).  There isn't even a fuse associated with the headlight circuit.

daeman
daeman SuperDork
1/6/22 5:33 p.m.

In reply to 1988RedT2 :

Fair enough, thats a bit of a bastard.  What year model is yours? Maybe someone  here can help you find a useful  diagram.

1988RedT2
1988RedT2 MegaDork
1/6/22 6:18 p.m.

In reply to daeman :

Found one here:

https://liveworkdream.com/2013/02/07/dodge-ram-headlight-cheap-fix/

I'm going to take another look at it tomorrow with a light instead of a meter.  I think driving a relay with the good side should work.

 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
1/6/22 7:40 p.m.

It might be getting power all the time and switching ground.

Or it might be switching power but a bad ground is causing you to see 12V on the ground side because the bulb is not the resistance in the circuit.

When I get home I will check the diagrams to see what kind of system it is. I won't (legally) be able to screenshot a diagram but I can describe the whats and wherefores.

einy (Forum Supporter)
einy (Forum Supporter) Dork
1/6/22 7:57 p.m.

A Chrysler product with a bad ground?  Nah ..... 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
1/6/22 8:41 p.m.

....for some reason I thought you'd said what year it was.

Assuming that it is a 2013, the black wire is ground, the white/dark blue wire is low beam, the white/light green is high beam.  Power is the switched side, comes out of the BCM, left side of dashboard.  I see no separate lighting fuses in the diagrams, but this does not mean they don't exist.  Given that the BCM is doing the heavy lifting then they'd be inside the car.

I'd check for power coming out of the BCM,  It's pin 3 in the C1 connector.  If there is power there, but not at the headlight, I'd look at the bulkhead connector I100 (ST/SLT) or I120 (Longhorn/Laramie) and the wiring close to it.  Wiring gremlins seem to love to hang out near connectors.

 

I've been known to bypass a bad BCM by finding power for a relay, and switching it off the other side's headlight feed.

1988RedT2
1988RedT2 MegaDork
1/7/22 10:03 a.m.

In reply to Pete. (l33t FS) :

Oooof!  My bad.  It's a 2006 with 125k miles on it.  6-speed manual.

1988RedT2
1988RedT2 MegaDork
1/7/22 4:32 p.m.

Who could imagine that at my current ripe old age, that I would today purchase my very first test light.  Made in America and sold under the Lisle name.

https://www.lislecorp.com/about-us/

Using the test light, all is as one would expect.  Switch on low beam, signal on one wire.  Switch on high beam, signal on other wire.  I'll be wiring the relay tomorrow, if all goes according to plan.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
1/7/22 5:59 p.m.

In reply to 1988RedT2 :

The voltmeter probably showed voltage because Dodge might be biasing a small amount of current across the headlight to determine if it has a bad bulb or not.  A voltmeter will pick that up but a test light will not.

This also means that they might not send voltage if it sees the wrong resistance...

 

Looked up an '06, wire colors are the same and it does go through the underhood TIPM.  Low beam is pin 4 of connector C5, no bulkhead connectors en route to the headlight.  C5 is the smallest of the row of six, has room for  only ten terminals.  According to what Dodge told Alldata, anyway.  

Right low beam, if you need to go there, is pin 3 in C7, white/tan wire.  C7 is the 14 pin connector next to C5.

 

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