Ghost Glow in the Right-Side Marker Lamps
2019 transit bus, 24 V chassis lighting, LED lamps throughout, body controller with low-side drivers. Complaint: right-side marker and clearance lamps glow dimly, about 5–6 V, whenever the left turn signal is active; the right front turn signal hyper-flashes; no complaint on the left side. With the left turn signal on: left front turn 23.9 V pulsing, right marker 5.4 V, right front turn 2.1 V, right rear lamp cluster ground stud to clean chassis 2.4 Ω, left cluster ground stud 0.03 Ω. Left turn lamp current 3.1 A. Disconnecting the right rear cluster's ground wire stops the ghost glow and slightly dims the left turn lamps. The right rear quarter panel was replaced three weeks ago, the harness re-clipped to the new panel, and twelve ring terminals are stacked on one frame-rail ground stud.
1. Explain the mechanism behind the ghost glow and why LED lamps show it while incandescent lamps usually do not.
Read the worked answer
The lamps are not receiving a stray feed; they are being fed by the left turn circuit through a shared and resistive return path.
- 3.1 A of left-turn current through the 2.4 Ω right-cluster ground raises that point roughly 3.1 × 2.4 ≈ 7 V above chassis. The 5.4 V measured at the marker is consistent once parallel paths and meter loading are allowed for.
- Everything bonded to that ground bus now sits several volts above chassis potential. Any LED whose positive side connects to an "off" driver output - pulled low but not a perfect open - sees enough differential to pass a few milliamps and glow.
- The 0.03 Ω left stud shows the left side returns properly, which is why the complaint is one-sided even though the bus bar is shared.
- An incandescent filament needs far more current to glow visibly, so the same leakage heats nothing. An LED lights at a milliamp, so a fault that was invisible on older buses becomes a visible customer complaint.
2. The parts clerk suggests a new right rear lamp assembly. Is that the right repair? Describe the repair and how you prove it.
Read the worked answer
No. The 2.4 Ω is a return-path fault and the assembly is a victim; replacing it leaves the bad stud in place and the new lamp will ghost as well.
Repair: remove the stacked terminals, clean the frame rail to bare metal, replace corroded ring terminals and star washers, apply anti-corrosion compound, torque to spec, and reduce the stack - twelve terminals on one stud is a design weakness, so split it onto a grounding bus bar if the OEM kit exists. The quarter panel replacement is the prime suspect: a stud installed over seam sealer or paint reads exactly like this.
Prove it under load, not with an ohmmeter on a clean joint. With turn signal and markers on, measure voltage drop from the lamp ground terminal to the battery negative post; it must be well under 0.1 V. Then confirm no ghost glow, restored left-turn brightness, and no hyper-flash. A resistance check alone can pass a joint that fails at 3 A.
3. What explains the right front turn signal hyper-flash, and what would you check if it continues after the ground repair?
Read the worked answer
On a body-controller bus, hyper-flash is usually an outage decision, not a flasher can. The controller measures current on the right turn output; the ground offset and the parallel leakage path lower the current it sees, so it reports a lamp out and flashes fast. Expect it to vanish after the return path is repaired - verify by comparing commanded versus measured current on that output.
If it persists:
- Check the right front lamp connector for corrosion and pin tension.
- Compare measured output current right versus left; a real lamp or connector fault will still show as low current.
- Confirm the controller's lamp-type and lamp-count configuration was not left at an incandescent setting after recent programming.
- Look for a second, independent ground fault at the front lamp.
- Inspect the harness where it was re-clipped to the new quarter panel for a chafe that intermittently shorts the turn feed, which would also explain an intermittent rather than constant symptom.