Reverse lights go when I turn on the parking lights/headlights. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

E.D.75GMC · 01-23-2014, 12:05 AM

An electrical gremlins just hit my truck... My reverse lights are coming on when I pull the knob to turn the lights on. I know little to nothing about electronics so I don't know where to start with sorting this out.

walkerb · 01-23-2014, 07:57 AM

do they go off when you hit the brakes?

chengny · 01-23-2014, 08:39 AM

If you recently changed any bulbs in the rear clusters - check that they are the correct style for the socket (i.e. make sure that there isn't a double contact in a single contact socket & vice-versa). This can cause cross feeding among the lighting circuits.

Unfortunately, if that isn't the problem, you are probably looking at a short between the lead to the parking lights and the lead for the back-up lights. Crawl under and inspect the wiring harness from the firewall back to the rear clusters. Any damage to the harness sheathing, that is bad enough to cause a short, should be easy to spot. Pay especially close attention to areas where it passes close to the exhaust system.

The cab wiring for the back-up lights is seperate and distant from the wiring for the other lights. It does not come near those other circuits until they all join together at the firewall (on the cab side). After that, the leads for the back-up, parking & brake lights are all bundled together in that plastic sheathing.

**hatzie** · 01-23-2014, 02:02 PM

Backup lights are very separate from the other circuits. If it was turn and parking lights at the same time I'd say look at the bulbs but...
Likely it's in the back and that's really quite simple. There are only 4 circuits with a common ground behind the LH tailight lens. Has U-Haul or some tool rental joint hacked up the taillight wiring for a trailer plug?

E.D.75GMC · 01-23-2014, 03:33 PM

Yeah there was a trailer brake installed but I removed it a couple weeks ago when I swapped the painted bumper for a chrome one. The reverse lights didn't start to come on with the other lights until last night though. I've removed the reverse light bulbs for now and I'll check out everything you guys have said when I get home from work tonight.

E.D.75GMC · 01-24-2014, 01:15 AM

I replaced the reverse light bulbs, now it's back to normal... I guess 39-year-old light bulbs don't work as they should sometimes haha

01-23-2014, 12:05 AM	#1
E.D.75GMC Registered User Join Date: Aug 2013 Location: Calgary Alberta Posts: 273	Reverse lights go when I turn on the parking lights/headlights. An electrical gremlins just hit my truck... My reverse lights are coming on when I pull the knob to turn the lights on. I know little to nothing about electronics so I don't know where to start with sorting this out. Attached Images

01-23-2014, 07:57 AM	#2
walkerb Registered User Join Date: May 2010 Location: Rochester NY Posts: 831	Re: Reverse lights go when I turn on the parking lights/headlights. do they go off when you hit the brakes? __________________ 1973 k20 Suburban 350 1974 Custom stepside c-10 250 1975 Chevy stepside 454 1986 K30 Crewcab SRW 350 R.I.P- glimmertwin01- board member and good friend, you will be missed

01-23-2014, 08:39 AM	#3
chengny Registered User Join Date: May 2007 Location: New Hampshire Posts: 349	Re: Reverse lights go when I turn on the parking lights/headlights. If you recently changed any bulbs in the rear clusters - check that they are the correct style for the socket (i.e. make sure that there isn't a double contact in a single contact socket & vice-versa). This can cause cross feeding among the lighting circuits. Unfortunately, if that isn't the problem, you are probably looking at a short between the lead to the parking lights and the lead for the back-up lights. Crawl under and inspect the wiring harness from the firewall back to the rear clusters. Any damage to the harness sheathing, that is bad enough to cause a short, should be easy to spot. Pay especially close attention to areas where it passes close to the exhaust system. The cab wiring for the back-up lights is seperate and distant from the wiring for the other lights. It does not come near those other circuits until they all join together at the firewall (on the cab side). After that, the leads for the back-up, parking & brake lights are all bundled together in that plastic sheathing.

01-23-2014, 02:02 PM	#4
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	Re: Reverse lights go when I turn on the parking lights/headlights. Backup lights are very separate from the other circuits. If it was turn and parking lights at the same time I'd say look at the bulbs but... Likely it's in the back and that's really quite simple. There are only 4 circuits with a common ground behind the LH tailight lens. Has U-Haul or some tool rental joint hacked up the taillight wiring for a trailer plug? __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful.

01-23-2014, 03:33 PM	#5
E.D.75GMC Registered User Join Date: Aug 2013 Location: Calgary Alberta Posts: 273	Re: Reverse lights go when I turn on the parking lights/headlights. Yeah there was a trailer brake installed but I removed it a couple weeks ago when I swapped the painted bumper for a chrome one. The reverse lights didn't start to come on with the other lights until last night though. I've removed the reverse light bulbs for now and I'll check out everything you guys have said when I get home from work tonight.

01-24-2014, 01:15 AM	#6
E.D.75GMC Registered User Join Date: Aug 2013 Location: Calgary Alberta Posts: 273	Re: Reverse lights go when I turn on the parking lights/headlights. I replaced the reverse light bulbs, now it's back to normal... I guess 39-year-old light bulbs don't work as they should sometimes haha



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