GMC P35 Back Up Light Switch Replacement Guide (1975-1978)
Fix reverse lights that don't work, flicker, or stay on with this guide to your truck's back up light switch.
- You must identify if your P35 has a manual or automatic transmission before buying, as the parts are completely different.
- For automatic models, the switch is part of the Neutral Safety Switch on the steering column; for manual models, it's on the transmission case.
- Symptoms are simple: lights don't work, flicker, or stay on. A no-start condition can also occur on automatic models.
- Always check the bulbs and fuses first before condemning the switch.
Is Your Back Up Light Switch Failing?
A bad back up light switch is a common problem on older trucks like the GMC P35. Over decades of use, the internal contacts wear out or the switch gets damaged. Before you assume the switch is the problem, always check the simple things first: make sure the reverse light bulbs are good and check the fuse labeled "TURN B/U" or similar in your fuse panel. If those are fine, the switch is the most likely cause.
Automatic vs. Manual Transmission: A Critical Difference
You must know which transmission your P35 has before ordering a part. The switch for an automatic transmission is completely different from the one for a manual transmission, and they are not interchangeable.
- Automatic Transmission: The part you need is a combination Neutral Safety Switch / Back Up Light Switch. It's a crescent-shaped component found inside the cab, mounted to the steering column under the dashboard.
- Manual Transmission: You need a smaller, screw-in style switch. This part threads directly into the side of the transmission case, located underneath the truck.
Common Symptoms of a Bad Switch
When the back up light switch fails, the symptoms are usually very clear. Here’s what to look for:
- No Reverse Lights: This is the most common sign. You shift into reverse, but the lights don't come on at all.
- Flickering or Intermittent Lights: The lights flash on and off, or they only work if you jiggle the gear shifter. This often points to worn internal contacts or a misaligned switch on automatic models.
- Reverse Lights Are Stuck On: Less common, but the switch can fail in the "on" position, keeping the reverse lights lit up even when you're driving forward or parked.
- (Automatic Only) No Crank / No Start: Since the switch is part of the Neutral Safety Switch on automatic models, a complete failure can prevent the engine from starting.
Choosing the Right Aftermarket Back Up Light Switch
Original OEM switches for a vehicle of this age are typically no longer available from dealers. The good news is that the aftermarket provides reliable replacements. Your choice will be between brands like Standard Ignition and Rostra.
- Standard Ignition (Standard Motor Products): Often considered a top-tier aftermarket brand, SMP parts are known for their quality and reliability. They are a very common and trusted choice for replacement electrical components on these trucks.
- Rostra Powertrain: Rostra is another well-known manufacturer of automotive electronics and transmission parts. Their switches are a solid and dependable alternative.
Pro Tip: Given the low cost of this part and its importance for safety, it is wise to choose a quality brand. The small price difference is worth the peace of mind knowing the part will fit and function correctly.
| Part or Service | Estimated Cost |
|---|---|
| New Aftermarket Switch (Rostra, Standard) | $16.17 - $53.37 |
| New OEM (If Found) | $75 - $150+ |
| Shop Labor | $100 - $200 |
Frequently Asked Questions (FAQ)
Where is the back up light switch located on my 1975-1978 P35?
If you have an automatic transmission, it's part of the Neutral Safety Switch on the steering column inside the cab, near the firewall. If you have a manual transmission, it's a screw-in switch on the side of the transmission itself.
Is this a difficult part to replace myself?
For most DIY mechanics, this is a very manageable job. For automatic models, access under the dash can be tight but requires only basic hand tools and usually takes less than an hour. For manual models, you will need to safely lift and support the vehicle to access the switch on the transmission.
Does the new switch need to be adjusted?
For manual transmissions, no adjustment is needed. For automatic transmissions, the new switch on the steering column is adjustable. You may need to slightly rotate it until the reverse lights only come on when the shifter is in the reverse position. Having a helper watch the lights while you make small adjustments is very useful.
Are there any recalls for the back up light switch?
No. Searches of the National Highway Traffic Safety Administration (NHTSA) database show no recalls for the back up light switch on the 1975-1978 GMC P35. A recall did exist for the 1978 model concerning the *parking brake* indicator light switch wiring, but this is an unrelated part.
My lights still don't work after replacing the switch. What now?
If a new switch, fuse, and bulbs don't fix the problem, you likely have a wiring issue. Check for power at the switch's electrical connector. Inspect the wiring harness for any obvious breaks, corrosion, or bad ground connections at the tail lights.
Technical Specifications
Location and type are dependent on transmission. Automatic transmission models use a combination Neutral Safety Switch mounted on the steering column. Manual transmission models use a screw-in switch mounted on the transmission case.
We Have This Part in Stock
The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.