1958-1959 GMC 100 Engine Oil Pressure Switch and Sender Guide
Don't guess about your classic truck's oil pressure; understand this critical part.
- A bad oil pressure switch can give false readings like zero or maxed-out pressure, or cause the warning light to stay on.
- Always check your oil level first to rule out a real low-pressure emergency.
- For this vintage truck, a new aftermarket part from a brand like Standard Ignition is the most practical choice.
- Replacement is simple, inexpensive, and can be done by most owners in under 30 minutes.
Is your GMC 100's oil pressure gauge acting strange?
A faulty oil pressure switch or sending unit is a common issue on classic trucks like the 1958-1959 GMC 100. Because the engines of this era, like the 270 cubic inch inline-6 or the optional V8, rely completely on steady oil pressure for survival, this small sensor is incredibly important. Ignoring its warnings can lead to catastrophic engine failure. Luckily, it's an inexpensive and easy part to replace.
Symptoms of a Failing Oil Pressure Switch
When this part starts to fail, it can give you a few different headaches. The signs are usually easy to spot if you know what to look for.
- Flickering or Constant Oil Light: The most obvious sign is the red oil warning light on your dash flickering or staying on, especially if you've checked your oil level and it's full.
- Incorrect Gauge Readings: If you have a pressure gauge, a bad sending unit can cause it to read zero, max out completely (sometimes over 80 psi), or swing back and forth wildly.
- Oil Leaks: The sensor itself can fail and leak oil. If you see oil dripping from the switch where it screws into the engine, it needs to be replaced.
Don't Ignore the Warnings
Never assume a warning light or low gauge reading is just a bad sensor. Always check your engine oil level immediately. If the oil level is good, the problem is likely the sensor, but it's critical to verify you aren't dealing with a real loss of oil pressure which can quickly ruin the engine.
Choosing the Right Aftermarket Part
For a vehicle as old as a '58 or '59 GMC 100, finding original factory parts is nearly impossible. New Old Stock (NOS) parts are very rare and can be expensive. The most practical and reliable option is a new aftermarket part.
Standard Motor Products (Standard Ignition) is a well-known brand that has been making aftermarket parts for a very long time. They manufacture their switches in a quality-controlled facility and it is often the only brand readily available for classic trucks like these. While any mass-produced part can have occasional defects, Standard is a trusted choice for these simple vintage components.
Cost of a New Engine Oil Pressure Switch
| Part Type | Typical Cost |
|---|---|
| New Aftermarket (Standard Ignition) | $16 - $20 |
| New Old Stock (OEM) | $50 - $150+ (if found) |
| Shop Labor | $45 - $90 |
Pro Tip: Get the Right Socket
These sending units often require a special, deep socket to remove and install them without breaking the new part. An oil pressure switch socket is inexpensive and makes the job much easier. The size is often 1-1/16 inch for these older GM vehicles.
Frequently Asked Questions (FAQ)
Where is the oil pressure switch located on a 1958-1959 GMC 100?
On GM engines from this era, the oil pressure switch is typically screwed into the side of the engine block. Look for a small, can-shaped sensor with a single wire attached, often located near the oil filter or the distributor.
What is the difference between an oil pressure switch and a sending unit?
A 'switch' is a simple on/off device for a warning light. It closes a circuit to turn the light on when pressure drops below a set point (e.g., 5-7 psi). A 'sending unit' or 'sender' is a variable resistor that sends a changing signal to a gauge, allowing the needle to show the actual pressure reading. Make sure you buy the correct part for your truck's dashboard setup (light or gauge).
Can I replace the oil pressure switch myself?
Yes, this is a very DIY-friendly job for most owners. It usually involves disconnecting the battery, unplugging one wire, and using a wrench or special socket to unscrew the old unit and install the new one. The job typically takes less than 30 minutes.
Do I need to use thread sealant on the new switch?
Yes, it is very important to use a thread sealant appropriate for oil and gas on the threads of the new switch to prevent leaks. Some new switches, like those from Standard, may come with sealant already applied.
Technical Specifications
Varies by engine (I6 or V8) and dash type (light or gauge). Verify the part matches your specific application. The part is typically a single-pin electrical connector with a threaded base for screwing into the engine block.
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.