Most classic car owners know if their car has factory gauges or original warning (aka “idiot”) lights. What they may not know is why this is an important factor in selecting the correct wiring harness and sending unit for classic car gauges or lights.
Automotive wire isn’t picky; it will send information to the back of your gauges no matter what. The usefulness of that information all comes down to the way sending unit makes its signal.
There are two basic types of sending unit, and both rely on grounds:
Warning Light
Warning light senders create an “open” in the circuit, so they are essentially an on/off switch. Contacts inside the switch are open (no current can flow) during normal operation, but close at a specific value- be it coolant temperature or oil pressure. When the critical temperature or pressure range hits, the circuit is completed, and the dash light turns on.
Gauge
This one is resistance to ground. Most classic car gauges sending units control the dash signal by decreasing the amount of resistance in the circuit as the sending unit heats or pressure changes, this translates to a higher reading on the face of the gauge. It’s not an on off switch, but more of a sliding scale dictated by the resistor inside the switch.
So if the wire itself doesn’t care, then why does this make a difference in the harness part number?
Two very important reasons
Classic car gauges and warning lights sending units may have different connection methods. A part designed for gauges may have a ring terminal, while the warning light harness of the same year uses a push-on blade. Both works, but they are certainly not interchangeable, and a factory harness will only have provisions for the correct, matching sender.
As you may suspect, car manufacturers usually must appeal to the largest customer base possible if they want to turn a profit. To do so, they include just a few stock gauges on the cars, trucks, and SUVs that leave assembly lines. While most customers want to see their speed, engine temperature, and mileage record, only a few find other information useful. Whether you are looking to add a set of gauges to a vintage vehicle, or add several key gauges to a newer vehicle, this article will give you pointers on both.
Stock Or OEM Instrument Clusters
Regardless of the type of vehicle you drive, your stock instrument cluster likely tells you about some important information on a modern vehicle. That is, you probably have a speedometer and odometer. Your vehicle will also come with a fuel gauge, and usually a temperature and oil pressure gauge. This has not always been true, however. In the past, many vehicles only featured warning lights that illuminated when something went wrong. This was very typical of cars in the 60’s to the late 80’s, many of which people are restoring or modifying today.
While OEM instrument clusters have come a long way in recent years, they are far from perfect. In fact, most gauges only tell you a bit of information about your vehicle and when vehicles are modified for performance, often, more information is needed for the driver.
Upgrading Classic Car Gauges
As stated before, many vintage cars, especially those from 1960-1990, used lights to indicate low oil pressure, or high temperature, which left a lot to be desired. Because of this, many car enthusiasts added a “triple set” of classic car gauges where they could, which gave the car 3 gauges that weren’t found on them at the time – Oil Pressure, Coolant Temperature, and a Voltage or Ammeter gauge. These same 3 gauges are still sold together today, and along with a Tachometer is usually all that’s needed to upgrade a vehicle of this era and give the driver a full scope on what’s going on under the hood.
When choosing aftermarket gauges for these vehicles, usually the most important question is whether you want these gauges to be mechanical, or electrical.
Mechanical And Electrical Gauges
Mechanical gauges have been around for nearly a century. These gauges have a direct connection to what’s being measured. That is, a mechanical gauge may use an oil line that allows the gauge to directly measure oil pressure, or a temperature probe that is screwed directly in a coolant passage.
On the other hand, Electric gauges function differently. Rather than featuring a direct connection, electric gauges operate using a sensor, or sender, which is screwed or plumbed directly into what’s being measured. While this is essentially the same as the mechanical device, the difference is that this sensor then picks up and sends the information via an electrical wire to the gauge, where the gauge then displays the information.
The difference in your decision depends on a few factors. Keep in mind that on either gauge, the lighting to illuminate the gauge at night will always require an electrical signal and ground, so regardless of which design you use, you will still be doing electrical work.
A Mechanical gauge will have less wires though, but you will need to run the materials through the firewall and dash that allow the gauge to operate – usually tubing or piping. The benefit though is that mechanical gauges tend to be slightly cheaper, as they don’t require the purchase of the sensor or sending unit to pick up the signal and make the gauge operate. With either gauge, you will need to know several specifics on the vehicle you are working on:
- Do you have an accessible port to thread in and attach the sender or mechanical tap for the gauge? What size is it? Many gauges come with, or you can purchase an installation kit that has adapters. Make sure you research all that you will need carefully.
- Is the sender for an electrical gauge available for the size port you need on your vehicle’s engine? Or is an adapter available to size correctly?
In the past, automotive enthusiasts believed mechanical gauges to offer better accuracy than their electrical counterparts. That is no longer the case. Provided you purchase an electrical gauge from a reliable manufacturer, you can likely assume it is as accurate as a mechanical alternative.
Installation
After you have decided on the mechanical or electrical gauge, or what varieties of gauges, styles, and lighting options you wish to add, you are ready to move on to installation. The first task you must accomplish is deciding where to locate your new aftermarket gauge. Picking a spot that allows you to easily read the gauge without obstructing your road view makes sense. One common item is the gauge “pillar-pods”, which mount a gauge or series of gauges on the driver’s side pillar in between the windshield and the driver’s door. Many other aftermarket companies and gauge companies make pod to hold the gauges in a variety of locations.
Keep in mind being careful when drilling into the dashboard itself, and remember that wherever you install your gauges, you will need to run a multitude of wiring to that location. All gauges, especially electrical ones, are going to need 2 sources of voltage from somewhere – one, they will need a 12V source from the headlight switch that is 12V when either the parking or headlights are turned on. This source will power the lighting on your gauges, to operate when the headlights or parking lights are turned on. Two, you will need a 12V ignition source, meaning, a wire than has 12V when the ignition key is in the RUN position. Finally, the lighting circuit, and often the gauge itself, will need a ground wire as well.
Final Thoughts on Aftermarket Gauges
Aftermarket classic car gauges are often an effective strategy for supplementing your vehicle’s stock instrument cluster. Whether you want to get as much performance as possible out of your machine or simply want to minimize your chances of a catastrophic mechanical failure, installing an aftermarket gauge may be the right approach.
As with other components, aftermarket classic car gauges, vary in reliability and quality. If you have decided to install one on your vehicle, be sure you choose a well-known and highly rated manufacturer. Also, when installing your new gauge, take care not to damage existing systems. With the right tools, a quality gauge, and a bit of effort, though, you can likely get the information you need to get the most out of your vehicle.
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OMG! This article really opened my mind on just how crucial gauges are in indicating the amount of pressure change takes place within our vehicle. Last night, my daughter noticed something weird on her car dashboard where its main gauge didn’t move at all. If you ask me, I’d simply refer to a technician right away to resolve the issue.