Preventative Car Maintenance is Easy With a Homemade Radiator Monitor

Know your car's cooling system is in good shape using MOTHER's homemade radiator monitor, including parts layout, circuit board, how it works, building the monitor, installation and bill of materials.
By TJ Byers
September/October 1985
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Diagram: MOTHER's homemade radiator monitor 2.
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You can drive with confidence using MOTHER's homemade radiator monitor. (See the radiator monitor photos and diagrams in the image gallery.)

You don't have to be a mathematical wizard to figure out that it's a lot less expensive to keep and maintain an old car than it is to regularly trade up to a new one. The drive-it-off-the-lot depreciation of $1,000 to $2,000 is just the beginning of the financial headaches awaiting the new-car addict. Nonetheless, millions of people buy a new car every three to five years just for the security of having a reliable vehicle.

Well, the key to feeling secure in (and making peace with) an older car—whether you're an accomplished mechanic or a complete klutz—is preventive observation. No matter who does the wrenching, it's always cheaper to catch a problem before it becomes a disaster . . . and nine times out of ten, simple anticipation will save you from being stranded. For example, a water pump seldom fails precipitously and costs only $30 to $60 to replace, but replacing an engine destroyed by heat—the inevitable result of continuing to drive with a bad water pump—could easily set you back ten times that amount.

Unfortunately, few cars are equipped with the gauges you need to properly monitor the condition of your auto's power plant. And those little red indicators on the dash are, in most cases, of little help in prevention. The threshold for such "idiot lights" is so high that they often don't come on until after damage has been done.

For these reasons, no experienced auto recycler would consider driving an "experienced" car that wasn't equipped with a water-temperature gauge; this simple instrument warns of all sorts of maladies (including some that go beyond cooling system troubles) before it's too late. But even the water temperature gauge gives only an indirect warning of low fluid level in your radiator.

One of the best ways to monitor the condition of your car's cooling system is simply to check the coolant level frequently; antifreeze loss is the primary sign that something is going wrong. And with modern-day electronics, we can take the concept of frequent checks to the extreme: With the simple radiator monitor I'm about to show you how to build, you'll have an electronic watchdog keeping an eye on your coolant level every second that you drive . . . helping you to get more years out of an old car or to protect a new vehicle against a premature death. You'll know instantly, courtesy of a flashing light, whenever the level drops below normal.

How MOTHER's Homemade Radiator Monitor Works

The concept behind the coolant-level monitor is very simple: Water is a better conductor of electricity than air is. Two probes discern the difference in resistivity (the measure of a conductor's efficiency) between the two mediums. As long as the two probes are covered by the water/coolant mixture, the resistance is low and the transistor (Q1) conducts electricity. If the level drops, however, the resistance increases to the point where the transistor stops conducting.

When there's power coming from the transistor, the integrated circuit (ICI) keeps the warning lamp off . . . but when the transistor stops transmitting, the IC switches the lamp on and makes it flash.

Building the Radiator Monitor

The components listed on the bill of materials should be attached to a printed circuit board. (I recommend this technique to avoid wiring errors and to speed up the assembly process.) You can make your own board using a Radio Shack kit (which includes materials for two boards) or order a prefabricated board from Danocinths, the address for which is listed in the bill of materials. [EDITOR'S NOTE: An article on page 136 of MOTHER EARTH NEWS issue 84 explains in detail how to make printed circuit boards.]  

Follow the parts layout (see Figure 1 in the image gallery) in placing the components on the board. Pay special attention to the transistors and integrated circuit: They can be physically oriented in more than one way, but only one way is electrically correct. When you're sure you've got them right, solder everything in place with a low-wattage iron and rosin-core solder.

Installation of the Homemade Radiator Monitor

Four wires need to be soldered to the right side of the circuit board (looking at it from the foil side). The "+" and "-" connections will probably be fairly short, since they'll be connected to your car's 12-volt electrical system somewhere under the dash and near the circuit board.

The two connectors marked "probe," however, need to be wired to metal rods placed in your radiator's coolant overflow (or recovery) tank and will have to be long enough to reach through the fire wall and to that tank. You can usually route these wires through the fire wall next to the steering column or a heater hose.

(Some cars aren't equipped with coolant overflow tanks, and you have to have one to use our monitor. It's a wise idea to install one of these tanks in any event, because it will allow your car's cooling system to purge and recapture coolant as the liquid expands during heating and contracts during cooling. What's more, it's a simple and inexpensive matter to install one.)

I've had good results using hot-dipped galvanized nails as probes. Pick a size that's long enough to reach two-thirds of the way down into the tank when the nails are inserted through holes drilled in the top on each side of the cap (as shown) . . . size 16d often works well. Wrap the probe wires (don't worry about polarity) around the heads of the nails and seal the heads of the probes to the tank with silicone caulk.

The "+" connector on the board must be attached to a 12-volt hot wire that's switched on and off by the key. There are a number of these under the dash . . . the radio power wire is an easy choice in American cars and many foreign ones. If your radio goes off when you turn off the engine, you're in luck. Another possibility is the hot wire from the switch to the ignition system, although you may need a color-code chart to identify this conductor. (Or, if you're feeling really fastidious, you can string a wire all the way to the fuse block.) The "-" wire should be connected to a screw that fastens to the car's chassis to form a ground.

Finally, install the circuit board under the dash, with the indicator lamp sticking out in a prominent location. (It's a good idea to wrap the circuit board in plastic or cloth so that none of the components will short against bare metal under the dash.)

As long as the coolant level is normal, you'll barely notice the monitor's warning lamp. Should the level drop, however, it will flash an insistent warning that says, "Stop now, or pay later!"


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