Testing for a current drain on an older car is very simple and straight-forward. You'll need a digital volt-ohm-ammeter and a pair of small jumper wires to connect the test probes without having to hold onto them. Remove the negative battery cable, then connect the meter's negative probe to the battery post, and the positive probe to the battery cable. Start with the meter set to the highest current scale. Once it has been determined a lower scale can be used, switch to it without fear of blowing an internal fuse from exceeding that range.
There should be no current flow with the ignition switch off and doors closed. If the radio has a digital display, it will have a memory circuit for the station presets and clock. That can draw around 10 milliamps. Some cars from the '60s and '70s had mechanical clocks but even those draw very little current. If you have one that makes a loud click about once every two minutes, that is the self-winder operating. Other than during that operation, they don't draw any current.
Be careful to not open any doors or turn on any lights. If you find current exceeds what is expected, remove fuses, one at a time, to see which circuit is responsible for that draw. Most cars will have the fuse box inside so you'll need to disable the dome light(s) to allow the doors to be opened. Many digital meters have a 2 amp internal fuse that may blow if multiple interior lights turn on.
As a general rule, a small bulb such as is used in glove boxes will draw about half an amp. A trunk or under-hood bulb draws closer to 3/4 amp. For higher current, look for such things as a sticking power window switch or power seat switch. It's somewhat common for those circuits to be protected with automatic-resetting circuit breakers. If you see a high current draw that occasionally drops to 0 amps, suspect a problem in that type of circuit. An additional clue is the circuit breaker will be hot. They trip from heat buildup and reset when they cool down.
If pulling fuses and circuit breakers doesn't stop the current draw, you'll need to look for things unrelated to the fuse box. Although not real common, unplug the generator / alternator. Those with built-in voltage regulators typically have a circuit connected directly to the battery for monitoring system voltage. The circuit may be protected from a dead short by a fuse link wire in the harness. Those can't be easily removed from the circuit like a regular fuse.
For more obscure problems including those where the amount of the current draw seems to vary quite often, look for corrosion between electrical terminals in the bulkhead connector or in any other connector that has a wire with full battery voltage when the ignition switch is off. Increased humidity or wetness from rain will cause more current to flow through that corrosion.
Some models with carbureted engines turned the air conditioning compressor on for two seconds when the engine was stopped. That was to put an extra load on it to reduce dieseling. That system operates when a disconnected battery is reconnected and can momentarily draw enough current to blow a digital meter's internal 2 amp fuse. You'll need to start on a higher scale to prevent blowing the fuse, then switch to a lower scale for better accuracy. Many meters require removing one of the test probe leads and moving it to a different jack. That will again initiate the operation of the compressor clutch and blow the meter's fuse. To work around this problem, treat it as a computer that has to time out, and follow the procedure under "Computers That Must Time Out".
Computers That Must Time Out
By the mid 1990s many manufacturers had at least one computer that would draw as much as three amps for up to 20 minutes after turning off the ignition switch before it went to "sleep mode". These systems present new challenges when measuring current drain because anything that creates an open circuit and removes battery voltage will wake those computers up again for another 20 minutes. You must insert the ammeter as shown in Figure 1 on a high enough range to prevent blowing an internal fuse in the meter. After the computers have timed out and current drain is ready to be measured, you must switch the meter to a lower range for the needed accuracy. Switching to a lower range is the problem. Most digital meters require one lead to be plugged into a special jack for the 10 Amp scale, then it must be moved back to the common jack for all the other ranges. Removing the lead breaks the circuit. When the meter lead is plugged in again, the computers will again draw high current until they time out.
Even if you can switch from the 10 Amp range without moving the lead, there's still a problem. Figure 3 shows a representation of the range switch in the meter. Note that the red switch contact is making the connection to the gold contact for the 2 Amp scale.
Once the computers have timed out and current draw has dropped, you must switch to a lower range for more accuracy. In Figure 4, the switch is being turned from the 2 Amp range to the 200 milliamp range. As it turns, the connection is lost first from the 2 Amp contact, then it is made to the 200 MA contact. This is referred to as a break-before-make" switch meaning it breaks one connection before contacting the other one. That momentary break in the circuit is enough to wake up the computers requiring another 20 minute wait. If the current draw exceeds the meter's internal fuse rating, the fuse will blow leaving you with an open circuit.
There's an easy way to avoid blowing fuses and waking up computers that have to time out. Start by removing the negative battery cable as before, but then connect it back to the battery post with a small jumper wire. A blue jumper wire is shown in Figure 5. This will wake the computers and they'll start their time-out cycle, but while that's occurring, you can connect the ammeter as before, in series with the battery cable and post. Since current takes the path of least resistance, it will flow through the jumper wire and bypass the meter.
Start with the meter on the 10 or 2 Amp range to be sure the rating of the internal fuse won't be exceeded. After the time-out cycle should be completed, remove one end of the jumper wire. The meter will maintain the connection so the computers won't wake up and draw high current. Once you verify the current drain has gone down, you'll need to switch to a lower range. First, reconnect the jumper wire to bypass the meter, then switch to a lower range. At that point remove the jumper wire again so all current must flow through the meter. Anytime you want to switch ranges, reconnect the jumper wire first. That way there will never be a break in the circuit.
In Figure 6 the jumper wire has been disconnected. Now the computers are still in sleep mode and all current drain is going through the meter to be measured. The meter probes are typically also connected with a pair of jumper wires so you don't have to hold them on the battery cable and post.
Basics of Batteries