Testing Shortcuts For Charging Systems
Some of the best ways to troubleshoot a charging system include an intensive look at apparently non-related equipment. For example, rear-window defrosters are habit-forming alternator killers in America’s cold weather country. These items can pull down a battery when the vehicle cranking motor needs all the help it can get on a zero-degree day.
Another climate-related habitual offender can be the vehicle air conditioner (a/c), which is placed on line just when the vehicle alternator is taxed by a thermal fan on many front-drive vehicles. The auxiliary cooling fan comes on with the a/c control and so does the compressor clutch, making life difficult for the biggest and baddest alternators.
Testing charging systems on many of today’s vehicles comes complete with headaches that are reminiscent of the early days of electronic engine control. Information about the system is important, and when "A" circuits suddenly become "B" circuits, it would be a good idea to have your own score card to protect the hardware and test equipment from frying.
With service manual in hand, go after a "light on" (no charge) condition with a simple digital volt/ohm meter (DVOM) test series, which begins with a probe of the alternator to battery output terminal on SI-style, or CS-style alternators, to very quickly eliminate the possibility of a wiring harness problem. A few of the new charging units use load-response control, so unless you’re going to go whole-hog, head-to-head with a dedicated bench tester, including a pulse frequency generator and a quad-trace 1,000 megahertz lab scope, your output tests should be limited to scoping the basics. If the belt is tight, the wiring harness is OK, and the battery is up to snuff, look next for a gremlin in the computerized engine management system that may be linked up to control field voltage and current.
A scan tool will be able to clue you in to problems with regard to voltage control by the electronic control module (ECM). You’re going to like the new multi-range amp/meter adapter that plugs into the volt-ohm port of your DVOM. By using the induction clamp on this unit, it is possible to watch amperes to the alternator field toggle according to electrical system load demands. Items to watch for include external duty cycle of cooling fans and how voltage regulation responds to instant on-loads. Digitally controlled, pulse- width-modulated alternators such as General Motors’ (GMs’) CS-130 will overcharge (or will seem to) up to 16 volts on a very cold sub-zero day. Monitoring with your scan tool and a good DVOM or lab scope at the battery output post will explain the voltage temperature relationship. As the temperature under the hood comes up, the voltage will drop well below 14 volts. This is a very normal condition. The real problem with this scenario comes when units, such as Ford’s IR unit, burn off the terminal lug during the warm-up phase.
A suitcase-enclosed electrical system analyzer can be a real boon for road service, as well as in the shop. It helps that you can test battery condition, alternator output, diode condition, and electrical shorts and opens. The portability of such a test unit makes it invaluable as a backup to roll-around testers.
After all is said and done, the hard-core testing of most of today’s automotive electrical systems is still accomplished with a tester that has a combination volt meter/induction amp meter with separate leads to perform voltage drop checks. The last few years have shown us that a full-fledged "carbon pile" load bank with 800-plus load draw capability can wring out some of the more stubborn electrical-related problems popping up in high-output systems. A battery test is still a direct readout with this equipment. A better option that is being readily used in shops, garages and even department store battery outlets, has a "load-decay" electrical system tester. With this piece of equipment, the "rate of decay" (controlled measurement of discharge cycle) of a battery is measured and a microprocessor compares the test battery to established general standards while compensating for temperature in most cases. The biggest advantage with this gear is that a battery that is low in charge can be evaluated with a great degree of accuracy.
Looking at the total vehicle system under test can lead to a problem diagnosis that will eliminate a problem or compensate for a vehicle defect. For instance, some ’92-’94 GM "C" and "H" cars can be upgraded from a CS-130 alternator that can operate in an environment with a 105-degree Celsius air inlet temperature under the hood, to a heavier, quieter and more adequately cooled CS-144 alternator that can handle over 120 degrees Celsius.
Testing for such a problem can be as simple as checking into inlet fan temperature at the alternator with an air contact probe and a thermo-couple, hooked up to your trusty DVOM. Low-output complaints at idle can be tested and resolved with a simple temperature reading and voltage comparison chart. By the way, a new voltage regulator connector body will be required to accept the CS-144 alternator.
Dodge Cummins diesel pickups will discharge profusely if the vehicle-heated air intake continues to draw high current after the vehicle has achieved warm-up. This can be due to a faulty control circuit, a sensor problem, or a sticking or faulty thermostat. Test for this problem with a traditional roll-around tester, a scan tool or a direct-reading submersible temperature gauge. Don’t be afraid to tap into the vehicle thermal sensing circuit with a lab scope to graph the behavioral characteristics of a known good controller vs. that of one suspected of malfunction.
On-the-vehicle testing still is the preferred choice of most good service test facilities. However, modern bench testers are in use in just about every shop in the country to help diagnose odd-ball problems with alternators. Remember that you must diagnose GM and other import alternators (newer than 1986 or so) with a 12-volt battery that is capable of recharge on the test stand.
Never rule out operational or functional errors that are non-electrical. Some Ford trucks can ruin serpentine drive belts and begin slipping after stripping a string-like belt section from the entire belt length. Measure for a mismatch at the a/c compressor or at the crankshaft pulley. Fix the problem by referring to several of FoMoCo’s technical service bulletins (TSBs) applicable to the specific vehicle under test.
Chrysler’s 1985 2.2L engine is computer-controlled and uses a long interface wire to control field current. This interface must be transistor-controlled for voltage and current flow for the system to correctly charge the vehicle battery. Using your DVOM and adapter for induction measurement of amperage, three things can be determined. One is whether the field control is toggled by the computer; second, if the logic and power module are in command; and third, if load demand is faking out the computer and creating a dead battery or an impossible "catch up" scenario for the alternator.
Testing for alternator and battery failure is one of those day-to-day jobs that good technicians never think too much about, but are always on the lookout for new slants to an old problem. Basic test equipment, along with good and clear service information, can be the difference between profit and nightmare with most of the current systems on the market. Taking the time to learn about new tools and techniques is most of the battle.
The preceding technical article is brought to you by AutoInc. and the Automotive Service Association (ASA).
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AutoInc. Magazine ®, Vol. XLIV No. 6, June 1996