CASE OF THE PESKY PASSAT
The REGITAR Tech Line received a call from a customer about a VW Passat that he had rebuilt an alternator for using our VRB254 regulator, which is a Bosch regulator with two terminals Lamp & DFM. Not long after the alternator was installed the battery went dead (two days), so the customer was calling the Tech Line to ask if it was possible the VRB254 regulator could kill a battery in two days. Well the short answer is yes, but it’s a lot more complicated than that. At any rate the customer had used a test light between the negative battery cable and the negative battery post and since the test light bulb glowed brightly, determined that there was some type of electrical drain. Well of course he was correct and after we spoke with him and suggested that he disconnect the battery terminal at the back of the alternator he did so and called back to report the light was still on but the bulb was very dim. We suggested he remove the alternator and test the rectifier for shorted diodes. He did so and found that the salvage rectifier he had used was indeed shorted on one phase when those diodes tested at above 900 mA of leakage. We sent him a REGITAR replacement rectifier and he felt that the problem was solved after he installed it. Now this bought up a good discussion at REGITAR among the so-called resident brains. For instance since the test light still illuminated even though dim, did that mean there was still an electrical drain? All this discussion led us to believe that now was probably a good time to dive into this a little deeper. The fact is that twenty five years ago you could still use a test light to determine if you had an electrical drain on a vehicle, back then rebuilders were used to seeing regulators that had failed and were leaking rotor current or shorted rectifier diodes. That really was the extent of most leakage problems with the exception of the occasional mechanical relay sticking on, so using a test light was a good diagnostic practice. With today’s sophisticated vehicles using a test light to diagnose and fix electrical drains will probably lead you either in a circle or down the wrong road. We ran a test using a power supply and a good quality test light to see how much current it takes to make the light bulb glow dimly and it’s about 3 to 4 mA. I know this is very subjective, but then that’s the whole argument about using a test light to accurately determine drain or parasitic loads. Well why is that you might ask. Well let’s take the Passat as an example; it has a 75 series battery that has a RC (reserve capacity) of 90 minutes and an Amp-Hour rating of 54. The electrical equipment on this vehicle requires a key off “parasitic load” for the “keep alive memories” of the PCM, radio and other devices. The typical parasitic load is considered to be 25 mA or 30 mA maximum. If the Passat battery was fully charged but not driven for 30 days 1
the parasitic load would discharge the battery below acceptable cranking voltage. The parasitic load is probably what caused the test light bulb to be on dim after the alternator was repaired and reinstalled. So how much electrical drain or load caused the battery to discharge in two days? It’s safe to say the drain was several Amps not only from the two shorted diodes (one positive and one negative) but the VRB254 regulator has a soft-start feature at a 30% duty cycle that means if the shorted diodes caused the regulator to go into soft start mode there was the additional drain of up to 30% rotor current. This type of unintended “start-up” of the regulator is more common with the later types of regulators that use a programmable controller for the field. Okay so if a test light is not appropriate for checking the parasitic loads on newer vehicles what should you use? There are obviously all types of expensive equipment available from a Pic-AScope with an Amp probe to a simple low cost digital meter (needs at least a 10 Amp DC current setting). We are showing you a fairly low cost alternative, which is a digital meter (approximately $28.00) and a Thexton 431 Parasitic Load Test Tool (available on E-bay Tools) for $29.00. There is even a how to YouTube video for this tool. What could be simpler! This tool allows you to control the battery current to the vehicle to have it on when you need to and to isolate the vehicle from all power when you need to. It will also help you if you have a fuse puller and some wiring harness diagrams and component locators.
Thexton 431 adapter; Note On/Off switch
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The first thing you need to do is make sure the battery is fully charged, then disconnect the negative cable and install the Thexton tool or use a good 12 gauge jumper wire between the negative battery post and the battery cable. It’s not a bad idea to pull the courtesy light fuse which will help protect your meter and allow you to work with the doors open while not having to worry about courtesy lamp current draw. If you suspect that courtesy lamps or timers are causing your extra parasitic load you can always reinstall the fuse and take a meter reading. Always start off your meter on the highest DC ampere setting until you know the actual parasitic drain of the vehicle. After you have installed the Thexton adapter start the vehicle and run it long enough to awake all the electrical devices and then shut it down and hook your meter across the adapter or your jumper wire. You will have to wait for all the circuits to go to sleep (usually about 30 minutes) then you should be able to read the parasitic load. If it’s above 30 mA then start with the alternator battery terminal and disconnect and look for the current to drop, if it’s minimal (3 to 5 mA) then reinstall the alternator B+ and start pulling fuses until you find the circuit that is the culprit.
Meter set on 200 milli-amp scale
2003 S10 with Gen 4 OBDII PCM; radio memory; chime module; keyless entry HVAC module
Thexton 431 adapter
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We have generalized some of our descriptions to keep the data simple, but keep in mind vehicles like Cadillac Seville’s and other luxury type vehicles can have a parasitic load of close to 50 mA and take up to one hour for all the circuits to power down after the key is shut off. There is much information available on the Internet just Google parasitic battery load. Also if you do a search for GM Tech Link Volume 4, #4 there is a good explanation of battery reserve capacities and Amp-Hour ratings and how to calculate what your battery has if you don’t know. A general rule of thumb that is used is if you know your battery’s reserve capacity rating in minutes then divide that number by four and the parasitic drain on your vehicle should not exceed that number (example, battery has a reserve capacity of 100 minutes, divided by 4 equals 25 mA). In closing below is a chart of some typical parasitic loads for different devices that should help as a guideline. HAPPY HUNTING!
COMPONENT PARASITIC LOADS (Ma) Typical
Maximum
Component
Parasitic
Parasitic
BCM ………………………….
3.6
12.4
PCM………………………..
5.8
10.0
RADIO……………………..
3.0
6.0
REGULATOR…………….
2.0
4.0
ELC………………………….
2.0
3.3
CPS………………………….
1.6
2.7
ILLUMINATED ENTRY
1.0
1.0
THEFT………………………
0.4
1.0
AUTO LOCKS……………
1.0
1.0
CHIME……………………..
1.0
1.0
HVACInformation MODULE………… Courtesy1.0 ALLDATA
1.0
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