This is Dave again. The question is now answered and the problem has been solved, so I’m posting this for those who might benefit in the future. I’ll tell you (1)what I believe the correct answer to the question to be, (2) how the problem was resolved, and (3) what I learned about current-measuring test instruments along the way. Since some asked, my Trailblazer is a 4x4 with no aftermarket options.
Item (1). Acceptable Current Draw: After a little research, the 25 - 35 milliamp figure mentioned above still looks like a good rule of thumb to me. AllDatadiy.com provides a list of GM service bulletins for this car, and one bulletin shows parasitic loads for six GM vehicles (none of them TrailBlazers) that range from 11 to 28 milliamperes (.011 to .028 amps). That matches pretty well with the figure I mentioned in my original question – about 25 - 35 milliamps. The source of that figure was an article written by someone named Larry Hammer, and can be found at this website: http://flashoffroad.com/electrical/CurrentDrain/currentdrain.pdf. If that does not work, Google “Parasitic Current Drain”. Now that the problem is fixed, my TrailBlazer draws less than 10 milliamps – after the computers on board shut things down (this takes about fifteen seconds or so).
Item (2). How the problem was resolved: I found that my (digital) ammeter had nearly the full battery voltage drop across it while reading 92 milliamps. I had expected the ammeter to have nearly zero volts across it. (The meter was just protecting itself.) This meant that the vehicle was not operating with normal voltages and the 92 milliamp measurement should be ignored. Time to start over.
I had already ordered a clamp-type ammeter but when it arrived I found that it could not be used as a “clamp meter” except in the very high current ranges (like 40 amps or more). Some research led me to the one I now have on order – an ExTech Model MA220 from Calright Instruments at calright.com. Cost is a around $100. However, I wanted to fix this car immediately if I could, so I went to Radio Shack and bought sixteen 1-ohm Ten Watt Resistors, and connected them together in a bundle to make a 1-ohm 160-Watt resistor. I then connected this 1-ohm resistor between the negative terminal of the battery and the black battery lead – with the resistor temporarily SHORTED OUT with a test lead with alligator clips. I DID NOT ATTEMPT TO START THE VEHICLE OR OPERATE POWER WINDOWS ETC. WITH THIS TEST SETUP IN PLACE. After giving the car a minute or two for the computers to go through their paces and settle down, I removed the SHORTING test lead and connected my digital voltmeter across the 1-ohm resistor. With this setup, 1 volt means 1 amp, 1 millivolt means 1 milliamp, etc. The computers need 10 volts or so to operate normally, so any reading above 2 or 3 volts means the car may not be operating “normally”. That is why a clamp meter would be so nice to have. (As I write this, mine is still in the mail.)
What I found was that the parasitic load was not 92 milliamps but more than an amp! It was the dome light. But the dome lights were not lit up. Why? Because the car has a dome light override switch in the lower left corner of the dash. Push it once and the dome lights do not come on when the car doors open. Push it again and they do. It was switched to the position where they do not come on. Meanwhile, the instrument panel lights “DIMMER” control was turned all the way clockise – which turns the dome lights on. Here’s the key to the whole matter: even with the override switch keeping the dome lights turned off, there is still a big current draw with the switches positioned as described. Voila! Big current draw through the dimmer/dome light path with the dome lights NOT LIT UP. Chevrolet are you listening? How about a change in the logic so this cannot happen? On future cars of course.
Item (3). Well, I answered most of this at the beginning of the section above. I plan to use the ExTech MA220 in the future if this ever comes up again. The 1-ohm resistor described is clumsy and one has to be very careful not to let anything short out by accident.
One other thought might be helpful. I can now tell if I have a parasitic current draw just by using my tricke charger (which has its own ammeter). First, I charge the battery for an hour or two or until the the trickle charger’s ammeter indicates that “almost zero” charging is going on. Remember where the meter’s pointer is – even if it has no numbers on the scale. Then let the car sit overnight or for 5 or 6 hours with KEY OFF and NO LIGHTS ON. If there is a light that illuminates the engine compartment when the hood is up, lower the hood until the light goes out or remove the bulb if necessry. Disconnect the trickle charger. Then the next morning, reconnect the trickle charger and watch the pointer closely when the charger is plugged in. The charger current should move up to maybe half scale or so but you should immediately see the pointer start falling back toward zero. In 10 or 15 SECONDS, mine is almost back to where it was the previous night when the battery was fully charged. That’s when there is no parasitic load problem. When my TrailBlazer had its problem, the trickle charger would need 15 to 30 MINUTES to get itself back to “almost zero” charging current. I think this is a good poor-man’s way to assure oneself that such a problem is fixed.
Let me say thanks to those of you who wrote with helpful information. This is my first use of the Car Talk chat line, and I was amazed by the response.
I have found via Google searches that people sometimes have a frustrating time getting these parasitic current drain problems fixed. I hope that these notes will be helpful to someone in the future.