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| Sanity check needed - chasing down electrical current drain https://slantsix.org/forum/viewtopic.php?t=48447 |
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| Author: | Fab64 [ Sun Apr 08, 2012 3:39 pm ] |
| Post subject: | Sanity check needed - chasing down electrical current drain |
Happy Easter to all, After taking some time off due to frustration, I'm back working on the car today. Some of you may recall that I installed an HEI system in February, then I ran into problems (the car stopped running). I had the control module checked out, and it was good. The last thing I did (three weeks ago) was wire in my old coil, in place of the new one, and was able to get the car running. Thus, I concluded that the new coil was bad. Unfortunately, I went back out the next day and the battery was dead. I first suspected maybe my 3-year-old battery had coincidentally chosen this time to die of natural causes. But I had it load-tested, and it checked out good. My next thought was that I must have wired something incorrectly when I re-installed my old coil. But I've double-checked all my wiring, and everything looks good. So, I'm now trying to diagnose the problem. Not being an electrical expert, I asked a friend and was told that if I disconnect one cable from the battery, then complete the circuit with a Volt/Ohm Meter (attach one lead to the disconnected battery cable, and the other to the battery terminal), a proper-functioning circuit should show little (if any) current flowing. Is this correct? Mine shows about 12-1/2 volts flowing. I assume this is my current drain. I've disconnected all the wires going to the HEI module and relay (i.e. no part of the new HEI system is connected), and still show current flowing. I've pulled all five fuses out of my main fuse block, and still show current flowing. Can anyone confirm whether or not I should show current flowing under these circumstances? Any ideas on what to look for next? As always, all suggestions are much appreciated. Roger |
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| Author: | emsvitil [ Sun Apr 08, 2012 4:54 pm ] |
| Post subject: | |
Put the meter on the amps setting not the voltage............ |
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| Author: | Slanted Opinion [ Mon Apr 09, 2012 1:23 pm ] |
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Also, on many meters there is a separate socket for the + lead to plug into when you are testing amperage (current flow). Remember that current flow is measured in series (in line with) the load, just as you are doing it now (by disconnecting one battery cable and making the meter part of the circuit). Voltage is measured across something. Warning: If you touch the meter across the + & - of the battery while it is in current measurement mode, the battery will flow hundreds of amps through the meter, instantly frying it (or frying a fuse in the meter, if it has one, some do). - Mac |
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| Author: | KBB_of_TMC [ Wed Apr 11, 2012 11:46 am ] |
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Often, if the load is large enough to notice, it is too large for many multimeters; I try a simple 12V test light in series 1st; if it lights up, it's a big load, if it doesn't, it's well within the range of most meters. Even if a DMM has a fuse like my Sears, it can and has burned out other hard-to-get components before the fuse blows. Another trick is to compare the meter's internal load with that of the drain. If you measure the voltage of the battery, then put the meter on voltage in series, and the there is a significant difference (> 10 mV), then the load is on the order of the meter's and it'll be very safe to measure current; if it's less you can't tell much, so use a test light. The formula is iLeakage = 1/(Rmeter*(1/Vmeter-1/Vbattery)). For example, if the battery reads 12.00V, and a 2Mohm meter in voltage mode in series reads 11.98V, then the leakage current is around 4mA, assuming the loads are purely resistive (usually a good-enough approximation). It is normal that the alternator diodes "leak" a little; there is a factory spec that I don't have in front of me, but I seem to remember it's around 0.5-1 mA. Radios with memory also draw some current; my Jenson AM/FM/cassette draws ~10mA, enough that I often disconnect the battery if I'm not going to drive for a long time (I like to keep the battery 90% charged, so 0.1*50A-hr/0.01A ~ 500hr ~ 20 days). |
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| Author: | Fab64 [ Sat Apr 28, 2012 1:45 pm ] |
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Thanks for the suggestions so far, guys. I did a bit more diagnosis today. With the dial set on 2000 ohms, I get a reading of 841 on the meter. I've tried disconnecting the alternator wires, the starter, all fuses are out, brake switch disconnected, made sure all dash switches are off, all wires in the HEI setup were disconnected - and still show 841 on the meter. I'm out of ideas, so am going to put a battery disconnect switch on it for now. At least, this will keep the battery from draining when it's not being driven. I have to wait until Tuesday, when the electrical "expert" at my garage will be back, and see if he will work on something this old. Roger |
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| Author: | Mroldfart2u [ Sat Apr 28, 2012 1:59 pm ] |
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Quote: Thanks for the suggestions so far, guys. I did a bit more diagnosis today. With the dial set on 2000 ohms, I get a reading of 841 on the meter. I have to wait until Tuesday, when the electrical "expert" at my garage will be back, and see if he will work on something this old.
Ummmm WHAT were you measuring with the VOM set to OHMS? Thats not the way to check for a voltage drain....
Roger |
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| Author: | Fab64 [ Sat Apr 28, 2012 2:36 pm ] |
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That's a good question - lol!! I think I'm losing it. In the previous suggestions, my eyes read "amps", but my tiny brain was thinking "ohms". Thanks, I will try it again. . . |
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| Author: | wjajr [ Sat Apr 28, 2012 7:49 pm ] |
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Quote: Ummmm WHAT were you measuring with the VOM set to OHMS? Thats not the way to check for a voltage drain....
There is not any voltage drain, voltage is potential and dose not flow. Ohm is a unit of resistance. He is trying to measure current, the flow of electrons, so as Ed said; set VOM on amps, but be careful performing this test, it does not take many DC amps to fry the meter. Reread Mac’s warning: Quote: Warning: If you touch the meter across the + & - of the battery while it is in current measurement mode, the battery will flow hundreds of amps through the meter, instantly frying it (or frying a fuse in the meter, if it has one, some do).
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