OK, I am confused. Probably not a surprise to most of you...

I finally got around to checking the current drain on the '90GT. Disconnect ground strap, hook multimeter on 200mA range between strap and ground.

No reading. Zero mA. This does not compute. Touch strap to ground to make sure that a relay didn't drop out, etc. - got a visible spark, so there darn sure is power flowing. Zero meter reading.

Swap meters. Same result. Repeat - same result on five different meters.

Try it on 10-Amp scale. No reading.

Finally hooked up an antique Bell & Howell digital meter that I got with a Heathkit 25' color TV kit. It shows 89 mA on the Nixie tubes...

Hooked up a #67 12-volt bulb between ground and strap - it glows brightly.

Obviously, I am making a simple mistake. Any comments?

 

Almost hate to ask, but you are plugging your positive probe into the proper jack on the multimeter, aren't you (since there's typically a separate, fused probe jack for current measurement)?

 

the leads of the newer meters may requier a different position......the positive lead.

 

stereo Ed

 

That was close, but a win for me, nevertheless.


BTW, Wally, when I do this test, I use an alligator-clip-terminated jumper from the ground strap end to a chassis ground point and keep it grounded until the multimeter is properly hooked up and I'm sure the interior lights have timed out, etc. Then disconnect the extra jumper and take the reading. If you don't do something like that, and depending on your multimeter and the range it's on, the initial spike (interior lights, etc.) might blow the fuse in the multimeter.

 

Doesn't sound like you made any mistake to me. I have had the same thing happen a few times. I blamed the amp meter. I also noticed at the same time that nothing lit up - I had the hatch opened - so that confirmed that no current was flowing. Then a little later I tried again and everything worked as you would expect. Like you, I was sure I had everything hooked up correctly the first time. I puzzled over it for a few seconds and then ignored the anomaly. Alan may have an explanation. Until you brought it up today, I had forgotten about it.

Did you find the source of the extra ~50ma?

Oh, and I do it the way Ed described (hook up the meter across the ground before removing the ground), but I believe I learned that from one of your posts long ago.

 

Bill, both you and Wally might be suffering from the "empty wallet" syndrome. That is... no current-see.

(Hopefully, my earlier two potentially helpful posts entitle me to get away with that.)

 

"Almost hate to ask, but you are plugging your positive probe into the proper jack on the multimeter, aren't you (since there's typically a separate, fused probe jack for current measurement)? "
Don't be afraid to ask - I'm open to all comments and questions. Black to "COM", red to proper 200mA jack, then moved to 10 Amp jack, no reading either place.

"BTW, Wally, when I do this test, I use an alligator-clip-terminated jumper from the ground strap end to a chassis ground point and keep it grounded until the multimeter is properly hooked up and I'm sure the interior lights have timed out, etc. Then disconnect the extra jumper and take the reading. If you don't do something like that, and depending on your multimeter and the range it's on, the initial spike (interior lights, etc.) might blow the fuse in the multimeter."
Did it that way - no reading. Touched strap to ground w/meter hooked up, got visible spark - removed strap from ground, no reading on 200mA or 10 Amp scales.

"Did you find the source of the extra ~50ma?"
I think that I have more than 89 mA flowing - visible spark, lights #67 bulb to full brightness. Started pulling relay - no change in bulb brightness. Started pulling plugs - Plug G dropped the bulb to very dim, Plug K put it out.

Now in the process of pulling the panel for complete cleaning and replacing the interior light relay socket that I pushed out of the panel...

 

One other thought... any big capacitors in your car? I get a nice big spark when I reconnect my ground strap due to a 1 F capacitor for the aftermarket audio amp. I've never measured it, but that initial capacitor-charging-spike is probably pretty big—enough, I'm guessing, to exceed the lower ranges (and maybe even the upper?) on common portable multimeters.

Now that shouldn't affect steady-state current flow once you're past that initial transient in any way that I can think of, but the initial transient could be enough to blow a multimeter fuse.

 

is the ground strap in good condition?

try the meter between a good ground and the neg batt post.

 

"One other thought... any big capacitors in your car?"
None that I have seen. Audio system is pretty standard - no high power amps. Again - current flow lights up a #67 bulb continuously.

"is the ground strap in good condition?
try the meter between a good ground and the neg batt post. "
Yes, replaced - no change. Tried the post - no difference.

 

Is the meter fuse blown? Easy to blow it on 200mA setting if the interior lights are on...

Then all currents will be 0

Alan

 

What Alan said: the fuse in the meter is probably blown. As I recall, there is a higher current surge when the ground strap is first reconnected that reduces to the nominal low constant drain (35-50 ma?) after a few seconds.

These fuses are usually a fast reacting type and are somewhat more difficult to find, as well as being a lot more expensive.

 

I'll vote for blown fuses in the meters. The #67 bulb is a .59 amp bulb, so must be drawing close to that to glow brightly. Plus the spark says way more than a small fraction of an amp, probably. But 5 different meters? Whp owns 5 different meters?

I have no reasonable explanation for the reading on the antique meter. Could there somehow be a big AC component? I can't think why, but I'm totally unfamiliar with the later cars; could be they did some weird things with the electronics.

 

when using an ammeter its absolute paramount to start on the most highest setting then reduce. This allows the meter some electrical safety from higher current values and is also why the meters are fused at lower settings thus preventing your $$$ multimeter from releasing its 'smoke'.