120 mA draw
 

All,

My battery went dead after about 10 days sitting in the garage. After getting the battery charged and taking it on a checkout run, I disconnected the negative battery terminal and connected my ammeter in between. I found 120 mA draw, which seems like a lot for an old-school car.

The wiring for the car is fairly unmolested but for a dash light fuse, modern stereo (now removed) and seat heaters.

I went to the fusebox and jostled fuses one at a time with no effect until the fourth from the back. When I pulled that one, current draw dropped to zero. The clock also stopped, and the glovebox light stopped working.

Based on what it operates, the wiring diagram says this is fuse 18 which doesn't seem to match either the fusebox cover or the counting of fuses from the front. (page 4, trace 42 here: http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_78SC_USA.pdf)

I know that wiring diagrams isn't completely up-to-date for a 1980 car. For instance I believe the Jetronic unit under the passenger seat is also on this circuit.

Any known bad actors on this circuit? What should I look for?

Thanks.

I had a similar problem with battery going dead. Pulled the fuse like you did and the clock and glovebox light quit working. I was puzzled till I found I had left the sliding switch for the glovebox light in the ON position! Ergo when I opened the glovebox the light was on BUT when I closed the door the light stayed on and I didn't notice it.
Just did a DOH! and slapped my forehead when I found my "problem".

excellent troubleshooting , I would say you are 9/10ths there.

that circuit powers interior lights as well as glove box light and luggage compartment light ,.

Are you sure you your luggage compartment light is goin out when you close it via the plunger switch ?
if you cant pin it down start popping light bulbs.
I bet you will have it figured out by tomorrow.

not sure it's applicable to your situation, but on my '86, my failed door switch (interior lights wouldn't come on when I opened my door) was the root cause of a VERY similar drain situation.

Turns out that on my car, the power windows remain energized (can be closed) after the car's shut off and before a door is opened. Since my door switch never "told" the relay that my door was open, my power windows remained active whenever my car was off and pulled a drain that would kill my battery after a week or so.

Easy to check -- can you operate your power windows with the key out and do your interior lights come on when you open the door?

Cleaning my door switch so it worked solved my problem

SC did not have the kermit palzer window relay thingie. 84-ish for start.

'84 don't got it either, more's the pity. Must b an '85+ thing.

For sure post SC. It's a multi syllable German doo-hickey relay that keeps the windows active until you pop the door open.

Perhaps "frammitz" valve?

(FWIW, all the silly names are from MAD magazine from the early 80's)

120 MA is pretty wimpy in broad speak.

Current draw.
 

I believe that 120 mils is very close to spec.

Check the glove box light switch.

Have a look at the specific gravity on each battery cell.

Good luck,

Gerry

pm me as needed.

Anything greater than 30 mA is suspect. How fast your battery drains is a function of the draw and the Amp Hour rating of the battery. Temperature affects the time also.

Like someone said earlier, pull the bulbs on the interior lighting circuit one by one until you find the culprit.

For a 55 Ah Optima Yellow Top, at 120 mA the maximum theoretical life before being completely drained is 19 days. That seems awfully short. Siegel's electrical book (https://www.amazon.com/Mechanic-European-Automotive-Electrical-Systems/dp/0837617510) gives 50mA as the standard, but it's not specific to any particular car. Ima go with Tim on this one.

Weekend plans have just been made! We'll see what comes.

FWIW, I wired a rocker switch in line (no cutting wires PLEASE!!!) with the clock's ground. When I park the car the clock gets powered down (via cutting ground). When I go to drive ithe vehicle again in a day or a week, etc. the correct time gets set and I turn the clock back on.

I have yet to check what the clock's specific mA draw is, but will do so for shyts & grins at some point.

For a basis of comparison, when I eliminated the parasitic draw on my 83 the steady state draw went under 10mA (I think it went to 7mA) and I haven't had an issue with battery drain since.

My draw was coming from the front windshield heater wiring. I don't have a front windshield heater option and I never found the exact source -- I just disconnected that wire at the fuse panel.

On my '83 fuse 18 has a black wire that goes to the trunk light and a red wire that goes to the interior lights, glove box and OXS relay. If you don't have an O2 sensor then you don't have the OXS relay.

Bentley
 

According to the Bentley, 0-100 mils is normal.

But, as previously posted, disconnect the battery
for a few days, and reconnect.

Let's be certain you do not have a defective cell
in your battery.

Best,

Gerry

I think the point is a draw may or may not be problem. If the car is a daily driver and you're driving it enough that it is constantly recharging, then it's not an issue. If you drive it once a week or every couple of weeks and it sits in an unheated garage and you use one of those little Miata batteries, it's a bigger deal.

My 85 does have that feature.

What would battery disconnection and reconnection do? Would that affect the draw somehow?

Depends on the time of year. When it's not raining, snowing, or salty I drive it. This time of year where I live, it could be a month between drives.

The battery is an Optima Yellow Top, so not a Miata battery. I think there's a problem that I need to chase down.

I can't imagine that the clock would make this kind of draw.

Battery.
 

An internal cell discharge issue.

The idea is to check the battery voltage prior to
disconnecting, then leave the battery disconnected
for a few days.

Shorted or defective internal cells can reduce the voltage.

But look, just take your car to a Sears shop & they will
check your battery under load conditions.

G.