Thanks for info. Hope to play with it tomorrow. I don't have a single aftermarket install, except FiTech EFI, right to battery, and I disconnected that before testing.

It does have factory spotlight, which I don't know if is fused. Maybe inline fuse? I haven't followed the wiring for that which I suppose runs into a factory police harness which could be fused to one of the ones labeled "accessories" and I pulled every fuse when testing. But I would certainly notice if the spotlight was coming on/off at times.

could it be possible the wiring in the pillar the spot light goes through may have the insulation cracking and just a strand or two touching the through tube causing a drain ?
just thinking out loud here.

Quick and dirty. Disconnect the negative battery cable. Install a test light between the negative post and the removed cable. If there is a IOD (ignition off draw) the lamp will light. Don't be surprised if it lights. Give it 15 minutes and see if everything goes to sleep. the light should go out. If it does there is no draw.
Doug

Thought the test was done the same way, but with negative cable. Or would it really matter much because the light is completing the circuit?

Either cable. We actually use a shunt at work. A shunt place in series between the battery and cable can show amp draw on a digital volt meter. You can also use an amp meter. But the problem is you may blow the milliamp fuse in the meter if the draw exceeds the fuse limit.
Doug

The fuse in most ammeters is 10A, if you have more than a 10A parasitic draw something is seriously wrong.

The face of your meter ought to be labeled for max current in ammeter mode.

Making the current measure can be tricky.

sample quote

You are using the amp meter the right way, but there is a problem you need to be aware of. Starting in the mid 1990's, most vehicles have at least one computer that needs up to twenty minutes to go to "sleep" mode. During that twenty minutes it can draw up to three amps. Logic would dictate you just have to wait for more than twenty minutes, then start the testing, but there is one more piece to the story. Anything you do to remove power to that computer, even for just a fraction of a second, wakes it up again, and then you have to wait another twenty minutes for it to go to sleep. Once the battery cable is removed, as soon as you connect the final meter lead, the computer will wake up and draw high current.

Most of the common, inexpensive volt/ohm/amp meters have an internal 2-amp fuse for the milliamps and 2-amp range, and they need to have the positive test lead moved to a special jack to use the 10-amp range. That 10-amp jack is rarely fused.

Most people will start the testing with the meter on the 10-amp range, then they can read that 3-amp drain until the computer goes to sleep. Once that happens, we need to switch to a lower range for more accuracy. That means removing the test lead from the 10-amp jack and moving it to the common volts/ohms/milliamps jack. Doing that breaks the circuit, and once reconnected, the computer will draw enough current to blow the meter's internal 2-amp fuse. "Frog fuzz!" Now the frustration begins.

If you are lucky enough to not blow that fuse, you will be on the 2-amp range, and once the computer goes to sleep, you will probably want to switch to a lower range, again, for more accuracy. All voltmeters with a rotary switch use a "break-before-make" switch which means as you slowly rotate the knob, the internal contacts break the connection to one range, then it makes the connection to the next range. That very minute gap is plenty to cause the computer to wake up again. Even if the fuse does not blow, you will get an over-range indication because the computer is drawing much more current than that range can handle.

I am fairly confident this is why you found 2.25 amps of current drain. You may have waited an hour, but you have to wait after making the final meter connection and selecting the range.

There is an easier way though to solve this.

Connect your meter like you did, and start on the 10-amp range. Wait a good twenty minutes or until you see the current drop significantly, then use a small jumper wire to connect the battery cable to the post. You can also jump between the two meter probes. Regardless how you do it, the meter is bypassed, or in effect, shorted out in the circuit. Voltage to the computer has never been lost and you are free to move the meter's probe to the milliamps jack. When you are ready, remove the jumper wire and read the meter.

If you see the current has dropped low enough to warrant switching to a lower range, put the jumper wire back on first, then switch the range. When you remove the jumper wire again, current will have no choice but to go through the meter to get measured.

Unless specified differently by the manufacturer, the industry standard is you're allowed up to 35 ma. (.035A), of current drain to keep all the computer memories alive. Chrysler says at that rate, a good, fully-charged battery will be able to crank an engine fast enough to start after sitting for three weeks. Cadillac is one brand I am aware of that allows up to 50 ma.

end quote

from

https://www.2carpros.com/questions/parasitic-drain-caused-by-altenater-diodes

This also explains why using a shunt to measure the DC current may be preferable.

Not even applicable to the OP's ride.

Nothing to go to sleep on my ride for sure. I'm sure there's a draw via the way tested. Then I left negative cable disconnected for several days, then connected charger to battery and it was still at 100%

I'm starting to wonder more about the spotlight. I want to dig into the dash a bit anyway because one or both of front speakers is crackling and annoying me, so I'm going to dig in a bit and follow that factory spotlight wiring and see where it leads. I've had everything out before to bare firewall, and just can't remember now, and whether there's inline fuse. I still need to test alternator too. I got busy with damn wiper grommets and putting linkages back on (and cutting up my arms) and didn't finish as quickly as anticipated

Is larry not lovin it anymore ?

I seem to remember someone offering you a reasonable sum of $$ to take that PITA off ur hands

with an older vehicle and an undiscovered milliamp level electrical fault,
use your eyes to look all over for “Green Death” corrosion deposits,
that horrible blue-green-grey colored stuff.

If you cannot find it with regular white light
sometimes using the Ultra-Violet flashlights that come with Freon leak spotting dye
in a totally dark garage will make Green Death glow faintly.

As a Christmas present I was just given a
Sharper Image Travel UV-C Sanitizing Wand
Item No 205847
that has a 4 watt UV-C tube
and an internal mercury switch that turns itself off it the light is not shined away from your eyes.

https://www.sharperimage.com/view/product/Travel+UV+Sanitizing+Wand/205847

It is way brighter than my UV freon dye flashlight
or my Hunter’s urine/blood spotting UV flashlight.

If you suspect the spotlight,
and do not want to tear into the dash
begin by disconnecting the two battery cables
remove the bulb from the spotlight
and use a good meter to measure resistance in Ohms
from positive + bulb socket contact pad to black - battery cable.

Using a “Megger” meter hand cranked at a higher voltage than the 9 volts of the typical multimeter would be even better.

https://www.youtube.com/watch?v=t0k8MnJzM_g

https://www.youtube.com/watch?v=WpANiRvB4tE

Using a megger is a good way to blow up any electronics. I would pass on that. Granted the M body in question doesn't have as much electronics as later model stuff but odds are good his Fitech and radio would be kaput unless isolated.

Odds are pretty good there is a disconnect in the kick -panel to unplug the spotlight.

When using the amp meter at the time the test is started we use a second cable run in parallel to the meter. That way any current flowing when modules go to sleep is uninterrupted. Start on the 10 amp scale and remove one end of the jumper. Read the scale. If the draw is low enough reconnect the jumper. Switch the meter to the lower scale. Now remove the jumper. This is why we use a calibrated shunt. Everything stays connected. You just read the millavolt scale on the meter I also find it helpful to latch the door latches. Or in the case of an older car remove the dome fuse or plunger switch. That way you can test with the doors open. I do these tests often. I work in prototype debug at FCA. It’s amazing what will cause an issue. Many times it gets down to removing fuses, modules, switches one at time waiting and watching. Can be very time consuming on the new stuff.
Doug

Oh yes, I was certainly advocating using the 4000 volt setting on the the Biddle, not the 20 volt.

My favorite setting is the 1000 volt setting,
which will bring earthworms up out of the lawn.

I have been working a lot and been trying to finish PITA job of replacing wiper linkage grommets and then put the linkage back on, clock the motor just right and attach to motor. I got it all back together, and wiper motor worked for a second then stopped. Now it's working intermittently (not in the way intermittent wipers are supposed to). I played with harness, cleaned connections, jiggled wires and tapped on motor with hammer. Only once did the motor start up again, and that was when jiggling wires. I'm wondering if wiper motor is the issue. I did pull that fuse when doing the testing so it should have dropped the amp draw on meter then, but I guess the draw could be intermittent. So next is fiddle with that motor and see if disconnecting changes draw, and still need to test alternator.

Remember, '89 Diplomat so not any modules to go to sleep. Also I did disconnect the FiTech power when testing the first time. I disconnected dome light plunger the first time testing, drivers door open, me at fuses, my brother at battery.

And I will be pulling the dash soon anyway to access front speakers, and I'll follow the spotlight wire and disconnect where I can.

The lowest voltage I have seen a megger test at is 100v. which will pop a car's electronics.

One could also disconnect the wiper motor to see if the draw goes away (kwik and dirty )
If you know where the SL wiring is, does it have 1 or two wires? If only 1 that would be power and the ground path would be though the s/l mounting. Isolating is would open the circuit. One could also cut the wiring (I wouldn't want to either). I kind of doubt it's the S/L myself. BUT ????
I'm assuming you completely disconnected the alternator with no change ?
The only other items I can think of are: glove box, trunk, under hood lites, But then being an ex cop car it could be a left over energized relay that is not hooked to anything. Keep us posted

Don't think this is the answer, but try adding a ground wire to your wiper motor.
I had to do this on my '89 D250, for some reason case wasn't grounding and motor wouldn't work.

I don't want to dogpile about using a megger but frankly I can't see what value it would be even if it could be used. Meggers are used to stress insulation to determine its condition. Using it in this case wouldn't provide any additional information over what a VOM would give. As to the problem, wow, even Hitler couldn't win with two fronts. Either get the wipers squared away or the leakage squared away. As to finding the leakage, make a copy of the wiring diagram from the FSM. Highlight the positive side of any feeders to individual components. Then start isolating them one by one. I have been down this road more than once and as shotgunning may be good for pheasants, it isn't for electrical troubleshooting.