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Parasites in my Fiat 850 Spider

I’m stumped with my 1973 Fiat 850 Spider. This little car has a grand total of 10 fuses, 8 below the dash board, and two in the rear where the engine is. So, it’s not that complicated, right? Wrong.

So here’s the story: I purchased this little car in Philly. Everything works, except after a few days, I noticed that the car battery was out. So I jump it and allow the alternator to recharge it. It runs fine for a few days until it goes dead again. It reaches the point where it’s completely dead, no service light, no radio, nothing.

I figured I bought a car with an old battery. I replaced it with a new one from Walmart. It runs great for a few days until gradually no charge again. I take it to a shop to fix something else, they test the battery and they tell me it’s bad, that I bought a brick. OK, bad luck, right?

I replace the Walmart battery for another one under the warranty. What can go wrong now? Well, the same symptoms appeared after a few days. By now, surely it cannot be the battery. So I know what you’re thinking. My little convertible has a parasitic draw. But remember, it has only 10 fuses, no power windows, no power locks, no computer, no software, no Bluetooth…gish, it has a push-button AM radio, for crying out loud!

So I proceed to recharge the battery. I trickle charged it for 18 hours or so. The battery is now back to new. I then connected my meter in series to the negative pole to measure the amps, expecting as you would imagine a draw of at least 4 amps. Well, the little devil has a draw of only .35 amps, well below the acceptable level for a car this type (right?).

So what gives? Does the car have gremlins or parasites? Am I going crazy? Can I be so unlucky that I have had three bad batteries in a row (even from Walmart)? Am I missing something?

Any, and I mean any guidance would be greatly appreciated. My hair will thank you.


Is it safe to assume you have checked the alternator and it is charging fine? You should check the volts at the battery with the engine off and then running and see 12–>14 volts I guess.

The way to find the parasitic draw is to pull the fuses one by one while checking the draw.

Your testing found you have a draw. 350 milliamps is enough to drain a good battery over a long weekend. I would expect less than 30 milliamps for a car like this. Start pulling fuses one by one. If that doesn’t lead you in the right direction unplug the alternator. You likely have a bad diode.

I know you don’t have them but I have had two intermittent drains. One from the door handle push button that turned the interior lights on and another to a faulty load level switch. The only way I found them was by accident and head scratching.

asemaster is correct. A car battery is typically 70 amp-hours. divide that by 0.35 amps and you get 200 hours or 8 days. It will be half gone in 4 days.

But I suspect you also have an alternator problem and the battery is not being charged.

+1 to @asemaster. 0.35 amps is way to high for a such a simple car. Do the “pull the fuse test” but I’d suspect an alternator, too.

Given the car is now 43 years old and the car was never known for its quality, the wiring is likely to give you LOADS of trouble in the future. Consider replacing all of it.

Thank you all for your ideas and suggestions.

So let me get this straight. 35 miliamps would be considered too high of a draw for this type of car? OK, I will then proceed to do the parasitic test by pulling the few fuses one by one and measure any drop in amperage. That’s easy to do.

As for asemaster’s comments, please pardon my ignorance, but what is a bad diode? Will I need to replace the alternator? What could be the cause of a bad diode (to try to prevent it in the future)?

Finally, a few more questions: Does the alternator also draw power form the battery when the car is idle? Hence the suggestion to unplug the alternator, do I understand this correctly?

Mustangman, yes this is a 43 year-old car, and yes, the Fiat 850 was not the most reliable car in the world, but to suggest that the wiring is probably going south form here onward, well, I feel that’s a little stretch, not that I do not appreciate the suggestion. A wire, after all, is a wire. I find it hard to believe that an electrical issue would be brand-specific. I’m just trying to talk myself out of spending a bundle to replace the entire wiring system, I guess…

In any event, I will test for parasitic draws and will report back.

If anyone can suggest otherwise, please let me know.


An alternator won’t draw power when the engine is shut off if it and the voltage regulator are working correctly. First have the charging system tested, parts stores will do this for free. Then do the current drain test like you did before, removing and replacing one fuse at a time to see if you can find the circuit that’s draining your battery.

And yes, the wiring on old cars like yours fails. It’s usually the insulation. You don’t have to replace it all right now, but be on the lookout for cracked and missing insulation. Also, check ALL connections for corrosion, clean them as needed. Especially the battery connections and the battery and engine ground connections.

No. 35 milliamps is what I would like to see. 350 milliamps is what you have and is way too high. You have something that is draining the battery.

A diode is a “one-way check valve” for electricity. It allows current to flow one way but not the other. There are three of them in the alternator and they allow current to flow out of the alternator to the battery but not back into it. The suggestion to unplug the alternator is to see if the draw goes away.

Cool car.
On a car this old I too would suspect a high resistance short to ground. After 47 or 48 years, it wouldn’t surprise me to hear that the insulation was cracked and even crumbling. Usually these high resistance shorts turn up at those spots where the harnesses go through openings in the body.

Do you have schematics and wiring diagrams for the body?
Have you checked on a “niche enthusiast” site for aftermarket harness suppliers? Cars like these usually have a cult following, and they can be a wealth of information.

I once owned a Fiat 850 Spider and got into the best shape of my life. How? I pushed that little car far more than I ever drove it.

My memory is very fuzzy on these cars but does this car not have an external voltage regulator with mechanical points in it?

There’s a set of cut out points inside the regulator which is designed to pass charging current to the battery and open when the engine is shut off. If the points do not open the alternator will charge the battery normally but the battery will drain through that same set of points.

.35 Amps = 350 milliamps, another example of a society no longer do math in their head, if the numbers look right, the fact that it is one order of magnitude off doesn’t register.

I had that experience with my Morris Minor too. Push it, hop in and pop the clutch, push it again and pop the clutch. Pushed it two blocks from the gas station once before it started again. What in the world was I thinking? And it was downtown and I had my fry cook whites on. Why didn’t my parents stop me? Used to deliver chicken in a little Fiat though. Pretty basic indeed.

Like @“oldtimer 11” stated, the current draw you showed the system is drawing is more than 10 times higher than it should be normally. It should be more like .020 amps, which is also stated as 20 milliamps, or even less current possibly. Hopefully by pulling fuses one at a time you will find the path the excessive current draw is on. Fuses almost always supply power to more than one circuit though so the trouble circuit may not be what is labeled on the fuse position.

Diodes allow current to flow in only one direction unless they short internally. The alternator has diodes inside it and they can go bad, especially when you try to charge up a dead battery using the alternator instead of a battery charger. It makes the alternator work harder than it normally should. If you work on the alternator be sure to disconnect the negative battery lead first. The main alternator lead is hot at all times.

Just to clear up one misconception, diodes conduct in BOTH directions. Its just that they have a low resistance in one direction and a high resistance in the other, but they do have a reverse current. This is why I think OK4450 is on the right track here.

The diode pack in an alternator is a full wave bridge rectifier, so the current must pass through 2 diodes in series. This minimizes reverse currents even further, but if one diode were to short out, the reverse current would be much higher and the output of the alternator would be lower and that could be another possibility. However, most cars of this era had a battery light or charge light. If one diode shorted, that battery or charge light would come on when the key was off and go off when the engine was running.

First check the light by turning the key to on without starting the engine to make sure the light is working.

All of my vehicles have had wye configuration alternators and so 2 diode trios, one trio for positive, one for negative cycle. Only one diode in series per winding for a given polarity. The reverse leakage current on these diodes is very minuscule, in the microamps and so by all rights, negligible. Unless it is damaged…

If you are wye connected, you need 8 diodes, if delta connected, you only need 6. Each phase uses 4 diodes, but each diode is used in more than one phase.

Edit: my bad, you only need 6 diodes for wye wound generator as well. But to discharge a battery through the coil of an alternator, you still need to go through two diodes in series with that coil.

The reverse current through a normally functioning diode would be so low it is basically meaningless in this and most other cases. It would be in the nano or microamp range.

Concur w/the others, 350 mA when the engine and everything else is off is considerably more than it should be. My Ford truck of similar vintage measures less than 15 mA. So whatever’s causing it, that’s almost certainly what is draining the battery. One of the functions of the alternator diodes – assuming you have an alternator and not a generator – is to prevent a current path from the battery through the alternator which would otherwise discharge the battery when the engine is off.