Parasites in my Fiat 850 Spider


The reverse current of a single diode is higher that you realize. The reverse current through a diode bridge is a lot lower because it has to go through two diodes in series, as well as two coils.

The only schematic I can access right now is for a Toyota alternator. It is interesting as it is an ungrounded wye wound generator, but the diode bridge is delta connected to the coils. When the engine is off, parasitic current through the alternator has to go through the three diodes on the + side, then through two coils per diode in series and then through the three diodes to the neg side. In other words, there are three parallel paths each involving a diode, two coils and another diode in series.

Now the book I am looking in has two alternators, one with an internal IC regulator and one with the mechanical external regulator. In the mechanical regulator, there is a set of contacts that open when the engine is off that takes the alternator out of the circuit, but it looks like a shorted diode on the + side might energize the coil and make the contacts.

The internals of the IC is not shown, but the alternator has a total of 12 diodes with the IC regulator. 6 are in the bridge, the rest support the IC.


I’m an electronic illiterate now but the Army Signal School, which is the only electronic training I have had, taught us that a diode is a one way switch. There may be some small current going the other way but the way we tested them was with an ohm meter to see if you got a reading one way but not the other. I guess its splitting hairs, but if I got a reading both ways I’d know it was bad.


All this discussion about diodes are fun. I used to solder them when rebuilding the alternators way back when in some other world. I doubt the OP is going to need to do that though. If the diagnosis is made, is a rebuilt alternator available for this car?


Bing, you no doubt used a low scale on the ohm meter so you got a couple of ohms in the forward biased direction but no needle movement in the reverse. If you had used a higher scale, you would have seen some needle movement on your Simpson 260.


Just out of curiosity, I looked up the specs on alternator diodes. Typically, they are:
Peak reverse voltage: 200 volts
Average forward current, max: 35 amps
Peak surge current: 200 amps
VF: 1.05 volts
IR at 200 volts: 50µA
RR time: 15µs

note that the IR is at 25ºC. It doubles for every 10ºC increase, so at 95º that would be 6 mA. But typical would be a lot lower, and also lower because actual voltage is 15 volts, not 200 volts.


@bing were you using a vtvm? (vacuum tube volt meter)?


Wow, I’m truly electronic illiterate, but I’m getting the hang of it. Thanks for the diode discussion.

So going back to the testing, I finally had a chance to pull the fuses one at a time. As I said, the Fiat 850 has a total of 10 fuses, 8 below the dash on the driver’s side, one on the rear with the engine (yes, the engine is in the back and it spins counterclockwise, btw), and another one on the right-hand of the steering wheel under the dash. I was unable to find the last two fuses, but the other 8 were pulled one by one and there was no significant drop in amperage.

I’m not sure if I can rebuild the alternator as galant suggests (and yes, this car does have an alternator AND a regulator). Replacing one will cost me about $300 plus labor, and very few mechanics are willing to even bother with these cars nowadays. Imagine that, no computers to tell you what’s wrong…shameful.

So, unless the draw is coming from the two fuses I could not locate, can I assume that the draw is coming from the alternator due to a faulty diode? Am I way off?

I will upload the entire electrical diagram if it helps to diagnose the issue. I will scan the user’s manual which includes one (pretty cool, if you ask me).

BTW oldtimer 11, my bad on the math. Yes, I know how to use math with multiples of 10. I grew up with the metric system, but then again, I’m human. Sorry about that.

Thanks again,


Disconnect negative lead on battery, disconnect alternator & regulator, reconnect battery and take your reading again. Potentially eliminates the finding of those two elusive fuses…


To see if the alternator is causing the excessive current draw just simply disconnect the wiring to it, and the external regulator if there is one. Since the alternator lead is HOT to the battery it would be best to first disconnect the negative battery lead before removing the main lead and make sure it is isolated from touching anything after it is removed. If the current draw is still there then you have proved the alternator is okay and the hunt is still on. Disconnect other things that you suspect may be causing the problem. Once you find and disconnect the path the problem is on the current will drop and you need to trace the wiring to the problem. Looking at the wire colors that has the power on it may help you locate the other end of the wiring and the problem.


Search for a fiat club near you , just like my neighbors corvette club the resources are there to be used.


Disconnect the main power lead to the regulator and check for any draw between that lead and the regulator terminal.

An electrical problem on an old Spider is really not that difficult to sort out.


I have added the original Fiat 850 Spider wiring diagram.

As you can see (if interested), it is not particularly complex by today’s standards.

The alternator has a a ground wire, a grey cable to the regulator, a brown one to the starer and the “Alternator field circuit relay” (not sure what that does), and a yellow cable to the “Battery charge indicator relay”.

The last connection tells me that there there could be some sort of battery indicator light, but I can assure you that there is none.

I will also look for the fuse that it is located between the Regulator and the “Alternator field circuit relay” (grey/black wire), and test it if I can pull it off.

In any even, I will proceed per Cougar’s suggestion above and will report back.

I’ll keep you posted and thanks for all your help thus far.


BTW, I now have problems starting it. It sounds as it is flooded. Oh well, that’s an issue for another topic…


Barky, never heard of a tube type so doubt that’s what they had. They were multi meters though and usually used the ohm meter part.


gallant: I’m not sure about alternator availability for the 1973 850 Spider. A friend had a 1968 but I think it may have had a generator. My 1981 Bertone (continuation production of the Fiat X 1/9) had a Bosch alternator. The 1973 could possibly have the same but with Fiat who knows?


Bing, VTVM’s were for bench use only, they has to have 110VAC. The Simpson was used in the field.


sgtrock21, my 1973 Spider uses a Marelli alternator. I found a remanufactured one on eBay for about $300. I will test amperage drain on the one installed later today if the weather cooperates. This will hopefully determine if it needs to be replaced (I certainly hope not :-).


If the alternator / generator turns out to be the problem, I suggest consulting with an auto electric shop. Most midsize and bigger cities have them. They are usually locally owned, often long-standing, and have a wealth of knowledge and new and old parts to repair or replace your component.


That’s a great idea, shanonia. I much rather pay a local guy to rebuild my old one. If what an alternator contains is nothing more than copper, diodes and other electrical components, surely it could be rebuilt. And yes, I do live in a large metro area. I’ll google it.

I’m still unable to test the alternator as it is raining, cold and nasty outside today…


If the trouble is with the alternator you could replace it with one with a built in regulator. It is a very simple install. Since you say that you have no charge warning light you could install one and run power from the ignition switch through the warning light and then to the L lead of the alternator. Add a battery sense lead from the old regulator wiring and that’s it. The hardest thing is getting something to match your mounting points on the engine. It shouldn’t cost much either.