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Low Resistance Between Separate Circuits

09-14-2022, 07:23 AM
Car is a 1986 Daihatsu Skywing (Taiwan only model, like a Mk2 Charade) which recently suffered reverse polarity. Alternator seems unsurprisingly dead, but my main current concern is the wiring loom.

I took all the 10 panel fuses out, and (with battery and alternator out and key on) checked continuity between the half-circuits

Excluding earths (some of which could of course have been shorts-to-earth) I found no dead (buzzer-triggering) shorts between circuits, which was nice

BUT there were quite a lot of worryingly low values of resistance, about 7 in the 3-400 ohm range and 7 around 100 ohms for the "upper" half of the panel (10X10 matrix minus 4 earths and selfs) and 4 around 700 ohms for the lower half of the panel. (10X10 minus 4 earths and selfs)

I assume this is not normal and that these are partial shorts, say between melted wires, or between tracks behind the instrument panel, but I havn,t done this before so am not certain.

Does this interpretation seem reasonable?

Visual inspection has so far revealed no obvious damage, but I havn't done much dismantling yet.

09-14-2022, 07:19 PM
It may be mostly normal, or there may have been a few devices with reverse polarity protection diodes that have now been compromised. In the worst case condition of what you measured though (100 ohms), those circuits would only be drawing around 140 mA. Those could be light bulb circuits, or that might have just been electrolytic capacitors that were discharged from the reverse polarity application. Were the measurements you took rather static, or where they moving around a bit while the meter was connected? If the readings were moving around quite a bit, that may have just been from the meter charging up capacitors.

If it were my car, I'd keep all the fuses out, then install a test light in series between the negative battery cable and the negative post of the battery and see if it glows brightly or if it dims after it's been connected for several seconds. It doesn't necessarily need to go out completely, but shouldn't be as bright as if you were to connect it across the battery terminals directly. Assuming it does dim, then I'd connect the battery cables and do a similar test across each of the fuse terminals in the fuse box. See if you find a few circuits that glow really brightly. If so, leave those fuses out for now and install the rest to see if they blow. I'd then use a wiring diagram to determine what is on the circuits that cause a bright glow of the test light. Start to track down the modules in those circuits and test the modules/components for low resistance to ground. If you find they are only bulbs, then it's probably a non-issue.


09-14-2022, 08:38 PM
DP sorry. See below

09-14-2022, 08:52 PM
Thanks for your reply.

Car has about a 4 amp key-on drain (measured with a DMM in series with a rather tired motorcycle battery, (used in a half-assed attempt to potentially limit current in a disaster, and because it gave me more fiddling space). I pre-tested this with a 10 amp fuse which didn't blow.

This drain is only very slightly affected by fuse removal. However, disconnecting the ballast resistor on ignition coil abolished it, making me thing it might be normal ignition circuit drain.

It seems to be stable, as did the connectivity measurements between un-fused circuit halves, but I didn't leave it connected for very long.

I will have another look at it in line with your suggestions this morning. I dont have good circuit diagrams for this car but its probably mostly the same as a Charade 2. It has a few taped off wires and open terminals of unknown function, suggesting its been messed with in the past, before I broke it.

Is there any special reason for using a test light here rather than a DMM (apart from not wanting to blow a DMM)?

Perhaps its desirable to load the circuit a bit?

09-15-2022, 06:50 PM
Using a test light will smooth the inrush current and provides a rather easy visual indication when the offending high-draw circuit is identified. It has the added benefit of not blowing several fuses during the diagnostic stage.


09-15-2022, 10:47 PM
Circuit 5 (15A wiper/turn) has 0.15A, with a clicking relay sound when energised
circuit 7 (15A Gauge) has 0.42A.

The latter fuse was blown, so I suppose this circuit is an especial suspect for short-to-earth damage. It is also the only circuit that shows continuity-to-earth on both halves of the circuit with the fuse removed.

Temperature gauge and radiator fan control switch relay earth circuit have been out for a while, before I reversed the polarity accidentally while investigating.

Similarly the fan relay solenoid live side was out, and I had lost one (out of 3) indicators on the drivers side, and 2/3 on the passenger side. Bulbs and bulb holder contacts seemed OK.

Now (post-reversal) I have no indicators working on the RHS and the hazard switch no longer does anything.

Cant really interpret the turn signal and hazard switch circuit diagram, but it seems more sophisticated than I'd expect a trad bi-metallic thermal switch to be and seems to show some (presumably rather small) "black boxes", labelled "Open detecting circuit", "oscillating circuit", and "relay driving circuit".

There are 3 capacitors shown but apparently no diodes.

09-18-2022, 08:41 AM
It is also the only circuit that shows continuity-to-earth on both halves of the circuit with the fuse removed.

What is meant by this? With the fuse removed are you measuring continuity to common ground on both fuse terminals within the fuse box? If so, that indicates that there is still something in the primary/feed side of the circuit that is shorted to ground. One side of the fuse should go directly to the battery, or possibly through a main fuse. If you're measuring a short to ground on both fuse terminals, then there's still another circuit with fuse installed that has an issue.

Regarding the turn signal switch diagram, it sounds like your car has a "Bulb Out" warning system that monitors current draw in the various bulb circuits and will provide an indication if it determines a bulb is not drawing the expected amount of current. That would be the "Open detecting circuit." The "oscillating circuit" would be the flasher, which might be electronic rather than a simple bi-metallic strip flasher. I'm not sure what the "relay driving circuit" would be, unless there was a trailer wiring circuit option as those often use relays to prevent the trailer lighting from loading the standard vehicle wiring. Using an electronic flasher and having a bulb out circuit would be pretty unusual for a 1986 economy vehicle.

Do you have a link to the wiring diagram you are referring to that shows 3 capacitors that you can share?


09-20-2022, 10:26 PM
Thanks for your reply. Yes, I thought the "two earths" thing suggested that one of them was probably a short, though I'm unclear why it would have to be via another fused circuit. I suppose the one with the higher resistance would be more likely to be the short.

Immediately after the polarity reversal, on (unwisely) restoring power to the whole system to test the alternator, there was a faint buzzing for a minute or so which sounded like the "lights on" alarm, followed by ominous silence.


Should link to an "album" on Google Photos with turn signal circuit diagram, fused circuit description and fuse panel images.

I don't know for sure the Skywing is exactly the same (diagram is Mk2 Charade), but externally at least, it seems identical.

Weirdly, the alternator (which showed slightly less than battery voltage post-incident so wasn't generating) has now apparently recovered. I took the voltage regulator off to diode-test the rectifier but can't see how that could have fixed it. One meter shows about 30VAC "ripple voltage"(assuming I'm reading it right) but the other shows 0, so I'm inclined to ignore that.

09-21-2022, 05:02 PM
If you're measuring a low resistance path to ground on both fuse terminals, with the fuse removed, that definitely would suggest there's still something wrong in another circuit besides the one that the fuse was pulled for. If all other fuses were out and the hot/supply side of the fuse was measuring low resistance to ground, then the issue would be somewhere between the battery positive terminal and the fuse. A higher resistance would be less of a short, so the primary concern would be a circuit that measures less resistance to ground.

According to the wiring diagram for the flasher circuit, there appears to be an 8-pin DIP integrated circuit (IC) that controls the turn signal flashing and hazard flashing. That block diagram shows a PNP bi-polar junction transistor (BJT) driver for the relay, and the collector of that BJT is tied directly to battery power via the 15 amp fuse when the ignition is on (or through the Horn fuse if the Hazards are active). There's a pretty good chance that BJT or other components within the IC blew from the reverse polarity.

Do you have access to the turn signal flasher to confirm you have battery voltage to terminal B, ground at terminal E, and a pulsed signal at terminal L when the ignition is in the Run position? You most likely have some power to terminal B and a ground to terminal E since the flasher is clicking. Since the hazard switch does nothing though it would seem you're never getting a pulsed signal to terminal L. Are you saying that at least some indicators on the LHS still work?

Regarding your alternator, I hope you measured more like 30 mVAC of ripple. 30 VAC of ripple, when rectified, would still be around 21 VDC (RMS) which would be dangerously high for the 12V charging system of your car.

09-24-2022, 01:17 AM
Thanks for your reply, and for the loan of your expertise.

After a bit of fiddling (cleaning, greasing contacts) the indictor status is

LHS Flash speed “Normal” RHS Hyper-flashing (presumably due to low R)

Front OK OUT
Wing OK OK (faint)
Tail OK OK
As before, Hazard flasher no longer works.

The two DMM's I have are undocumented in English, so I dunno what the units are, but if the scale is normally in mV that's probably what they'll be.

Rhino DT700D meter has VAC switch positions labelled 750 and 200, and reads 31 post to case, 30.8 at bat. (on 200 range.)

The Hila DM-3200 has VAC switch positions labelled 600 and 200, and on the latter reads 0.1 (mV?) so I’m inclined to ignore the “ripple voltage”.

Unfortunately I'm going to be out of the country for a few months, so getting the heap through inspection (or not) will have to be deferred until my return, assuming it isn't scrapped by the authorities meantime.

Thanks for your help, which is greatly appreciated.

09-25-2022, 10:38 AM
The hazard switch pretty much only determines where the flasher circuit gets power and directs power to both sides of the vehicle. It appears to be purely mechanical, so if the hazards are not working, that's probably not due to the reverse polarity of the battery. Similarly, since bulbs on each side of the vehicle work for the turn signals, that further suggests the electronic flasher is not the problem, but rather something in the wiring.


09-25-2022, 10:45 PM
There were pre-existing issues with the wiring loom, which I was "investigating" when I managed to reverse polarity.

I had to run an additional wire from the coolant temperature switch to restore cooling fan control via the relay, but then the power suppy to the relay failed. Temperature gauge stopped working, and my horn, though the latter works when directly jumpered. It was starting to look like a progressive wiring loom failure.

Whether the polarity reversal has accelerated the deterioration, perhaps by causing shorts between circuits, I'm not sure.

To pass inspection, I could possibly "piggy back" supply to the dead lights off the functioning circuits, via a relay if necessary.

The alternative seems to be a fairly extensive rebuild of the wiring loom, which may be unavoidable.

02-03-2023, 10:40 PM
Back in Taiwan trying to get car ready for its overdue inspection

I replaced one working but badly silvered LHS bulb with a 12V LED because that was all the shop had, and the disintegrated wing bulb holder with some small (I think 5ma 3.2 V) LED's in series with a 200ohm resistor in a connection block. 4 LED's worked jumpered directly to a fully charged battery but could only get 2 in situ on the car, presumably due to voltage drop through the dodgy wiring. These LED substitutions didn't seem to affect the flash rate

Piggy backing a non-working side RH light off the front light for Power and Earth caused both sides to flash, I suppose because this earth wire was shorted to LH power. Giving it its own earth wire to a nearby body screw stopped this happening.

However, the flashers are also supposed to be running/sidelights, I dunno how this function is achieved but on the RHS it isn't

The hazard switch now works. Activating this in the running light position gives a non flashing, but rather dim, running light on both sides.

That;s almost enough, but the inspection people may not agree.

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