Electrics FAQ Options

[lapuwali]

General problem solving

A few things to remember:

All electrical circuits start at the battery + post and end at the battery - post. A particular circuit may take a very convoluted route to get from one to the other, but they ALL do this.

The battery - post is directly tied to the body, so the entire body acts as the "ground" for the system.

Paint is a great insulator. Even a small amount of paint will completely break the circuit.

Air is also a good insulator. Connections can't be loose.

Corrosion and dirt are good insulators. Connections have to be clean.

90% of the time, a problem is caused by a bad connector, not a bad wire.

The car runs off the battery. The alternator is just there to keep the battery charged.

The 914 (and most Porsches and VWs) follow a few simple rules in coloring wires. Brown wires are ground wires (from an item to ground, usually the body). Red wires are unswitched power wires (wires that don't go through the ignition switch). Black wires are switched power (wires that do go through the ignition switch). Note that some combinations don't follow this pattern. Red wires with a white stripe, for example, provide switched power to the gauges.

Divide and conquer is generally the best strategy. If something isn't working, start at the item and check the connections directly at it. Then follow the wires in each direction (one to ground, the other to + [perhaps through a switch]), checking each intermediate connection. Just looking at connections often isn't enough, as connections can frequently look fine, but not actually be properly connected (this is esp. true if the problem is intermittent). Try flexing any connections to see if insulation is broken, or if the problem item starts/stops working. If the wire disappears deep into a wiring bundle, just look at the other end of the bundle for that wire. As stated earlier, wires in the middle of a bundle are almost never the cause of a problem. On the 914, most of the wiring is fairly well exposed, and can be inspected all along its length.

If there are no problems with connections to be found, then the problem may be in a switch in the circuit. You can check to see if the switch is getting power by disconnecting the power in wire (there will be one, perhaps red, perhaps black, perhaps some other combo) and see if a voltmeter shows 12v between that wire and ground. If the switch is getting power, then check to see if the switch is passing power through it when turned on (check the power out terminal on the switch for +12 to ground, same as the power in). If it's not, the switch is the problem. If the switch isn't getting power, then the problem is "upstream", closer to the battery, so keep travelling up the wiring chain. This may lead to the fusebox.

Wiring diagrams can be hard to follow, but the divide and conquer approach works here, too. Just look for the item you're interested in (there's a key, all of the items are numbered), then follow the connections on the diagram to see where they go. A typical chain is:

battery + to fusebox to light switch to dash lights to dash gauge body to ground wire to ground stud

Once you find the dash lights on the diagram (crossed circles), you can follow the wiring from battery + to ground and see all of the intermediate connections. It may help to photocopy the diagram and use a highlight marker to follow the wires in one circuit.

If several things fail at once, look for common connections. For example, the dash gauges share a ground between the lights and the gauges themselves. The tach and the fuel gauge need a ground to work (the speedo is mechanical). There are two power connections, one for the tach and fuel gauge, one for the lights. If the tach and the fuel gauge and the lights don't work, it's more likely to be the ground, as there's only one ground, but two separate power connections. If the dash lights don't work, but the gauges themselves work, then it can't be the ground (or the gauges won't work), so it must be the power to the lights. If the headlights work, then headlight switch must be getting power, so the problem has to be inside the headlight switch itself.

[lapuwali]

The turn signal circuit is very confusing. How does it work?

(Many thanks to Jim Kelly [jk76.914] for this writeup)



There are three parts of the flasher. The primary relay (I'll call it) switches on and off and provides an on/off voltage to ANYTHING connected to it. This is controlled by the second part, a solid state timing circuit. When a load is detected, the timing circuit starts pumping out a little square wave to the coil of the primary relay, causing it to turn on and off.... By doing it this way, the frequency of the on-and-off is independent of the load, because the load on the timing circuit (ie, the relay coil) is always the same.

Now it gets interesting. Notice that the output of the primary relay actually passes in series THROUGH the coil of the secondary relay. This coil is a special kind of coil that is normally used in magnetic circuit breakers rather than relays. It is a CURRENT COIL, and the secondary relay will close only when enough CURRENT is passing through it. Relays generally have voltage coils that close when the correct minimum voltage is applied. The current coil is designed to minimize voltage drop, so it is only a few turns of relatively heavy gauge wire.

After it has passed through the coil of the secondary relay, it exits the flasher to be used for anything in the car that needs to flash- directional signals, hazard warning, emergency brake light, or (at least in my car) the fasten seatbelt light.

The secondary relay will create a second flashing output, but only if enough current is passing through the primary flashing relay. The threshold current that trips the secondary coil will cause a secondary flashing output when the turn signals or the hazard flashers are engaged, but not when the emergency brake light or the fasten seatbelt light is engaged. They still flash because they are attached to the primary relay, which is oblivious to load, but their load isn't enough to cause the secondary output to flash.

Why bother? Well, the law required a cockpit-visible indication of a burned out direction signal bulb. So if you turn on your left signal and the front bulb, say, is burned out, the rear signal will still flash, but there will be no secondary flashing output. And, you guessed it, the secondary flashing output goes to the indicator light on the dash.

You can actually hear it if you sit in your car with the engine off, but turn the ignition on. Pull up on the emergency brake and listen, and then turn on one of the direction signals. The click gets audibly louder because both relays are clicking instead of just the one.

I drew the '73.5-'76 circuit first, because it's easier to figure out. This secondary flashing output goes directly to the only turn signal indicator in the tach (via the blue/white wire). The other side of that indicator is grounded.....

So, the driver-visible indication of a burned out bulb is- when the turn signal indicator works fine for one direction, but remains dark when you switch to signal for the other direction.... There is a bulb burned out on the latter side. Or something else that can reduce the current flow below what is required to trip the secondary relay... Like corroded contacts, corroded socket, wrong type bulb (not bright enough), bulbs replaced with LEDs, whatever that reduced the current below the flashing threshold.



Thankfully, the flasher works the same as in the later models that only have one indicator, which was covered in my earlier post.

When you look the indicators, it gets weird- The same blue/white wire goes to the tach, and it hooks to BOTH bulbs. The other lead of each bulb, instead of going to ground, actually goes to the turn signal circult that goes off to the bulbs themselves. BUT WAIT!!! The LEFT indicator is connected to the RIGHT circuit, and the RIGHT indicator is connected to the LEFT circuit.

If you study what's going on, you see it. When the left turn signal is on, the flashing signal trailing out to the bulbs is +12v/0/+12v/0/+12v/0 and on and on. The same signal is coming out from the secondary relay to both indicator lights. This signal is in phase with the primary flashing signal, so the indicator that is connected to the left turn signal WILL NOT FLASH because it will see +12v on both leads at the same time, or 0 on both leads at the same time. There will never be a voltage differential that will cause it to light up. And which indicator do you NOT want to see flash when the left signal is on? That's right- the RIGHT one.

So what happens with the left indicator? It flashes! Because one lead is connected to the same secondary flashing output that the other indicator is connected to, but its second lead, which is connected to the RIGHT signal circuit IS GROUNDED THROUGH THE TURN SIGNAL BULB FILAMENTS THEMSELVES! (let sink in)

This grounding scheme works because the indicator bulb is much lower wattage (meaning much higher resistance) than the main signal bulbs, so it lights up and limits the current in the circuit to a level below which the main bulbs turn on.

So that's pretty tricky, but it's not over yet! What happens when a bulb is out? Well, the secondary relay in the flasher stays open, as before. But what a weird-ass result that has- BOTH INDICATORS FLASH TOGETHER when the side with the missing/wrong/corroded/LED/whatever problem is turned on.

If you look at the circuit, you can see what's going on- with the output from the secondary relay floating, you have both indicator bulbs, in series, completing the circuit from the side that's on (+12v or thereabouts) to the side that's off, and grounding through the filaments of the big signal bulbs again! The indicators are a bit dimmer, because they're wired in series, but they flash together.

If you replaced your indicators with LEDs, this cannot happen. The back-to-back series LEDs will block current flow. In this case the indicator that a bulb is out is that the indicator LED on the side with the bad bulb will remain off when that side's turn signal is turned on.

So that's about it. These two installments show the before and after circuits for incorporating early dual indicators into late tachs... Plus enough info to dispel myths that are drifting around about grounding problems, diodes in the harness, etc, etc, etc.....

You can now also figure out how to diagnose problems better. For example, if the dash indicator light is on, but doesn't flash, it has to be a bad flasher. If the signals flash, but the indicator is steady, it's the secondary relay in the flasher. If both stay on, it's the primary relay in the flasher (probably) or the solid state timing circuit in the flasher (unlikely). etc, etc, etc....