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Volkswagen

Volkswagen Electrical System Service: Cures for Common Ailments

Edited from an article by Bob Dowie, ImportCar Magazine, February 2000

You can’t argue with the success of the Volkswagen in the United States. Many import car makers have not been able to stand this test of time. VW has endured by providing its owners with a good, reliable product for the right price. Throw in German engineering and a good dealer network and you have a winner. Volkswagen also has been flexible enough to change directions and provide the right vehicle for the time. For example, when tightened emission controls was forcing out the air-cooled bug, the then trend-setting, transverse engine, front-wheel-drive Rabbit was just the right car for the gas shortage years. And, with all the excitement over the new water-cooled Bug, it’s safe to say that the Volkswagen nameplate will thrive for years to come, providing several repair opportunities for shops devoted to servicing the nameplate.

Although the VW electrical system is basically very reliable, factors such as age, weather and usage can take its toll on the vehicle’s central network, prompting a variety of potential problem areas. Some can be simple to diagnose, while others can be a real challenge. In this feature, we’ll take a look at some of the more common ailments.

The first thing your customer wants the car to do is start. Volkswagens have a couple of unique problems to watch out for in this area. First, the wires that go to the front doors may be broken. These are the wires that sense that the passive seat belt is connected to the door. After the door is opened and closed a couple thousand times, the wires inside the black flex hose in the door jamb tend to break.

The easiest place to diagnose this problem is behind the sidekick panel (this problem is more prevalent on the left side). After the panel is removed, look for the wiring that leads to the door. The wires in question are the two brown wires usually covered with a black plastic tube, and that plug into the main harness with a white plug. Unplug the wires and jump the plug at the harness. If the starter cranks, you’ve found the problem. A light tug on the wires going to the door will usually uncover the broken wire. Wires not broken? Check for bad connections or a bad switch in the seat belt receiver on the door, although the wires are a much more common problem. Of course, this problem would affect the starter operation of any vehicle — gas or diesel, standard transmission or automatic. The next one is exclusive to automatic transmission-equipped cars.

NO-CRANK CONDITION
The problem with the automatic transmission-equipped VW is a no-crank condition when underhood temps are high. I believe the problem results from a VW design change that involved moving the starter from a nice, cool, easy-to-get-at location on the front of the engine, to a hard-to-get-at, hot spot under the exhaust manifold on the back of the engine. The complaint will be something like, "I was on the interstate and stopped for gas. The car did nothing when I tried to start it, although it didn’t act like the battery was low." Your customer may have even tried to jump it to no avail, but then 20 minutes later, the car started like there was nothing wrong.

Chances are you have a voltage drop in the solenoid circuit. The problem with diagnosing this one is that you probably won’t be able to duplicate the problem in the shop. If you can get it to happen, give it this quick check. The first step is to locate the solenoid wire connector under the hood. It’s usually on the top of the bell housing. There you should see a black, heavier-gauge wire with a red stripe and a white connector that provides easy access.

If you can get the car warm enough not to crank with the ignition switch, jump from the positive terminal of the battery to the wire that leads to the starter. Your starter button is the perfect tool for this job. If the starter cranks with the button, but not the ignition switch, you have a voltage drop problem. If it doesn’t crank either way, and the battery and cables are in good condition (don’t forget to do the voltage drop tests under load — while trying to start car), you can suspect a bad starter/solenoid. A word of warning — use only the highest quality rebuilt unit and be careful when looking up the labor. There is a big difference between the standard and automatic labor.

If you couldn’t confirm the problem or found a voltage drop in the starter signal circuit, what’s the next step? Suggest to your customer that you add a starter relay in the solenoid circuit. To install the relay, you’ll need a simple four-wire relay. We use one that is rated for 30 amps and comes with a bracket. You will also need some wire and connectors. Two of the wires for your relay are the black/red wires we used for the test. The other wires will be battery positive and chassis ground.

The black/red wire coming from the ignition switch will go to the terminal that operates the relay, with the other side of that circuit getting your new ground wire. The black/red wire that goes to the solenoid will go on one of the switched terminals of the relay, with your new battery positive wire going on the remaining switched terminal. Remember that our goal is to get full battery power to the starter terminal of the solenoid. Use heavy enough wire on the switched terminals to prevent any voltage drop.

CHARGING SYSTEM
In order for the starter to operate properly, a fully charged battery is important. On VWs, you can’t always rely on the charging system warning light to let the customer know that the system is not operating properly. Many times when the alternator fails, the failure will disable the lamp, or the lamp will go out but the voltage drops will appear. It doesn’t do much good to make the power and not have it get back to the battery.

When testing a VW charging system, the first step is to confirm that the battery light is lit with the key on/engine off, and goes out when the vehicle is started. If you get no lamp with the key on, go to the back of the alternator and locate the blue wire that tells the alternator/regulator that the key is on. If there’s power at that lead (I test with an LED test lamp here), short the wire to ground. If the charge lamp is now lit, you can bet you have problem with the alternator or regulator.

It is easy to remove the regulator/brushholders on most of these Bosch units. Once the regulator is removed, it will be obvious if the brushes are worn. You will have to decide if it makes sense to replace brushes, install a new regulator or sell the customer a rebuilt unit. Of course, if you inspect the slip rings in the alternator and they’re all grooved and worn out, the new brushes won’t last long. I usually get the customer involved in the decision and many times he or she is more comfortable with the rebuilt unit.

No power at the blue wire? You will have to search for an "open" in the wire. As with any wiring problem, a diagram will be a big help. But before you get into it too deeply, look around the battery area for wires that have been damaged by corrosion or, in some cases, by the battery not being secured enough, allowing movement to damage nearby wiring. While you’re in the area, it’s a good time to take a look at the battery terminals and perform any cleaning or service that’s required.

What if we had proper warning light operation, but still had a low charge rate at the battery? In this case, VWs are no different than any of the cars we work on. The first step is to check voltage output at the heavy recharge wire on the alternator. Output OK there? If so, you have resistance in the recharge wires. Be certain that the positive terminal on the battery is clean and extra careful that the wires on the clamp bolt are clean and making a good connection. Check volts before and after the connection. Another possible problem can be the fuse links in the harness. You’ll find them in the main harness next to the battery. Again, watch for battery acid damage.

If output was low at the recharge terminal? Hook your positive lead from the voltmeter directly to the alternator case. If you found that missing voltage, it’s time to provide a ground circuit for the alternator. In most cases, a wire from the alternator case to the engine will suffice, but don’t forget that the engine needs a good ground to the battery and the body. Make sure you put the charging system under a load while performing these tests.

One last thing on the charging system. If you have a customer complaining of intermittent, low electrical power in the rain, be sure that the plastic shields that protect the belt are in place on the low-mounted alternators. Some of the early vehicles didn’t come with them, while others get left off during service. If you are in doubt, call your dealer and ask if one is available.

FUSE BOX/CONNECTION PROBLEMS
A quick look at a VW wiring diagram will show that the fuse/relay box is the heart of the wiring system. Everything is wired through this unit. Remember that blue wire that tells the alternator that the key is on? If you didn’t find an "open" under the hood, your next place to check is at the fuse box. It’s not really a bad idea, as it could give the tech a good place to perform many electrical tests at one location.

The problem is that getting to the fuse box can be tough. Access in some cars is worse than in others. The vehicles with passive seat belts have a knee pad under the dash that makes those cars a special challenge. Another problem with the fuse box location is it has made the fuse box susceptible to water damage when there are leaks in the windshield area. This problem will affect the relays, connectors and the fuse box itself. This was a bigger problem on the older cars that used the ceramic fuses. The newer boxes with the ATC type plug-in fuses seem to be more reliable, but problems are still being reported. If you’re diagnosing a problem that uses a relay located in the above-mentioned area (that may have been wet), remove the cover to check for water damage. While some look good from the outside, they can be such a mess internally that you’ll wonder how they worked at all.

Bad connections can also be a problem. In severe cases, you can replace the fuse box, but replacing the plugs on the harness is not an option. You have to rely on chemicals and small brushes to get them as clean as possible. A little dielectric grease will help protect them. Of course, the worst connections will be damaged by the heat from the excessive resistance of the bad connect. Careful examination of the wiring diagram may give you an optional circuit to use. If all else fails, a relay can be added to the box to control the affected circuit. Many older cars have had fuel pump relays relocated to the top of the fuse box.

Voltage drops are another problem that stems from this fuse box arrangement. If a customer complains that his or her lights aren’t as bright as they used to be, or that any other accessory is not working up to snuff, one of your first diagnostic steps should be to do a voltage drop test. If you find a drop, add a relay to the circuit like we did with the starter. Be sure to add overload protection (a fuse) to your new circuit.

One last thing on the chassis wiring. VW uses a relay-controlled circuit, or what it calls an X circuit, that provides power to certain accessory items including the blower fan and wipers. Its relay is sensitive to voltage and will open when voltage drops too low. If the complaint includes problems with accessories that are in the X circuit, be aware of the relay and what it does. The relay could be doing its job and the real problem is low charging rate.

DRIVEABILITY PROBLEMS
It looks like we’re down to electrical problems that will affect the way the car runs. Like most import vehicles today, many of the driveability problems on VWs are the result of bad connections. For example, the ground wires that attach to the intake manifold are prone to breaking. When repairing the wires, be sure to extend them to allow for engine movement.

Another problem area involves the familiar Bosch plugs that are used on the fuel injection system. Be sure that the wires are fully seated in the plastic housing. One method is to remove the plug for a visual inspection to ensure that the wires are in all the way. This is the preferred way to check since it gives you an opportunity to inspect the connectors for corrosion. An easier way to perform this check when the connector is not very accessible, is to carefully remove the rubber boot to expose the back of the connector. A wire that’s not seated will be obvious, and a slight tug on the wire will confirm if it’s loose. The coolant temp sensor is a critical input for all the injection systems. Always remove the plug to look for signs of coolant leakage. Replace the switch if there is any hint of coolant.

Now, I’d like to share a couple of things that I’ve learned from experience. The electrical part of the ignition switch will fail to return to the run position, resulting in a start-and-stall complaint. The tip-off here is that when the car stalls, the warning lamps either won’t be on or there will be a delay before they light. Sometimes this condition can be so intermittent that you will have to teach the customer what to look for.

The rheostats for the dash lights are also prone to failure, and since they are part of the headlamp switch, you’ll have to replace the switch. It simply pulls out of the dash. If the blower motor resistor fails (high-speed fan only), replace the blower motor and the resistor. It’s not very expensive from a parts or labor perspective. The reason the resistor fails is high resistance in the blower motor, so if the motor is not replaced, the new resistor will fail.

The last system is the control unit. We all know how reliable the Bosch computers are, in that they hardly ever give us problems. That is unless they get wet. VW has put its ECUs under the hood in the cowl area in front of the driver, right under the vents in the hood. To protect these units, there are a couple of thin plastic protective covers that direct water off the ECUs and heater to the cowl drains. There is no problem with this system as long as it’s in place. The problem comes when the covers are damaged or left off. The control unit won’t last long without it. If you remove the cover, be careful not to break it. If it’s broken or missing, replace it.

Hopefully, you’re better prepared for Volkswagen electrical service. Just remember that the fuse box is the heart of the system. Review the wiring diagram, noting that all the wires go in and come out of the fuse box. Volkswagens are popular and durable. That means that even as they get older, customers will want them to be fixed.

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