VW Diesel / Glow Plug System

Contents:

• Overview
• Details
• Operation
• Trouble Shooting
• References

Overview:

The VW Diesel glow plug system consists of a number of components:

• Glow Plugs
• Bus Bar
• Fusible Link
• Glow Plug Relay and socket
• Engine Temperature Sensor
• Ignition Switch

The purpose of the glow plug system is to pre-heat the combustion pre-chambers in the indirect injection engine for easier starting when the engine is cold. Once the engine is warmed up, the glow plugs are no longer needed and the system turns them off. On my '81 VW diesel, the glow plug relay is located under the dash, but it is not part of the main fuse panel.

There are two types of glow plug systems used by VW, commonly referred to as "slow-glow" and "fast or quick-glow". The parts to both systems are distinctively marked and should not be inter-mixed (see Details section, below):

• The "quick-glow" system is factory installed on all normally-aspirated diesel Jettas and '82-'84 Pickup and Rabbit models.
• The "slow-glow" system is factory installed on '81 and earlier Pickup and Rabbit models as well as all turbo-diesel (TD) engines. Its use on the TD engines is to avoid excessive smoke after a cold start that the "quick-glow" system can cause.
• This description was from a '77-'84 Bentley VW Diesel Service Manual, it seems that most, if not all, VW diesels had the fast glow system instaleld from '85 onwards.

Details:

There are two basic glow plug systems used in VW diesels, commonly referred to as Slow and Fast glow. Early engines and turbo-charged engines use the slow glow system while later, non-turbo sengines use the fast glow system.

Glühzeit-Steuergerät (in English "Glow Timer")

• Slow Glow:
  • Plugs:
    • VW p/n:
    • Bosch p/n:
    • Current draw: 9A/plug, silver colored terminal
  • Relay:
    • VW p/n: 321-911-621
    • Bosch p/n:
• Fast Glow:
  • Plugs:
    • VW p/n:
    • Bosch p/n:
    • Current draw: 12A/plug, brass colored terminal
  • Relay:
    • VW p/n: 171-911-261A-E
    • Bosch p/n: 0.281.003.099

Operation:

The glow plug relay (or timer) is a fairly simple circuit built around a quad low-power voltage comparator (LM2901), a high current relay, a couple of transistors, diodes, resistors and capacitors. The circuitry is designed to provide a couple of variable delays, after application of power via the glow and start positions on the ignition switch, based upon engine temperature, as follows:

1. The first delay controls the illumination of the glow plug LED on the instrument panel, used to indicate when the glow plugs have reached starting temperature.
2. The second delay is used to keep the glow plugs energized for a few seconds after the LED goes out, to provide supplemental pre-heating of the intake fuel-air mixture during starting.
3. A third delay is triggered when the ignition switch is turned to the start position, turning the glow plugs (back) on during the starting cycle. I have observed some relays that do not turn off after the starting cycle is done.

A temperature sender, mounted on the cyliner head, sends a variable resistance signal to the relay as an indication of the engine temperature. This is a fairly low resistance device, a few hundred ohms when cold, with a lower resistance as the temperatures rises. One potential problem arises if the sensor, wire or connection to the relay fails open, the relay may "think" the engine is cold all the time, and potentially run the glow plugs too long, shortening their life.

The glow plugs are electrically heated devices. They typically pull a very high current (30-40A when cold, tapering off to 9-12A when hot) at 12V and will glow red hot within a few seconds of applying power. A good test of operation is to use a 10A (or larger) battery charger to power the plug temporarily. If a red glow or high current is not observed, the plug has most likely failed or burned out.

TroubleShooting:

There are basically two methods for testing the glow plug system, in-place and out of the vehicle. I have experienced failures in almost all the components, so its best to isolate and test each component when problems are present. First off, signs of glow plug trouble include:

• Difficulty starting, increasing with decreasing temperature
• Rough running engine when cold, with lots of white/gray smoke smelling of unburnt diesel

These symptoms are signs of other problems, too, so more testing is warranted before replacing parts.

Fusible Link:

First item to check is the fusible link on the engine side of the firewall. Follow the heavy wire from the glow plugs to the fuse housing. Pull off the cover and inspect the link. I've had a burned out link and also plain old loose screws cause starting problems. If you have to replace this part, why not go ahead and pick up a few extra, they are inexpensive and might come in handy one day.

Glow Plugs:

There are several ways to test glow plugs. The way I use when I suspect one or more are bad is to remove them (disconnect the injector plumbing between the pump and injectors) and clamp a 10A battery charger on the plug. If its not glowing red hot in the correct time (7 or 30 seconds) it gets chucked in the trash. If it does glow, I keep it, but if any plugs have failed, I usually replace all 4 and keep the working units for spares. I also clean the copper bus bar surfaces before putting everything back together.

Relay:

The Glow Plug relay is in a 7-pin socket undder the dash, with pinouts as follows:

T = Temp Sensor, Input
L = Glow Plug LED, Output
30 = Battery Power, Input
50 = Starter Signal, Input
85 = Ground, Input
86 = Pre-glow Signal, Input
87 = Glow Plug Power, Output

All signals can be tested at the socket, with the relay removed. One somewhat common problem is that the plastic support tabs inside the socket can break, letting the contacts they support push inside the socket when the relay is inserted. This can be easily detected when you test for signals at the socket. Use the meter probe to push on each contact and see if its loose. If so, you can use epoxy to repair the broken support and then be careful when you re-install the relay.

References:

Here are some other references to the VW Diesel Glow Plug System I've found on the net:

• Martin Jägersand has an excellent collection of VW/Diesel information including the glow plug system