6.2L & 6.5L GM Diesel Glow Plug System Diagnostics

How to Troubleshoot the Glow Plug System on a 6.2L/6.5L Detroit Diesel

If you're 6.5L Detroit/GM diesel is hard to start, experiences a long crank condition, experiences a no start with white smoke condition, or produces excessive white/black smoke while cranking, you should suspect a glow plug system issue. White or black smoke indicates that the engine is getting fuel, but is lacking the heat necessary for autoignition (and therefore combustion) to occur.

NOTE - Jump starting a 6.2L/6.5L diesel should be avoided with the exception of emergency situations. Jump starting has been known to frequently cause glow plug system component failures.

6.2L and 6.5L Diesel Glow Plug System Diagnostics, Operation, and Troubleshooting

1982 - 1984 6.2L GM Diesels

The glow plug system on 1982 to 1984 model year 6.2L diesel engines consists of a thermal glow plug controller, a glow plug relay, a glow plug inhibit switch, and 8 individual glow plugs. The thermal controller is threaded into the water jacket and its circuitry utilizes a series of switches to cycle the glow plug system on and off until the engine is started. If the key is switched to the "run" position, the system will continuously cycle on, then off until the engine is started; the controller's purposes is to ensure that the glow plugs are cycled for the proper, safe period of time during the pre-heat cycle. The glow plug controller energizes the glow plug relay, which allows current to flow to each individual glow plug.

The glow plug controller senses pressure in the cooling system and disables the system entirely once the engine has started, save for a short post-heat (also after-glow, post-glow) cycle under certain conditions. Under normal conditions, the glow plugs are activated in 8 to 10 second periods during the initial pre-heat cycle. A glow plug inhibit switch mounted in the passenger side cylinder head at the rear of the engine disables the glow plug controller if the engine coolant temperature is greater than 125 °F. This switch/sensor is a common failure point and is often disabled by owners as the OEM part numbers are becoming obsolete. Although its function is to prolong glow plug life by disabling the system when the engine is warm, it can be safely bypassed by simply splicing both connector wires together; it is a normally closed switch.

1985 - 1993 6.2L & 6.5L GM Diesels

Beginning in 1985, the 6.2L diesel utilized a combination glow plug relay and controller. The same controller is used on all 1985 through 1993 6.2L and 6.5L diesels as well as all 1994+ naturally aspirated, non-electronic (fully mechanical injection pump, no PMD/FSD) engines. It is a 4 pin controller, which differs from the 3 pin controller found on 1994+ 6.5L turbodiesels with an electronic injection system (the controllers are NOT interchangeable). The glow plug controller is mounted on the rear of the driver side cylinder head, near the firewall. 1985 through 1989 model year engines also feature the glow plug inhibit switch, which disables the glow plug system if the engine coolant temperature exceeds 125 °F (see 1982 to 1984 above for additional information).

1994+ 6.5L GM Turbodiesel (electronically controlled injection pump, PMD/FSD)

1994 and newer 6.5L turbodiesels utilize a 3 pin glow plug controller, relay combination similar to that found on 1985 through 1993 model year engines. The controller is mounted to the driver side cylinder head at the rear of the engine (near the firewall). This 3 pin controller is not interchangeable with the 4 pin controller found on earlier models. Simply remove the electrical connector and count the male pins on the controller to verify you have the correct replacement.

For 1994+ 6.5L turbodiesels, the ECM uses the engine coolant temperature reading to assign an appropriate glow plug pre-heat cycle time. These engines feature two coolant temperature sensors; one is used for the temperature gauge in the instrument cluster while the other is used by the ECM only (located on the coolant crossover tube downstream of the thermostat housing). When the key is turned to the "run" position, the ECM activates the glow plug controller/relay for the determined period of time. The wait-to-start (WTS) light is illuminated during this time.

Note that under certain conditions these controllers will cycle the glow plugs AFTER the engine has started and is running. This is known as a post-cycle or post-start sequence (also after-glow in certain contexts). Each event should last 1 to 2 seconds and the WTS light should illuminate during these short cycles.

6.2L, 6.5L Glow Plug Troubleshooting

First and foremost, perform the following inspections:

• Check for loose or corroded connections/terminals on the glow plug controller (replace controller is present).
• Check for pitting or melted plastic around the terminals on the glow plug controller (replace controller if present).
• Check the fuse for the glow plug system (if applicable, refer to owners manual for fuse position).
• Inspect all wires and connectors. Replace any wires or connectors that are burned, melted, or otherwise bad.
• Relay chatter coming from the glow plug controller (glow plugs cycling on-and-off rapidly) is almost always a faulty controller.

The following tests can be performed in order to eliminate or identify possible glow plug system related problems. Process of elimination is your greatest asset in diagnosing glow plug system concerns.

Test Procedure



Test current draw during glow plug pre-heat cycle using inductive ammeter on either negative battery cable.

40 - 50 amps (nominal)

This tests only reveals whether or not the glow plug system is cycling or not. In our experience, you should read 40 - 50 amps at the negative cable of either battery during the pre-heat cycle. Current draw will vary by glow plug manufacturer and this is only valid for ACDelco glow plugs. If there is little to no current draw from the battery, the glow plugs are not receiving power. This is a good preliminary check. If more than 50 amps are being drawn, do not be alarmed. If less than 40 amps are being drawn, check each individual glow plug as they should draw a minimum 6 amps each at the start of a pre-heat cycle.

Test current draw during glow plug pre-heat cycle using inductive ammeter on each glow plug wire or individual bank of glow plugs.

6 - 10 amps per glow plug (nominal)

ACDelco 60G glow plugs (specs will vary for alternative manufacturers) draw between 6 and 10 amps during the start of a pre-heat cycle. You can therefore test each individual glow plug wire or the supply for each bank of glow plugs. This is useful in chasing a failed fusible link.

Disconnect ECM coolant temperature sensor (sensor on top of crossover pipe near thermostat housing, 1994+ model years only).


Perform this test only if you suspect that the glow plugs are not cycling as long as they should when the engine is cold. This sensor provides coolant temperature to the ECM and determines pre-heat cycle time. Disconnecting the sensor should cycle the glow plugs for the maximum possible duration.

Disconnect connector on glow plug inhibit switch and jump connector wires (pre-1989 model years only).


If the glow plug system works with the connector wires jumped, the inhibit switch needs to be replaced or bypassed. This is s normally closed switch which opens when the engine coolant temperature exceeds 125 °F.

Test for 12 volts at input terminal of glow plug controller (large lug, red wire. Check voltage from negative battery cable to lug).

12 v

No voltage to the 12v input terminal of the glow plug controller indicates a disconnected wire or blown fusible link on this circuit. This circuit should always have 12 volts, even with the ignition in the off position; it is fed directly from the battery.

Fusible link test - for any fusible link, test the resistance across the fusible link.

Negligible resistance

The resistance across the fusible link should be negligible. An unusually high resistance and/or no continuity indicates a blown fusible link. There is a fusible link on the glow plug controller, on each bank of the glow plug harness coming off the controller output, and on each individual glow plug wire.

Test the resistance/continuity between the left and right bank individual glow plug wires. Probe any glow plug wire on the left bank and any glow plug wire on the right bank.

Negligible resistance

If there is high resistance or no continuity between any individual glow plug wires on the left and right banks, a fusible link on the left or right bank harness (coming off the output of the glow plug controller) has likely failed.

Test the resistance/continuity between individual glow plug wires and the output terminal of the glow plug controller (only if the test procedure above has been performed first and the system passes).

Negligible resistance

If there is high resistance or no continuity between any individual glow plug wire and the output terminal of the glow plug controller, that glow plug's fusible link has likely failed.

Test resistance across each individual glow plug (across spade connector of glow plug and good ground on engine block).

0.8 - 1.8 Ω

If the resistance across any glow plug is out of spec, replace all 8.

Glow Plug Selection & Fusible Link Information

Two of the most important recommendations we can make is to 1) always use OEM ACDelco glow plugs, part number 60G. The quality is unmatched and aftermarket plugs not manufactured to the same quality standards or specifications have a tendency to swell and/or break off in the combustion chamber. For all intents and purposes, the OEM glow plugs are not particularly expensive. 2) Always replace all 8 glow plugs simultaneously. In theory, you need only replace a glow plug that is bad. However, if a single glow plug fails, the rest are not likely far behind. Even if there is not a failure, consider replacing the glow plugs preventively as a maintenance item. GM recommends replacing them at 100,000 mile intervals, however we prefer to cut that interval down to a maximum 60,000 miles. There's nothing worse than extracting a glow plug with a swelled tip or that is otherwise seized in the cylinder head.

A fusible link is nothing more than a fuse. It's a section of wire smaller in diameter than the circuit it is protecting and features specific material properties such that it will protect the circuit by "burning up" in much the same fashion a fuse does when the current draw for that circuit exceeds the protection limit. The fusible link will fail do to the heat created by the excessive current draw and create an open circuit, protecting equipment downstream of its location. The fusible link specifications for the 6.5L GM diesel glow plug system are as follows:

Fusible Link Location

Wire Gauge

Beginning at the spade connector for each individual glow plug

18 AWG

Left bank glow plug supply

14 AWG

Right bank glow plug supply

14 AWG

Glow plug controller input (circuit that is always hot)

12 AWG

Note - based on 2000 model year engine. Exact fusible link locations, colors, and specifications may vary. Always replace fusible links with the identical AWG size and length as that removed.

Fusible links should never be soldered; always splice the fusible link into place and cover the splice with heat shrink. Ensure that the length of the fusible link nominally matches the length of the wire that is being replaced.

Glow Plug System Part Numbers for all 6.2L, 6.5L GM/Detroit Diesels

Part Description

Part Number

Glow Plug

All model year 6.2L, 6.5L

ACDelco 60G

Glow Plug Controller

1982 - 1984 6.2L

Airtex 5S2126

1985 - 1993 6.2L, 6.5L

Airtex 1R1332

1994 - 2000 6.5L

Airtex 1R1372

Glow Plug Relay

1982 - 1984 6.2L

Standard SS591T

Glow Plug Inhibit Switch

1982 - 1989 6.2L

GM 23504005, ACDelco D1863CS [1]

ECM Temperature Sensor

1994 - 2000 6.5L

ACDelco 213-4396

[1] Part numbers may be obsolete for the glow plug inhibit switch - recommend bypassing switch altogether.