Distributor Ignition (DI) System Check. GMC Truck GMC C-Series Medium Duty Truck B7 1998

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 6.0L and 7.0L 🢒 Distributor Ignition (DI) System Check

Diagnostic Aids

A tach filter could cause an ignition system problem. If a voltage measured at the distributor 2 terminal connector (terminal C) with the ignition ON increases when the tach terminal connector is disconnected, the filter is faulty. Replace the filter.

Test Description

The numbers below refer to the step numbers on the Diagnostic Table.

Two wires are checked in order to ensure that an open is not present in a spark plug wire.

A spark indicates that the distributor cap, the rotor, or the coil output wire is the problem.

Normally, there is battery voltage at the C and + terminals. A low voltage would indicate an open or a high resistance circuit from the distributor to the coil or the ignition switch. If C terminal voltage was low but the + terminal voltage is 10 volts or more, the circuit from C terminal to the ignition coil or ignition coil primary winding is open.

This test checks for a shorted module or grounded circuit from the ignition coil to the module. The distributor module should be turned OFF, so the normal voltage should be about 12 volts.

If the module is turned ON, the voltage would be low, but above 1 volt. This could cause the ignition coil to fail from the excessive heat.

With an open ignition coil primary winding, a small amount of voltage will leak through the module from the battery to the tach terminal.

Applying a voltage (1.35 to 1.50 volts) to the module terminal P should turn the module ON and the Tach terminal voltage should drop to about 7 to 9 volts. This test will determine whether the module or coil is faulty or if the pick-up coil is not generating the proper signal in order to turn the module ON. You can perform this test by using a DC test battery with a rating of 1.5 volts. (Such as AA, C, or D cell.) The battery must be a known good battery with a voltage of over 1.35 volts.

This check should turn OFF the module and cause a spark. If no spark occurs, the fault is most likely in the ignition coil because most module problems would have been found before this point in the procedure. A module tester (J 24642-F ) could determine which is at fault.

Distributor Ignition System Check

Step

Action

Value(s)

Yes

Did you perform the On-Board Diagnostic (OBD) System Check?

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Check the spark plug wires for the following items:

•	Open circuits

•	Cracks in the insulation

•	Improper seating of terminals in the following areas:

-	Spark plugs

-	Distributor

-	Ignition coil

If you find a problem, repair as necessary.

Is the action complete?

Go to Step 3

If a tachometer is connected to the tach terminal on the ignition coil, disconnect tachometer.

Is the action complete?

Go to Step 4

Check for spark at a spark plug with a spark tester (J 26792 ) while cranking the engine (If there is no spark at one wire, try another one).
A few sparks and then a no spark is considered a no spark.

Is there spark?

Go to Symptoms .

Go to Step 5

Remove the distributor cap.
Crank the engine.

Does the ignition rotor rotate?

Go to Step 6

Go to Distributor Ignition (DI) System Check

Disconnect the 4 terminal distributor connector.
Check for spark at the ignition coil tower with the tester J 26792 while cranking the engine with a known good coil wire.

Is there spark?

Go to Step 7

Go to Step 8

Check for opens, damage, or wear in the following areas:

•	The distributor cap

•	The ignition rotor

•	The coil wires

Repair as necessary.

Is the action complete?