ERRATIC LIGHTS/DIGITAL DISPLAY ELEC. SYSTEM DISTURBANCES

MODELS AFFECTED: 1980-1985 CADILLACS EQUIPPED WITH DEFI, DFI, CCC OR CIMARRON EFI

A number of engine performance conditions, usually of an intermittent nature, can be caused by disturbances in other parts of the vehicle's electrical system. Voltage surges, intermittant opens and voltage transients can affect ECM or DFI system operation. Symptoms may include flashing or erratic telltale lamps and/or digital displays, engine quits, idle surges, poor performance, or disengagement of Cruise Control. This condition is sometimes called an ECM reset since these disturbances may cause erratic ECM operation until the ECM recognizes the fault, and "resets" itself, and resumes correct operation. Such erratic operation is usually of a temporary nature and does not damage the ECM.

This bulletin summarizes possible causes of ECM resets, and provides specific information relating to each of these possible causes. The following causes of ECM resets may not be all inclusive, and are not meant as a step-by-step diagnostic procedure. Use your knowledge of the vehicle's history and observation of any unusual vehicle equipment or replacement parts as a guide in diagnosis.

When diagnosing an ECM reset, you should first assure yourself that it is not an authentic intermittent code in the case of "Check Engine," or "Service" lights, or a simple ECC or fuel data circuit problem if the comment is intermittent displays.

When diagnosing an ECM reset, keep in mind that all resets originate as:

1. Power and ground loss to ECM.

2. Large voltage variations on ECM connected power and ground wires.

3. Large voltage variations or transients causing false signals on ECM connected low voltage signalline.

The vehicle history will often lead to a fix. Try to determine:

1. When did the condition begin?

2. Were any accessories recently added?

3. Was the car recently "tuned up"?

4. Was the car rebuilt after a theft or collision damage?

5. Has the car had a starter, generator or battery replaced recently?

6. Has there been heavy engine work done that may have disturbed power, grounds or sensor connections?

CONDITION:

---------- 1. "Check Engine," "Service Now," "Service Soon" or "Coolant" lamps flickering or intermittent. No codes or multiple codes. Commonly occurring with flickering "Check Engine" or "Service" lamps are the following stored codes:

A. 23 (1980-1985 DFI) B. 25 (1982 DFI) C. 51,52 (1982-1985 DFI) D. 42 (1981-1984 CCC and Cimarron EFI)

2. MPG or fuel data display goes blank or is erratic, fuel used and average MPG may reset to zero. If outside air temperature is being displayed, ECC head may blank (all DEFI and DFI, 1980-1985).

3. In diagnostics, the ECC head goes blank, may exit diagnostics, display only two dots ("snake eyes"), may reset to ".7.0" .or ".1.8.8" (all DEFI and DFI, 1980-1985).

4. Intermittent rough running, idle surge or high idle speed, random engine quits (all DEFI and DFI, EFI, CCC, 1980-1985).

5. Intermittent cruise control disengagement accompanied by one or more of the above symptoms (all DFI 1981-1985).

CAUSE:

----- All of the above can be caused by ECM resets due to:

A. Loss of power or ground to ECM. B. Large voltage variations or transients on ECM power or ground wires. C. Large voltage variations or transients on ECM connected signal wires such as MAP, BARO, EST, or TPS.

CORRECTION:

----------- A. Loss of Power or Ground to the ECM

Check:

1. Loose battery cable (all models).

Additional symptoms may include radio clock reset, headlamp dimming or slow cranking. Check for proper tightness of both positive and negative connections, and check for leakage at battery terminal or terminal corrosion.

2. Loose junction block connections; the point at which the fusible links are connected can create arcing when loaded or when opened intermittently on bumps, acceleration, and deceleration.

a. The 6.0L (368) engine, the 1981 4.1L V-6, and 1982-1985 four-cylinder Cimarron all use starter solenoid battery post as a junction block. Check the fusible links for corrosion and proper termination, and check the solenoid stud for tightness.

b. Check the jump start junction block underhood on 1982 V-6 and 1982-1985 HT4100 equipped Cadillacs. Check for tightness and check the fusible links and cables for corrosion and proper termination.

3. Loose engine grounds. False codes 23 (DFI), 25 (1982 DFI), 42 (CCC and EFI), 52 (DFI), 13 (all models) can be set.

a. Check both sets of engine grounds at engine right front under the generator. Check the braided ground strap on the ground stud toward the front of the car. (1980-81 V-8 and 1981-82 V-6).

b. Check engine grounds at cruise power unit (1980-1981 V-8 and 1981-1982 V-6).

c. Check engine grounds on engine right side. One ground stud is at the front of the engine under the rocker cover, one ground stud is on the AIR valve bracket. (1982-85 Longitudinal V-8)

d. Check for loose or improperly connected engine and body ground attached to the engine cradle near the starter and grounds attached to generator. (1985 FWD "C")

e. Cimarron; check grounds on engine above front side of transmission, on the rear of engine below distributor, and on cowl left side at tach filter.

f. All models - ground to fender from battery negative cable.

4. Check ECM edgeboard or header connectors for proper termination and verify all connectors are securely latched.

a. Check for steady battery voltage on ECM power circuits with a digital voltmeter, engine running.

b. Check for voltage on ECM grounds, engine running. Measure voltage with voltmeter; place one probe on the ground terminal of the ECM and one at the main engine ground. The voltage should be 0 volts +/- .10 volts. As a rule of thumb, voltages out of this range indicate poor connection to main engine ground, see checks in Item A, Step 3 above. Examine the ECM ground circuits at the bulkhead connector,look for pins bent over or backing out of the connector.

B. Large Voltage Variations on Vehicle Power and Ground Wires

On all models, check for:

1. Non-OE electrical accessories such as radios, theft alarms, horns, etc., that are tied to vehicle power and ground. Large power surges can affect ECM operation.

2. Monitor generator output at generator. Large or rapid variations in generator output can cause ECM resets. Generator overvoltage or voltage spikes due to poor regulation will be difficult for the ECM's voltage compensation circuits to follow. Rapid voltage variations or high frequency "noise" may not show up on a voltmeter. Generator output voltage may be observed using "special patterns" function on many shop tune-up scopes. Look for signs of generator faults such as "no charge" lights, battery undercharge (black test indicator or eye) battery overcharge (yellow test indicator or leaking electrolyte).

3. Inoperative or intermittent switches, relays or solenoids.

a. Actuate switches or use output cycling feature of self-diagnostics and try and relate the symptom to solenoids such as EGR, AIR, TCC, Cruise, CCP or A/C compressor turning on.

b. Actuate switches that control relays such as power seat, rear window defroster, fog lamps, etc. If the symptom occurs as a solenoid or relay turns on or off, turn the ignition key off, unplug the suspect solenoid or relay and repeat the test.

c. Replace any suspect solenoids, relays or switches, check circuits to suspect solenoids, relays or switches for intermittent open. An open in a power or ground supply to a solenoid can cause an inductive surge or "noise" on ECM connected wiring.

C. Large Voltage Variations on ECM Connected Signal Wires

An arc or power surge in a wire near an EC M connected signal wire can "induce" a voltage that will travel back to the ECM. The ECM may shut down or "reset." The engine can quit due to a "reset" or due to false sensor signals provided to the ECM. On all models, check:

1. Ignition Secondary -

a. An arcing plug wire, spark plug, cap, coil, or rotor can cause transients in EST lines, MAP lines, TPS wiring, etc.

b. EST, MAP, TPS, BARO sensor wires misrouted near spark plug wires or distributor cap. Check EST wire shielding continuity to ground. Compare wire routing to known good car, correct as required.

c. Non-OE ignition "tuneup" parts. Not all aftermarket ignition parts may not meet GM specifications for transient suppression. Check for:

i. Non-OE or non-resistor type spark plugs.

ii. Non-OE or non-OE equivalent plug wires, especially those which are a solid core wire or non-suppression type wire. An OE or OE equivalent plug wire may degrade and cause arcing with time and miles. Replace suspect wires before continuing diagnosis.

iii. Distributor rotor should have a dark colored suppression coating on the rotor tip. A rotor without the coating will have a brass or silver colored tip and could induce transients on the EST wires.

iv. The brush that provides contact from distributor coil to rotor on V-8 and V-6 engines should be a carbon material with a copper tip. The brush sits in the cap beneath the coil.

A brass colored metal brush or brass brush with a dark coating can cause transients in EST wiring of a sensitive car. These low resistance metal brushes come with many aftermarket and OE distributor caps.

Measure the brush resistance using an ohmmeter. Place one test lead on the copper tip and one test lead at the spring that contacts the coil. The brush resistance should be about 4000 ohms. If the brush resistance measures less than 3000 ohms, discard and replace with a carbon type brush.

Check the brush and rubber coil seal for proper installation. Install the brush into the cap, then the seal and then the coil.

Brushes are available through GMWDD as "Brush, Distributor Resistor," P/N 1989880.

From about 2/83 to 4/84, service stock of the brush included with "distributor cap" P/N 1977046 and the "Distributor Resistor Brush" P/N 1875967 included less than 3000 ohms resistance. The P/N 1875967 brush was superseded on 4/84 by P/N 1989880 with proper resistance.

Check service stock when replacing the brush or distributor cap on a DFI equipped Cadillac.

See also Service Information Manual, Engine Electrical Section.

d. On all models, check for an excess of silicone grease inside the distributor cap, on the distributor tower inserts. The grease is applied inside the plug wire boots for ease in installation. An excess of grease can run down inside the cap onto the spark tower inserts and promote arcing.

e. Check HEI coil secondary grounds. On all DEFI, DFI and V-6 CCC models, check the secondary ground path per Serviceman Bulletin T84-52, filed in Section 6C. On Cimarron, check coil mounting and engine ground on engine block near distributor, check phillips head screw retaining the EST harness in the distributor base.

2. Check MAP and BARO Sensor Mounting -

If the bracket is grounded, it may act as an antenna for radio signals, especially 450 MHz and 800 MHz from radio and radiotelephone systems. If electrical interference is suspected, change MAP and BARO from Bendix to Delco styles if applicable. See Serviceman Bulletin T84-9 for early 1984 Eldorado and Seville 50 MHz interference correction.

3. Check on-board HAM, CB radio and radiotelephone installation. Route power feeds and antenna wires away from sensitive sensor wires.

4. On 1981 DFI cars, check O2 sensor for proper connection, proper ground or intermittent connection. In the 1981 cars, if the ECM sees a negative O2 sensor voltage, the check engine light will flicker. See Serviceman Bulletin T82-46.

5. On 1982-1984 Cimarrons and 1985 FWD-C cars, check for an arcing electric EFE grid.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.