ISC/TPS ADJUSTMENT SYMPTOMS AND SYSTEM OPERATION
MODELS AFFECTED: 1980-1983 ELDORADO AND SEVILLE 1981-1983 DEVILLE AND FLEETWOOD BROUGHAM
Dealer comments and warranty claims indicate that a number of ISC-TPS adjustments are being performed on 1980-1983 Cadillacs equipped with Digital Fuel Injection. This bulletin provides information concerning what symptoms can be caused by a misadjusted TPS or ISC, and some symptoms that are not related to misadjustment but are often misdiagnosed. It also provides information on ISC-TPS system operation and proper adjustment procedures.
I. ISC-TPS SYSTEM OPERATION
The information provided below will assist in diagnosis of ISC and TPS misadjustments:
A. TPS voltage is used to limit minimum and maximum ISC plunger travel. This keeps the throttle lever from touching the minimum air rate screw at the low end, and from ratcheting at the high end.
B. Below 10 MPH, idle RPM is controlled by the distributor reference signal within the TPS limits described above.
C. The TPS voltage is used to set the throttle angle when:
a. the vehicle is above 10 MPH with closed throttle switch, or
b. whenever the engine is not running.
D. Changes in the TPS voltage are used for acceleration enrichment similar to an accelerator pump on a carburetor.
E. The TPS voltage is used for power enrichment. Since power enrichment occurs before wide-open throttle, the ECM will still command power enrichment by the time wide-open throttle is reached, even if the TPS is misadjusted several degrees.
F. Minimum air rate is used as a reference throttle angle to set TPS. The throttle position should never reach the minimum air rate setting during normal operation.
II. SYMPTOMS LEADING TO UNNECESSARY ADJUSTMENT
Currently, many ISC-TPS adjustments are made because of symptoms that are not related to ISC-TPS misadjustment. Listed below are common symptoms leading to unnecessary adjustment and the reasons why ISC-TPS misadjustment is not related.
A. Rough, Surging or Erratic Idle:
Idle speed is controlled by RPM, not TPS, when the vehicle is not in motion. A misadjusted ISC could possibly cause constant high idle, but not an unsteady idle.
B. Poor Performance, Hesitation and Detonatio
The TPS is used to set ISC position at speeds above 10 MPH during closed throttle. Acceleration enrichment is controlled by how quickly the TPS voltage increases. Therefore, a misadjusted TPS will not affect performance. On 1981 DFI vehicles, however, hesitation may result from TPS misadjustment due to the non-linear resistance curve of the 1981 TPS.
NOTICE: 1983 Service Information Manual page 6C-3, Hesitation Diagnosis, Step 7 and 1982 Service Information Manual page 6-3, step 9 should read "check for binding TPS", not "check TPS adjustment."
Diagnosis of these symptoms, which are unrelated to ISC-TPS adjustments, may be found in the 1983 Service Information Manual engine diagnosis section, pages 6C-1 through 6C-4. This diagnosis is applicable to an 1980-1983 models (some components are not used on past model vehicles).
III. SYMPTOMS OF A MISADJUSTED ISC-TPS
Although symptoms caused by ISC-TPS misadjustment are infrequent, there are certain symptoms that indicate this condition:
A. TPS Related
1. Fast Idle, Ratcheting, Code 30: A low TPS voltage may cause a fast idle by allowing the throttle angle to reach the low TPS limit too soon, preventing the ISC from retracting any more.
A low TPS voltage may also cause ratcheting when the engine is not running. Because the ISC must extend to the desired TPS value, a low TPS voltage may cause the ECM to extend the ISC until it ratchets without ever reaching this desired TPS value.
B. ISC Related
1. Fast Idle, Code 30: ISC adjusted out too far. The ISC is unable to retract far enough to obtain the commanded RPM.
2. Ratcheting, Code 30: ISC adjusted in too far. The ISC is unable to extend far enough to obtain the commanded TPS reading when the engine is not running.
C. Minimum Air Related
1. ISC Inopeative, Possible Code 27: Minimum air screw is adjusted out too far. This prevents the throttle lever from contacting and closing the ISC throttle switch. Under these conditions, the ECM does not use ISC for idle control. The ISC will not extend unless engine reaches the "save stall" condition. Use of the DFI throttle switch test and status light will verify proper throttle switch operation.
2. Minimum Air Adjusted Too Low: No affect on operation of the vehicle but could cause incorrect TPS setting when adjustments are made.
IV. ISC-TPS ADJUSTMENT PROCEDURES
The symptoms above are the only symptoms that indicate misadjustment of the ISC or TPS. If ISC or TPS misadjustment is suspected, verify the following conditions exist before checking or adjusting the ISC or TPS:
A. Verify that no codes are displayed. Unless the trouble tree for the code calls for checking the ISC or TPS adjustment, repair any codes before adjusting the ISC or TPS.
B. Repair any vacuum leaks before checking minimum air. Leaks such as those caused by a loose intake manifold will affect idle speed. Refer to Serviceman Bulletin #83-88 (May, 1983), Group 6A, for additional information on HT4100 intake manifold leaks.
C. Make sure that all switch tests pass.
D. Turn air conditioning off - Disengage the A/C clutch as follows:
1. When not in diagnostics, turn the Electronic Climate Control to "off."
2. When in diagnostics on 1982-1983 models, depress the "outside temp" button to turn off the "outside temperature" indicator (A/C compressor clutch status light).
3. When in diagnostics on 1980-1981 models, the A/C clutch is off.
E. Check engine timing and verify it is correct before adjusting ISC-TPS.
If any of these conditions above are not met, an incorrect minimum air reading and consequently an incorrect TPS adjustment may result. On 1982-1983 models, a quick check of TPS readings can be made. Refer to 1982 Service Manual 6C8-8 and 1983 Service Manual 6D3-12. By retracting the ISC and using the DFI parameters, the minimum air rate, TPS and ISC settings can be quickly verified on most cars without the need for tools or meters.
The tamper resistant welds must be removed to adjust the TPS. The procedures for TPS adjustment differ for each model year, and it is essential that the correct procedure is used. These procedures are summarized as follows:
1980- File tamper resistant spot welds and adjust per 1980 Service Information Manual 6C-166.
1981- Remove throttle body, use 5/16" drill bit to remove tamper resistant spot welds and adjust per Serviceman Bulletin 81-5A.
1982- Remove throttle body, use 5/16" drill bit to remove tamper resistant spot welds and adjust per 1982 Service Information Manual 6C8-8.
1983- Loosen adjusting screws and adjust per 1983 Service Information Manual 6D 3-12. (There are no welds.)
1981-1982 - Service units with build date code after 2-244 do not have welded TPS screws.
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.
