1991 ford f-250 7.5 engine scanner gives code 63 signal voltage too low and code 81. Check engine light comes on while letting off the accelerator and sometimes while idleing. the light will always go off by accelerating. Sometimes while idleing the the the engine will cycle in and out of this condition as if a solinoid were kicking on and off. When the check engine light is on the idle increases to 1200 rpm and there is low bottom end power, sometimes the engine will stall when at low rpm and demanding power. Have replaced the throttle position sensor, Oxygen sensor, coolant temp sensor and the MAP.

1991 is pre-OBD2. code readers will not work.
sorry.
however, all is not lost. Many cars have an alternate way of reading codes from the computer.
Toyota has a block under the hood and you short couple wires together and count engine light flashes
Chrysler you can turn the ignition key on and off a few times and count engine light flashes
in your case for GMs from 1982 to 1995
1: find the 12 pin connector under the dash and short the upper two rightmost terminals.
with the ignition "ON" and engine not running. The system will enter the diagnostic mode. In this mode, the computer will display a Code 12, three consecutive times, by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" or light. A Code 12 consists of one flash, a short pause, then two flashes in rapid succession.
2:. After Code 12 is displayed, any stored trouble codes will be displayed by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" light. All codes are displayed 3 times each then a code 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.

Tech -- GM 2 Digit codes for 82 and up (non OBD-II)
code - code definition
12- diagnostic mode
13- 02 sensor or circuit
14- coolant sensor or circuit/ high temp indicated
15- coolant sensor or circuit/ low temp indicated
16- system voltage out of range
19- crankshaft position sensor or circuit
21- throttle position sensor or circuit- voltage high
22- throttle position sensor or circuit- voltage low
23- mixture control (m/c) solenoid or circuit (carbureted models)
23- manifold air temperature (mat) sensor or circuit (1990 and earlier models)
23- intake air temperature (iat) sensor circuit (fuel-injected models)
24- vehicle speed sensor (vss) or circuit
25- manifold air temperature (mat) sensor or circuit - high temperature indicated (1990 and earlier models)
25- intake air temperature (iat) sensor or circuit - high temperature indicated (1991 and later models)
26- quad driver module circuit
27- quad driver module circuit
28- quad driver module circuit
29- quad driver module circuit
31- park/neutral position (pnp) switch circuit
32- baro sensor or circuit (carbureted models)
32- egr circuit (fuel-injected models)
33- manifold absolute pressure (map sensor signal voltage high
33- mass air flow (maf) sensor or circuit - excessive airflow indicated
34- manifold absolute pressure (map) sensor signal voltage low 34- mass air flow (maf) sensor signal - low airflow indicated
35- idle speed control (isc) switch or circuit (shorted) (carbureted models)
35- idle air control (iac) valve/ circuit
38- brake switch circuit
39- torque converter clutch (tcc) circuit
41- no distributor signals to ecm, or faulty ignition module ( carbed models)
41- cylinders select error- mem-cal or ecm problem ( fuel-injected models
41- cam sensor circuit (3.8 engine)
42- bypass or electronic spark timing (est) circuit
43- low voltage at ecm terminal l (carbureted models)
43- knock sensor circuit
44- oxygen sensor or circuit - lean exhaust detected
45- oxygen sensor or circuit - rich exhaust detected
46- power steering pressure switch circuit
48- misfire diagnosis
51- prom, mem-cal or ecm problem
52- calpak or ecm problem
53- egr fault (carbureted models only)
53- system over-voltage (ecm over 17.7 volts)
54- mixture control (m/c) solenoid or circuit (carbureted models)
54- fuel pump circuit (1986 and later models)
55- oxygen sensor circuit or ecm
55- fuel lean monitor (2.2l engine)
61- oxygen sensor signal faulty (possible contaminated sensor)
62- transaxle gear switch signal circuits
63- manifold absolute pressure (map) sensor voltage high (low vacuum detected)
64- manifold absolute pressure (map) sensor voltage low (high vacuum detected)
66- pressure sensor or circuit air conditioning

P1647 ECM HO2 Sensor control
malfunction -
Mahindra Scorpio P1832 Possible Solution :
The crankshaft sensor signals the fuel injection computer or the ignition control when the cylinders are firing. This causes the ignition coil to provide a spark and the injector to inject fuel into each cylinder at the right time.If either sensor isn't working correctly, the car will run rough and the engine will be less efficient. In later car models, the car's computer can usually keep the vehicle running, but the engine warning light on the dashboard goes on to warn the driver.

Well code 63 says the signal from the TPS is too low. The signal voltage is low when the throttle is closed and high when it is open.
A mechanic would check the reference voltage coming from the computer, and the resistance in the sensor with a meter.

CODE 11 No distributor referene signal detected during engine cranking....Check circuit between the distributer and the pcm.

CODE 12 Problem with battery connection.Direct battery input to pcm disconnected within the last 50 ignition key-on cycles.

CODE 13** Indicates a problem with map sensor vacuum system.

CODE 14** map sensor voltage to high or to low.

CODE 15** Problem with vehicle speed sensor signal.No vss signal detected during road load conditions.

CODE 17 Engine is cold to long.Engine coolant temp stays below normal operating temp(check thermostat).

CODE 21** Problem with oxygen sensor signal circuit.Sensor voltage to computer not fluctuating.

CODE 22** Coolant sensor voltage to high or to low.Test coolant temp sensor.

CODE 23** Indicates that the air temp sensor input is below the minimum acceptable voltage or sensor input is above the acceptable voltage.

CODE 24** Throttle position sensor voltage to high or low.Test the tps.

CODE 25** Idle air control (IAC) valve circuits.A shorted condition is detected in one or more of the IAC valve circuits.Or a vacuum leak has been detected.

CODE 27 One of the injector control circuit output drivers does not respond properly to the control signal.Check the circuits.

CODE 31** Problem with canister purge solenoid circuit.

CODE 32** An open or shorted condition detected in the EGR solenoid circuit.Possible air/fuel ratio imbalance not detected during diagnosis.

CODE 33 Air conditioning clutch relay circuit.An open or shorted condition in the compressor clutch relay circuit.

CODE 34 Open or shorted condition in the speed control vacuum or vent solenoid circuits.

CODE 35 Open or shorted condition detected in the radiator fan low speed relay circuit.

CODE 37** Open or shorted condition detected in the torque converter part throttle unlock solenoid control circuit.

CODE 41** Problem with charging system.An open or shorted condition detected in the generator Field control circuit.

CODE 42 Fuel pump relay or auto shutdown relay (ASD) control circuit indicates an open or shorted condition.

CODE 43** Multiple cylinder misfire detected.Peak primary circuit current not achieved with the maximum dwell time.

CODE 44 Battery temperature sensor volts malfunction.Problem with the battery temperature voltage circuit in the pcm.

CODE 46** Charging system volts to high>Computer indicates that the battery voltage is not properly regulated.

CODE 47** Charging system voltage to low.Battery voltage sense input below target charging voltage during engine operation and no significant change in voltage detected during active test or alternator output.

CODE 51** Oxygen sensor signal input indicates lean air/fuel ratio condition during operation.

CODE 52** Oxygen sensor signal input indicates rich air/fuel ratio during engine operation.

CODE 53** internal pcm failure detected.

CODE 54** No camshaft position sensor signal from distributer.Problem with the distributer synchronization circuit.

CODE 55 Completion of fault code display on check engine lamp.this is end of message code.

CODE 62 Unsuccessful attempt to update EMR mileage in the controller EEPROM.

CODE 63** Controller failure.EEPROM write denied.Check the PCM

You'll have to do a scan on the computer to find the trouble codes.
Your model can be done without a scan tool, research "check OBD 1 code Ford" online for the procedure, or you can find it in most repair manuals.

Code 63 means the signal from the throttle position sensor is low - meaning the sensor says the throttle is closed when it is actually open.
Could be a faulty sensor or wiring problem.

This sounds like a voltage regulator problem, the voltage regulator is built into the altenator, check your connections to the altenator specfically the ones that plug in, make sure they are secure. The plug in connections provide signal for how much output is needed from the altenator, the regulator provides the output. Check the battery voltage with a voltmeter, turn on everything electrical, rev up engine and see if voltage goes over 14.5 volts, it should be steady. Hope this helps, let me know.

Sorry this one doesn't have a good answer. After 1996 cars use what's called OBD2 (on board diagnostic version 2) This allows anybody to use a handheld scanner to test any car for "check engine" light. But before 1996 each manufacturer had their own OBD. So only the dealer could check it or you would have to buy a specific scanner for Fords only. I have a OBD2 scanner. You might ask your local autoparts store what they have to read your code. Otherwise you have to take to dealer or mechanic that has the right scanner. I wish you well.

check the engine ground wire (grounds from the engine to the body.) I had this problem on a ford ranger.