Where does the tach get it's signal? Occasionally my tach will hang and not operate, and also I have noticed when this happens my transmission will act up a little with the shift solenoids. I think that they have to be related in some way as it never happens until the tach starts acting up. The speedo seems to work fine. Anyone have any ideas...I'd like to get rid of the bug. It only happens once or twice a month, but is annoying.

 

The Tach signal comes from the negative side of the coil. The "trigger" mechanism for that is the PIP (stator) inside the distributor and the Ignition Control Module (ICM/TFI).

If you have a CCD-style ignition:

Courtesy of FordFuelInjection.com

If you have a Push Start - style ignition:

Courtesy of FordFuelInjection.com

 

I'll check for a lose connection....but it could just be this somewhat unreliable gauge clusters. I am wondering how all of this would affect the shift solenoids. I know they get there signal from the pcm, but what sensors are used by the pcm to tell the shift solenoids it is time to shift?

 

Cut from my Ford EVTM:

PIP

Description:On gasoline engines, the Profile Ignition Pickup (PIP) signal is produced by a Hall-Effect device in the distributor. It tells the PCM the engine rpm and the crankshaft position.



Programmable Speedometer/Odometer Module (PSOM)

Description: The Programmable Speedometer/Odometer Module (PSOM) receives input from the Rear Anti-Lock Brake System (RABS) Speed Sensor, which is mounted on the rear axle differential housing. The PSOM processes this input signal information and relays it to the Speed Control module PCM. This signal tells the PCM the vehicle speed in MPH.

Transmission function: Used as an input in determining shift scheduling and Electronic Pressure Control (EPC).


Vehicle Speed Sensor (VSS)

Description: The Vehicle Speed Sensor (VSS) is a variable reluctance sensor that sends a signal to the PCM. This VSS signal tells the PCM the vehicle speed in MPH.

Transmission function: Used as an input in determining shift scheduling and Electronic Pressure Control (EPC).


BOO Switch

Description: The Brake On/Off (BOO) switch tells the PCM when the brakes are applied. The switch is closed when the brakes are applied and open when they are released.

Transmission function: Disengages torque converter clutch when brake is applied.


Air Conditioning Clutch (ACC)

Description: The Air Conditioning Clutch (ACC) is an electromagnetic clutch that is energized when the clutch cycling pressure switch closes. The switch is located on the suction accumulator-drier. The closing of the switch completes the circuit to the clutch and draws it into engagement with the compressor driveshaft.

Transmission function: Used as an input to determine EPC when the ACC is engaged to compensate for the additional load on the engine.


MAP Sensor

Description: On gasoline engines, the Manifold Absolute Pressure (MAP) sensor senses atmospheric pressure to produce an electrical signal. The frequency of this signal varies with intake manifold pressure. The MAP sensor sends this signal to the PCM. The PCM uses this signal to determine altitude. The PCM then adjusts the E4OD shift schedule for the altitude.

Transmission function: Used as an input to determine shift schedule and EPC for altitude operation.


Low Range Switch 4x4

Description: The low range switch is located on the transfer case assembly. It provides an indication of when the 4x4 transfer case gear system is in the LOW range.

Transmission function: Modifies shift schedule for 4x4L transfer case gear ratio.

TPS

Description: The Throttle Position (TP) Sensor is a potentiometer that is mounted on the throttle body on gas applications and on the fuel injection pump lever on diesel applications. The TP sensor detects the position of the throttle plate or lever and sends this information as a voltage signal to the PCM.

Transmission function: Used as an input to determine shift scheduling and EPC


TOT Sensor

Description: The Transmission Operating Temperature (TOT) sensor is located on the solenoid body in the transmission sump. It is a temperature-sensitive device called a thermistor. It sends a voltage signal to the PCM. The voltage signal varies with the transmission oil temperature. The PCM uses this signal to determine whether a cold start shift schedule is necessary. The cold start shift schedule lowers shift speeds to allow for the reduced performance of cold engine operation.

Transmission function: Used as an input to determine shift schedule and EPC for temperature effects.


MLPS

Description: The Manual Lever Position Sensor (MLP) sends a signal to the PCM. This indicates the position of the shift lever (P, R, N, (D), 2 or 1). The MLP is located on the outside of the transmission at the manual lever.

Transmission function: Used as an input to determine desired gear and EPC.



TCIL

Description: The Transmission Control Switch (TCS) is a momentary contact switch. When the switch is pressed, a signal is sent to the PCM. The PCM then energizes the Transmission Control Indicator Lamp and the Coast Clutch Solenoid (CCS), applying the coast clutch to provide engine braking and cancels fourth gear operation.

NOTE:
TCIL will also flash if the EPC circuit is shorted.

Sensor: Transmission Control Switch (TCS)

Transmission function: Disable fourth gear operation.