For the tachometer input circuit function, see Figure
“Tachometer Input Circuit Function”
. The ECM provides an output for a remote tachometer with a 0 — 12 volt digital signal that indicates engine speed. The frequency sent by the ECM is 1/5th of the actual engine r/min.
Figure 1. Tachometer Input Circuit Function
Section 42.1.1
Fault Detection Management
No fault detection is available for communication between the ECM and the remote tachometer.
For the tachometer input diagram, see Figure
“Tachometer Input”
.
Figure 2. Tachometer Input
Key on, engine off voltage checks at the ECM are listed in Table
“Key ON Engine OFF Voltage Checks at ECM”
. These checks are performed with the breakout box installed and the ignition key ON, engine OFF.
| Test Points | Spec | Signal | Comments |
| 60 to 11 | 5 ± 1.5v | Tach B | The signal is pulled up by the ECM with the key ON, engine OFF. |
| 59 to 11 | 5 ± 1.5v | Tach A |
Connector checks to Ground at the ECM are listed in Table
“Connector Checks to Ground at ECM”
. These checks are performed with the breakout box installed and the ignition key in the OFF position.
| Test Points | Spec | Signal | Comments |
| 60 to 11 | > 1,000 Ω | Tach B | Less than 1,000 Ω indicates a short to ground either through the harness or internal in the ECM. Disconnect the ECM from the breakout box and measure to ground again. If the short is still present, repair the harness. |
| 59 to 11 | > 1,000 Ω | Tach A |
Harness resistance checks are listed in Table
“Harness Resistance Checks”
. These checks are performed with the breakout box installed and the ignition key in the OFF position.
| Test Points | Spec | Signal | Comments |
| 60 to C4 | < 5 Ω | Tach | Resistance from 60-pin connector to 31-pin connector – Tach signal. |
| C4 to Tach | < 5 Ω | Tach | Resistance from ECM connector to Tach input at instrument panel – Dash Tach. |
| 59 to Tach | < 5 Ω | Tach | Resistance from ECM connector to Tach input – Optional Owner/Operator Tach. |
Key on, engine running signal checks are listed in Table
“Key On Engine Running Signal Checks — Tachometer”
. These checks are performed with the breakout box installed.
| Test Points | Spec | Comments |
| 60 to 11 | 5 to 7v / 140 to 540 Hz | Tach signal from the ECM is a frequency that is engine r/min + 5. |
| 59 to 11 | 5 to 7v / 140 to 540 Hz | Tach signal from the ECM is a frequency that is engine r/min + 5. |
Fault Code Descriptions = NONE.
Note: The instrument cluster tachometer does not use these outputs. See the ATA communication for instrument cluster tachometer diagnostics.
Section 42.1.2
Tachometer Input Signal
The following information is only for auxiliary tachometers installed by the body builder or vehicle owner. It does not apply to the instrument cluster tachometer that is operated from the ATA data line.
The ECM receives a signal from the CMP sensor and calculates engine speed (rpm). The ECM sends the calculated engine speed as a digital buffered tach signal from the ECM connector to the owner installed tachometer.
| Series 40E Troubleshooting Manual – 6SE241 |
| Generated on 10-13-2008 |