Vehicle and engine tachometers · Dash-mounted engine speed gauges in SAE 2-1/16 in and 3-3/8 in sizes, fed from ignition, ECU, alternator W-terminal or magnetic flywheel senders, programmed for the pulses per revolution, with 12 or 24 V supply and lighting
A tachometer shows engine speed in revolutions per minute on a vehicle, machine or boat dashboard, so the operator can keep the engine in its working range and avoid over-speed. Electronic tachometers count pulses from the ignition, engine controller, alternator or a flywheel sender. Ask three things first: what size and supply the dashboard uses, which signal source the engine offers and how many pulses it gives per revolution, and what range, lighting and environmental sealing are needed.
Dash-mounted electronic and mechanical engine tachometers for trucks, off-highway machines, marine and industrial engines, with their signal and programming options. Handheld optical or contact tachometers, complete instrument clusters, engine speed sensors and speedometers are outside this page.
01
Isspro lists its OEM gauges, which include tachometers, speedometers, pyrometers, pressure and temperature gauges and voltmeters, with SAE standard 2-1/16 in and 3-3/8 in mounting diameters, glass-filled nylon cases that resist corrosion and vibration, and 12 V DC operation, with 24 V DC applications available on request. [1]
VDO's tachometer instruction sheet describes a 3-3/8 in housing that needs a panel opening of about 3-7/16 in, held by a supplied clamp and nuts, with clearance behind the panel for wiring. Terminals are switched 12 V power (+) after the fuse box or a 5 A fast-blow inline fuse, signal input (S), and ground (−) at a point not shared with other electronics; lighting has its own feed through a 1 A fuse. VDO notes that the gauge may need programming before installation. [2]
Editorial calculation: 2-1/16 in is about 52 mm, 3-3/8 in about 86 mm and a 3-7/16 in hole about 87 mm. [1] [2]
Give the supplier the dashboard cut-out size, supply voltage, engine type and maximum rpm, signal source (ignition coil, ECU tach output, alternator W-terminal, magnetic flywheel sender), pulses per revolution or alternator poles and pulley ratio, dial range and colour, lighting colour, sealing needs, and quantity.
02
For diesel engines with a low-frequency signal from the alternator W-terminal or a camshaft-driven generator, VDO's instruction sheet sets DIP switches on the back of the gauge from a chart, and notes that with a W-terminal signal the number of pulses equals half the number of alternator poles, a figure obtained from the alternator maker. [2]
For a high-frequency magnetic flywheel sender in the bell housing, the pulses equal the number of flywheel teeth; VDO's procedure subtracts 50 from that number and sets the DIP switches to the result, for example 104 teeth gives 54, then uses a fine-adjustment screw to trim the pointer. The magnetic sender is non-polarised, so either wire can go to the S terminal. [2]
03
Ask any supplier for dial range, accuracy, supported signal sources and programming method, supply voltage, current draw, lighting, sealing rating, operating temperature, case material, mounting size and clamp, connector type, hour-meter or other built-in functions, and warranty.
04
Murphy's specification sheet for its Selectronic MT90 digital tachometer gives the electrical and environmental values that the reviewed analogue sources leave open. It accepts a signal of 4 to 35 Vrms from a magnetic pickup or alternator and can be set for 3 to 999 pulses per revolution. Battery power is 8-40 VDC (12, 24, 32 volt), with current of 4mA @ 40 VDC with the backlight off and 25 mA @ 40 VDC with it on. Operating temperature is -20° to 70°C, storage temperature -25° to 85°C, and the mounting hole is 3-7/16 in. (87 mm). [3]
Murphy states that the MT90 can take its power from a magnetic pickup at the flywheel ring gear, but the backlight needs a battery source. Calibration uses rotary switches on the back, set from the number of ring gear teeth for a magnetic pickup, or from the alternator-to-engine pulley ratio and the number of alternator poles. The case is a polycarbonate/polyester blend with a polycarbonate lens and a #430 stainless steel bezel, and the display is a 4-digit LCD. [3]
05
Dynalco's SPD-100 product data sheet (March 2009) describes a digital tachometer that takes both signal and power from a magnetic pickup or pulser, with no power-supply connections; lighted versions need 12 or 24 Vdc. It needs a minimum signal of 2.5 Vrms, accepts up to 15 Vrms, limits pickup signals at about 10 V peak-to-peak and takes input frequencies up to 20,000 Hz. The four-digit LCD reads 0 to 9999 with 1 rpm resolution and accuracy within 0.5%, with 0.1% (typ.) at room ambient. The case fits SAE 3-3/8 in (86 mm) panel openings, is gasketed and spray-proof, and operates from −20 to +80 °C. The sheet lists CSA Class I, Division 1 approval with named Dynalco pickups, and American Bureau of Shipping type approval for use in classed vessels for the SPD-100. [4]
Editorial calculation: at the 20,000 Hz input limit, a 120-tooth ring gear read by a magnetic pickup corresponds to 10,000 rpm (20,000 × 60 ÷ 120); check your tooth count and maximum engine speed against the limit of the gauge you buy. Editorial note: for hazardous areas or classed vessels, ask for the approval certificate covering the exact gauge and pickup combination. [4]
06
| Part | Role | Fix in the order |
|---|---|---|
| Case and clamp | Mounts the gauge in the panel. [1] [2] | SAE size. |
| Signal input | Receives engine speed pulses. [2] | Source type. |
| DIP switches and trim | Set pulses per revolution. [2] | Programming. |
| Power and ground terminals | Supply the gauge. [2] [1] | 12 or 24 V. |
| Lighting | Illuminates the dial. [2] | Lamp feed. |
07
Signal level and pulses per revolution decide whether a gauge will read your engine at all; the MT90 and SPD-100 sheets publish both, while the VDO and Isspro sources give the source types and sizes. [3] [4]
| Field | What it means and published values | Ask the supplier for |
|---|---|---|
| Measuring Range (rpm) | e.g. 0 to 9999 rpm on a four-digit LCD, 1 rpm resolution. [4] | Dial range. |
| Accuracy (%) | e.g. within 0.5%, 0.1% (typ.) at room ambient (SPD-100). [4] | Accuracy. |
| Operating Voltage (V DC) | e.g. 12 V DC; 24 V on request; 8-40 VDC (MT90). [1] [2] [3] | Supply. |
| Operating Temperature (°C) | e.g. -20° to 70°C (MT90); −20 to +80 °C (SPD-100). [3] [4] | Range. |
| Storage Temperature (°C) | e.g. -25° to 85°C (MT90). [3] | Range. |
| Ingress Protection | No IP code stated; the SPD-100 case is gasketed and spray-proof. [4] | Rating. |
| Display Type | Pointer gauge with fine adjustment; 4-digit LCD on digital models. [2] [3] [4] | Analogue or digital. |
| Backlight Color | Separate lighting feed. [2] | Colour. |
| Housing Material | e.g. glass-filled nylon. [1] | Material. |
| Weight (g) | Not stated in the reviewed sources. | Weight. |
| Mounting Diameter (mm) | e.g. 2-1/16 in (52 mm) or 3-3/8 in (86 mm). [1] | Size. |
| Signal Input (V) | W-terminal, camshaft generator or magnetic sender; e.g. 4 to 35 Vrms (MT90), 2.5 Vrms minimum and 15 Vrms maximum (SPD-100). [2] [3] [4] | Signal level. |
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Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Engine: [type; max rpm] Signal: [W-terminal; flywheel sender; ignition; ECU] Pulses: [per rev; alternator poles; flywheel teeth] Size: [2-1/16 in; 3-3/8 in] Supply: [12 V; 24 V] Dial: [range; colour; lighting] Quantity: [gauges]
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Isspro lists SAE standard 2-1/16 in and 3-3/8 in mounting diameters. [1]
VDO says pulses from the W-terminal equal half the alternator's poles; set the DIP switches accordingly. [2]
VDO subtracts 50 from the number of flywheel teeth and sets the DIP switches to the result. [2]
Murphy's MT90 sheet gives a 4 to 35 Vrms signal input, 3 to 999 pulses per revolution, 8-40 VDC battery power, current of 4mA or 25 mA at 40 VDC depending on backlight, and an operating range of -20° to 70°C. [3]
Dynalco's SPD-100 data sheet describes a digital tachometer powered entirely by the magnetic pickup or pulser signal, needing at least 2.5 Vrms; lighted versions need 12 or 24 Vdc. [4]
Sources checked 2026-10-11. Isspro and VDO make vehicle and engine gauges.
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