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[Datsun 1200 encyclopedia]

Tachometer Wiring

From Datsun 1200 Club

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Category: Wiring

There are two popular kinds of tachometers, using different wiring. Datsuns used both kinds depending on the year.

Contents

Overview

There are two popular kinds of tachometers:

  • Induction wiring. There is no actual physical connection of the tachometer to the signal wires. This is what early Datsun 1200 uses.
  • Voltage signal wiring. The tachometer wire is connected to the '-' terminal of the coil (e.g. to the points). This is what later 1200s and most aftermarket tachometers use. Simply wire this in without changing/cutting the existing wiring.

174_5841ff3be1456.jpgPost 482595

Factory Tachometer

The early Datsun 1200 tachometer is wired differently than most tachos because it uses induction signal. This signal is indirectly wired to the tachometer via a transformer inside the tacho body. Also known as a current-sensing loop, inductive link, inductive coupling or impulse loop. This type works with standard points ignition or with electronic ignition, but not with multi-spark discharge ignitions.

23241.jpgAlbum 23241

  • B - ground - to back of Instrument Cluster
  • YR - IG power - to back of Instrument Cluster
  • RG - lights (two lights, two wires. Not shown in photo above) to back of Instrument Cluster
  • BW (Black with White stripe) - signal loop - to BW wires in dash harness

174_5c54dd0e61c81.jpgPost 490479 26338.jpgAlbum 26338

Conceptually the Induction LOOP works like this:
2708.jpgAlbum 2708

The BW loop of wire is the current induction coil. The ends of the loop, logically speaking, go in between the IG terminal of the ignition switch and the single-terminal (non-coil) side of the Ballast Resistor. Note that all IGN current goes through this wire, so connect it up in series using the factory connectors (yes, even cars without tachometer have the wiring in place). This is a BW wire along the top of the instrument cluster, seen here inside the yellow circle:
5043.jpgAlbum 5043
Pull the BW dash harness wires apart at the connectors, and plug them into the BW loop of the tachometer.

On the back of the instrument cluster circuit board -- marked as #1 in the photo above -- are three terminals on the back of the speedometer, just below the left turn signal indicator connector. From top to bottom:

  1. Lights - RG connect to male spade connector
  2. Ground - B connect to round flat slide-on connector
  3. IG power - connect to round bullet connector

This applies to the Round Gauge dash. The square type has only the Light connector, but a tach can be grounded to one of the gauge housing screws.

13937.jpgAlbum 13937 13938.jpgAlbum 13938

NOTE: If a B110 or B210 (120Y) is equipped with a factory tachometer, the factory tachometer is wired as a part of the ignition system and sends out a pulse to keep the engine firing (induction loop), so that without the tachometer connected the engine will not run with the key in the ON position (but will fire on the START position) of the ignition switch. If someone was to remove the tachometer the two black wires with white stripes that connect to the tachometer must be connected together.

Service Bulletin
24401.jpgAlbum 24401

With ECU

The stock early tachometer (the one with the loop connection) needs no mods to work with an EFI system:
26261.jpgAlbum 26261

NOTE: There is a tach signal OUT wire on many ECUs, but the early 1200 tachometer doesn't use this.

If you have converted to multiple coils it will still work. It works the same way whether you have 1, 2 or 4 coils:
26262.jpgAlbum 26262

NOTE that this type of tachometer works by sensing the intermittent current used by the coil. So anything connected to the coil wiring (such as anti-diesel valve) may make the tachometer read incorrectly (bouncing needle). The good news is that a failing tacho of the current-sensing type will not cause the ignition to fail.

Late 1200 Tachometer

LATE MODEL UTE
Newer utes do not have the tacho wiring in the harness.

1. You can use a Datsun 620 tachometer, it is a bolt-in and looks appropriate. 620 tacho is wired like any aftermarket tachometer.

2. You can use an early 1200 tachometer, but splice the wires into the COIL circuit. See POST Tacho not working WHY

Early Tacho in Newer Truck

If the newer truck doesn't have a dash harness LOOP, you can:

  • Find the BW wire in the dash, cut it, and connect to the tacho loop

or

  • Run new wires from coil back into the cabin and connect to tachometer. Discussion follows.
Instead of this normal wiring:


IGN+ --------------ballast--|+ coil

START ----------------------|

Remove the BW wire from the Ballast Resistor, and connect to new tacho wires:



tacho ==========|-|
IGN+ -----------| |ballast--|coil +

START ----------------------|

Which BW wire on the Ballast Resistor to remove? The problem is the Resistor has two BW wires. They are all BW!



IGN+|BW-----------BW<ballast>BW--|+ coil

START|BW---------------------BW--|

You can test this by removing all wires from the resistor, turning key to ON, then test which BW wire has 12V. That's the one to remove and tie into the tachometer.

Datsun 620 Tachometer

The tachometer from a Datsun 620 pickup fits exactly into a 1200. The colors and font are more like the PB110 or late model ute.

Wiring is different. The 620 tachometer is triggered by voltage from the negative side of coil (just like most aftermarket tachometers).

TachWireforDatsun620.jpg

620 Tach With ECU

26263.jpgAlbum 26263

Whether or not you need the resistor depends on the ECU, so ask the ECU vendor.

Note that some aftermarket tachometers won't work with digital signal from an ECU, and will need a converter box (basically a Relay with a 10K ohm resistor).

Aftermarket Tachometer

Most non-Datsun tachometers have four connections:

  • coil '-' . usually red or yellow wire
  • IG '+' (hot only when key is on) - usually red or white wire
  • ground - black wire
  • light - often blue wire


So where to connect the wires?

  • You can connect IG, Ground and Lights to the three connectors on the back of the speedometer (see above in the Factory Tach section)
  • There is no factory interior connection for the coil '-' signal. So you'll need to feed a wire through the firewall and connect it to the coil

Electronic Ignition

Some tachometers have problems with some EI units. 1980s Datsuns used a resistor from the coil feed.

Part Type 25950 
24280-N8200
1 watt 
2.2 k ohms (22,000) 

6d76e371.jpg 

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