# tips on finding/tracing tach signal from ICE



## McRat (Jul 10, 2012)

There are several different tach schemes. Many early cars do not have tachs at all. If it does not have an ECM, it probably doesn't have one.

Cars with ECM's (digital engine computers) generate the tach signal from inside the computer.

There is a crankshaft or camshaft reluctor wheel with multiple lobes AND a gap in the lobs to indicate TDC on #1. There is a Hall Effect sensor that relays those pulses to the ECM, which generates a tach signal. The digital dashboard module "listens" to the ECM to get this signal if you have a tach on the dash. There is also a wire from the engine bale connector that has this signal as well, but it is coming from the ECM.

There are a LOT of teeth on that reluctor wheel. 20-60?

If you are removing the engine and ECM, there is no tach on these.


----------



## dtbaker (Jan 5, 2008)

McRat said:


> There are several different tach schemes. Many early cars do not have tachs at all. If it does not have an ECM, it probably doesn't have one.
> 
> Cars with ECM's (digital engine computers) generate the tach signal from inside the computer.
> 
> ...



the car in question is a 2002 nissan altima. I have to cut the bundle of wires from engine at some point prior to the firewall while yanking motor, and am trying to find out if I am better off trying to find the wire(s) that are coming from the motor to the ECM to feed in the new signal from a speed sensor on electric motor (3 per rev, I assume), or if I will be best off skipping all the way to the dashboard, finding wire(s) to the tach, and sending the pulses from new speed sensor directly... skipping ECM.


----------



## McRat (Jul 10, 2012)

You will probably need a Nissan expert, but ...

Since 1996, all cars are OBDII. This means they have an advanced engine control computer (ECM).

The dashboard has it's own computer for controlling the needle movements.

It gets a feed from the ECM and moves the needles accordingly. 

ie - The ECM figures out the Ohms on the fuel level sensor, does the calculations, then makes the information available on the CANbus. I can make an fuel needle do weird ass things.

And, in the ECM, I can adjust the speedometer. I can tell it to double the actual sensor reading if I want.

Same is true for the tach. The ECM takes the crank position sensor and creates an RPM number and makes this available to all the other computers on the CANbus.

Does that make any sense?


----------



## dtbaker (Jan 5, 2008)

McRat said:


> You will probably need a Nissan expert, but ...
> 
> Since 1996, all cars are OBDII. This means they have an advanced engine control computer (ECM).
> 
> ...



getting there.... the modified question now is; assuming I have a pulse from hall effect sensor on electric motor at 3x/rev (to match assumed crank output with v-6 motor), should I plug that signal into the ECM to translate for the dash tach, or can I skip the ECM and send 3x/rev pulse directly to standard dash tach input.


----------



## gdirwin (Apr 7, 2009)

I would trace out the following sensors and wires:
- hall effect sensor (into the engine - as discussed there is an internal toothed wheel with some teeth removed for timing - near the front/cam)
- temperature/coolant sensors (you may want to re-use these)
- oil pressure (can also be re-functioned perhaps)
- fuel gauge (probably not in the engine compartment, but good to trace it)
- A/C (if you plan on re-using) and fan connections (also possibly useful)
- ground connections (to engine block or chassis)

You can also check other wiring to see if there are useful 12V supply lines (already fused etc...) that you can connect to an vacuum pump, coolant pump etc... if they are the correct gauge. A heavy wire alternator wire can hopefully be reused for a DC/DC convertor, or for a power steering electric pump. It may be easier to simply add your own 12V distribution fuse panel...

I kept the hall effect sensor, bolted a toothed wheel onto my motor coupler, then imbedded the sensor (ie keeping the ECU etc... seemed easier).

You should invest in a good wiring diagram for your specific make/model - it will list all connectors, wire colors, connections at the ECU and gauges etc... For example you may want to pull the check engine light connection to the ECU, and reconnect it to your motor controller... Or add resistors to your existing sensors to recalibrate for a motor temp sensor or coolant sensor... I would say this is essential before you start cutting.


----------



## piotrsko (Dec 9, 2007)

I'd go buy a good used shop manual that has a multi page diagram of the wiring harness. trace from there. Some are even online for free

Most of the good manuals have connector locations in addition to wire colors. IMHO whacking the harness to remove stuff is just asking for future gremlin issues.

some companies use the spark module, some use the camshaft, crankshaft, some have a flywheel sensor, or some combination of all 3.


----------



## sergiu tofanel (Jan 13, 2014)

McRat is correct. The instrument cluster is driven via the CAN bus. All sensors are tied into the ECM, and data is shared via the network. I have not messed with instrument clusters myself, but many people have hacked these devices with varying degrees of success. Here is on of them:

http://www.youtube.com/watch?v=FTOrUbbu_RU


----------



## dtbaker (Jan 5, 2008)

dtbaker said:


> Problem is, I don't know which sensor on ICE is the tach, so I don't know what to trace back.....



shop manual is on order.... I found a .pdf version on eBay for $12 that *should* tell me what I need to know if it is any good. I have not had good experience FINDING this type of info in previous shop manuals. i.e. the schematics may show the circuitry, but then don't show the physical location where a particular bundle of the loom actually goes thru the firewall, etc.


Question remains what kind of signal the stock tach display expects from the ECM. I suspect that the signal from ICE cam position sensor is not just a simple 3xrev square wave, and probably is some multiple to send a pulse for every valve plus top-dead center which gets used by the ECM to figure out timing and diagnostics before simplifying and sending a signal to the tach display. I am hoping that what comes OUT of the ECM is a simple 3xrev square wave similar to what I will get from the standard speed sensor on the Warp tailshaft that I can just parallel to a zilla hairball for over-speed protection without even having to go thru the ECM at all, thus using the simple 3 pulse per rev signal from the new sensor.


----------

