isuzu 55

[mizpah2]

Hi ,

ihave an isuzu 55 in my boat , when i came to run the engine , the tacho lights up and shows hours run but does not work ,when I put the ignition on the needle jumps a little but when I start the engine nothing .

It has two alternators on it , one 80 amp for the starter battery and an 110amp for the leisures .the 80amp is only showing 12.4 volts so I presume is faulty .

could the two problems be connected ? how is the tacho driven ?

[blackrose]

Take the panel off and check all the connections on the back. The tacho connector may have come loose or fallen off.

[ditchcrawler]

Hi ,

ihave an isuzu 55 in my boat , when i came to run the engine , the tacho lights up and shows hours run but does not work ,when I put the ignition on the needle jumps a little but when I start the engine nothing .

It has two alternators on it , one 80 amp for the starter battery and an 110amp for the leisures .the 80amp is only showing 12.4 volts so I presume is faulty .

could the two problems be connected ? how is the tacho driven ?

At a guess the revs are picked up from the W terminal of the alternator, If the alternator is not charging that could well be your problem

[mizpah2]

Take the panel off and check all the connections on the back. The tacho connector may have come loose or fallen off.

I have checked the connections , all seem ok ?

At a guess the revs are picked up from the W terminal of the alternator, If the alternator is not charging that could well be your problem

 

Ta , I have a wiring diagram but the plug on the back of the tacho is just letters it does not give a clue how it works .

[ditchcrawler]

I have checked the connections , all seem ok ?

 

Ta , I have a wiring diagram but the plug on the back of the tacho is just letters it does not give a clue how it works .

It will probably have a wire connected to the W terminal on the back of the alternator where it picks up the AC output as the alternator spins, if the alternator is not generating as you suggest there will be no AC output.

[Chewbacka]

Hi ,

ihave an isuzu 55 in my boat , when i came to run the engine , the tacho lights up and shows hours run but does not work ,when I put the ignition on the needle jumps a little but when I start the engine nothing .

It has two alternators on it , one 80 amp for the starter battery and an 110amp for the leisures .the 80amp is only showing 12.4 volts so I presume is faulty .

could the two problems be connected ? how is the tacho driven ?

In answer to the question of how is the tacho driven? - The alternator is an AC generator the output frequency being directly proportional to the speed of rotation. (For battery charging the AC is then put through a diode pack to turn it into DC). The oscillating voltage present on the W terminal is fed into the tacho where the clever circuit converts the frequency of the voltage into signal to drive the pointer of the tacho. Also the alternator rotation is a fixed multiple of the engine rotation - ratio is the difference in the size of the engine pulley and the alternator pulley. So if the pulley ratio is (as an example) 3:1 then the output frequency from the alternator will be 3 times greater than if it were directly driven. This is also programmed into the tacho. So the tacho can calculate engine rpm from the alternator (rotation) frequency.

[Tony Brooks]

As you have checked the connections on the back of the revcounter check the W terminal on the alternators. They are often 6mm blades and they do work loose. There will also be a multi-plug or two in the main engine harness. make sure this is clean and tight.

 

Just for correctness the revcounter is driven by half wave rectified AC so sort of DC and I do not know what the result of feeding it pure AC will be. If you have a multi-meter that can read frequency try measuring what comes out of the W terminal. The frequency should go up and down with engine revs. I also think you should get around 6V on the DC scale ot of the W terminal but stand to be corrected on that.

Edited November 21, 2016 by Tony Brooks

[Top cat]

The tacho on my Izusu packed in a few years ago after checking that the altenator was charging and the the signal was reaching the rear of the instrument I put it down to an internal failure, then a few hours later the hour meter failed too. Haven't bothered to replace the tacho and bought a separate hour meter.

 

Top Cat

[Chewbacka]

As you have checked the connections on the back of the revcounter check the W terminal on the alternators. They are often 6mm blades and they do work loose. There will also be a multi-plug or two in the main engine harness. make sure this is clean and tight.

 

Just for correctness the revcounter is driven by half wave rectified AC so sort of DC and I do not know what the result of feeding it pure AC will be. If you have a multi-meter that can read frequency try measuring what comes out of the W terminal. The frequency should go up and down with engine revs. I also think you should get around 6V on the DC scale ot of the W terminal but stand to be corrected on that.

Agreed about the half wave bit as I was avoiding going into too much detail. So it is an oscillating voltage but not a sine wave. I suspect in a modern tacho the w terminal output is converted inside the tacho into a square wave pulse train and then digitally processed, but this is a guess.

[ditchcrawler]

But from his first post he says there is no output from the alternator, its just sitting at the battery voltage. No alternator no signal to the tachometer

[mizpah2]

Well ,

thanks all for the input i have been refered to a guy close to my mooring who can repair the alternator so will take it to him . Now i and others i have told , know how it works .

Anyway back to the virtual pub .