Isuzu tachometer issue

[Snakey]

Our narrowboat has a 38hp Isuzu engine and the standard Isuzu/HMI instrument panel. Starter alternator is a 70A Prestolite. Historically, when the ignition is switched on the tach reads 500 rpm and the ameter shows 12v. On starting the engine the ameter would increase to 14v and the tach would record actual rpm. Now, however we seem to have a fault... 

Switching on the ignition nothing has changed. But now it takes anything from 15 minutes to half an hour running before the tachometer and ameter start reading correctly. So why?

Is this an alternator problem or an instrument fault? Where do I start looking? Has anybody had anything similar?

[Tony Brooks]

What does the charge/ignition warning lamp do please? In many/most cases the current through the warning lamp "wakes the alternator up", so no current = no charge, and no charge = no signal to the tacho so the tacho stays as it was, but I would have expected it to drop to zero when the ignition was turned on.

 

Also, what do the other instruments and warning lamp do when you switch the ignition on.

 

First physical checks - check all connections on the alternator are tight and clean. Check the large multi-way connector(s) in the main engine wiring harness are clean, tight and secure.

 

As very preliminary guess, I would say the symptoms suggest possibly worn brushes in the alternator or a loose connection.

 

Edited to add: a photo of the back of the alternator would help so we can see if it is a six or nine diode machine, because they are "woken up" in different ways.

Edited March 17 by Tony Brooks

[blackrose]

10 hours ago, Snakey said:

Historically, when the ignition is switched on the tach reads 500 rpm and the ameter shows 12v. 

 

If it's showing a voltage then surely it's a voltmeter?

 

The voltmeter (for the start battery) on my deluxe Isuzu panel stopped working and so did the LCD engine hours counter in the middle of the revcounter, although the revcounter itself is fine. I believe this is a common problem. I just bought a couple of cheap meters on eBay. I took out the voltmeter from the panel and replaced it with a mechanical engine hours counter of the same diameter and cut a small hole in the aluminium panel for the new voltmeter. Obviously they both needed to be connected behind the panel.

 

1 hour ago, Tony Brooks said:

First physical checks - check all connections on the alternator are tight and clean. Check the large multi-way connector(s) in the main engine wiring harness are clean, tight and secure.

 

Check all the electrical connections on the back of the panel are properly plugged in enawll. It could be that the contacts are dirty and the voltage needs to rise to 14v before there's anything getting through to the meters. Disconnecting and reconnecting the plugs several times might be enough to clean the contacts, or you could buy a can of electrical contact cleaner on eBay and spray a tiny bit onto the connections.

Edited March 17 by blackrose

[Tony Brooks]

I would suggest that asking a poster that we have no idea of their practical abilities or the design of their boat to look behind a control panel would not be the first thing to do, because in many cases that involves taking the panel out of the boat and that can, all too easily, lead to connections being pulled off.

 

If it is easy to get at then fine, but if not, let's get the results of what I said before going any further.

[blackrose]

Well in that case we also don't know that the OP's practical abilities extend to checking the alternator and engine harness connections as you suggested Tony. Also I never actually said that checking the panel connections was the first thing to do....

 

As always we can only advise what the potential problems and solutions might be. It's always up to anyone asking for advice to decide whether they have the practical abilities to take on the job.

Edited March 17 by blackrose

[Tony Brooks]

1 minute ago, blackrose said:

Well in that case we also don't know that the OP's practical abilities extend to checking the alternator and engine harness connections as you suggested Tony.

 

As always we can only advise what the problems and solutions might be. It's always up to anyone asking for advice to decide whether they have the practical abilities to take on the job.

 

There is a big difference between looking and feeling connections that are normally in view, be it somewhat restricted, and those that may need pulling cables out of the boat.

[blackrose]

2 minutes ago, Tony Brooks said:

 

There is a big difference between looking and feeling connections that are normally in view, be it somewhat restricted, and those that may need pulling cables out of the boat.

 

Why would getting behind a display panel to check connections involve pulling cables out of the boat? 

 

There's almost no extra cable length going to my display panel but it's not difficult to unscrew the panel and lift it a few inches to check behind. The cables can't be pulled out from the back of the panel as they clip in - unless you yanked the panel away using brute force!

 

It seems like you'd rather I hadn't suggested checking the connections at the back of the panel at all Tony, which seems odd to me since in my experience there's a fair chance this is the cause of the problem. I never said it was the first thing to do but it's definitely "a" thing to do if your suggestions yield no results.

[cheesegas]

Probably not the case here as it looks like charging and tach problems are linked, but it's worth noting that some older Isuzus (before they were blue!) had a tacho driven from a hall sensor from the overhead camshaft. Screws into the top of the valve cover at the alternator end at the engine.

 

The cables join the same loom as the alternator exciter cable and go through the same multiplug, so check this first in case it's come partially undone.

[Snakey]

2 hours ago, blackrose said:

First physical checks - check all connections on the alternator are tight and clean. Check the large multi-way connector(s) in the main engine wiring harness are clean, tight and secure.

Photo of alternator taken a couple of years ago when I attempted to replace it. A direct replacement wasn't available so I had the brush box/reg changed by a specialist. At that time the factory (HMI) loom had a leg servicing the starter motor and alternator which was tight across the cooling pipework; I made up a new slightly longer run exactly as per the HMI wiring diagram. The tachometer wire isn't included in the multipin (again, as per the factory spec). The system worked fine until recently.

The ignition warning light behaves like it should. I have checked connections are tight, but not with a meter as I don't know what values to expect!

And for 'ammeter' read voltmeter. 60 years of messing with cars has left its mark...

Edited March 17 by Snakey

[Tony Brooks]

Many thanks. That looks like a nine diode alternator, so uses the warning lamp to elite it. But as the warning light is working as it should, coming on with the ignition and going out as the alternator energises, the fact that you have no rise in voltage or revcounter is a bit of a mystery.

 

Definitely have a look at the multi-way plug in the main harness and then do a Blackrose suggests and check the connections on the control panel.

 

The alternator looks like a Lucas A127 clone to me, which is very common.

 

Test results for testing the alternator connections.:

 

B+ to B-, thick brown to thick black cables, should read battery voltage and charging voltage once the engine is started and revved.

Note: The engine battery master switch needs to be turned on.

 

D+ or warning lamp wire. The thin cable that looks like brown and white running to the blue bullet connector. Disconnect (in your case, the blue connector is probably easier than the stud on the alternator) with voltmeter between the cable (not the alternator) and any negative. The meter should read zero with the ignition off and battery voltage, with no warning lamp when the ignition is on.

 

W or rev counter wire that I think is the thin black one. This passes pulses, so you need the meter set to Hz if you can, or possibly AC volts. I can't give a definitive figure, but the meter should rise as you rev the engine.

 

A further thought - if the warning lamp is an LED or uses a grain of wheat dolls house bulb, then it needs a parallel resistor to help energise the alternator so if that has failed or become detached the alternator will not energise. I don't know what type of warning lamp you have, but if it is one of these, the bulb should just stay on with a faulty parallel resistor.