Thornycroft 108 Wiring / rpm gauge

[wrisberg]

Hi guys.

 

Just finishing up my rebuild, new wiring etc.

 

My old ( noname ) rpm gauge, has been diconnected, and a Thornycroft gauge has been fitted.

 

When the engine is running, it's pretty much at a standstill at, about 500rpm. No matter how many revs i give the engine.

 

I tried to mount my old rpm gauge, to the wires, to see if i have made a mistake, but it runs as it should.

 

Soo, here is my question.

 

The old one gets ( G ) / signal, from the alternator. Does any of you know if its the same with the old thornycroft rp, gauges ? Either the gauge is broken, or it needs to get a signal, from somewjere else, thats my conclusion.

 

The wiring is okay, and is working with the old one, just not with the thornycroft . . . Anyone has an idea?

 

Kind regards.

 

 

 

 

[nicknorman]

No knowledge about your specific tacho but in general they need +ve, -ve and the W output (single phase tap) from the alternator. Since different alternators go at different speeds depending on the pulley ratios etc, most electronic tachos have a calibration function where you can compensate for the different pulley ratios. Perhaps you need to alter the calibration, but you'd probably need to find an instruction manual to work out how.

Edited March 31, 2016 by nicknorman

[wrisberg]

There is no + connection on this tachometer. How do I give it +? Maybe that's why it isn't working properly.

 

Jonas

Edited April 1, 2016 by wrisberg

[nicknorman]

If it only has 2 connections it's probably the wrong type of tacho. Not all work by the alternator w connection, sometimes they are run from a tacho generator and thus are effectively a voltmeter, or some other technology. Does it have a VDO part number visible? I cant see from the screenshot. Why do you think it is the right part for your engine? Are there any markings on the two screw terminals we can see in the picture?

Edited April 1, 2016 by nicknorman

[wrisberg]

There is a - ( minus ) and a G ( giver, i presume )

 

You are proplably right, it could be that its not alternator ( W ) Driven . . . Dammit!

 

Is it possible to change the Tachometer, so its possible to recieve signals from alternator?

 

I have heard of tachometers, thats been changed,. the other way around.

 

I bought this one second hand, as my old one, was a mess, and this is a Thornycroft, so it would fit the engine make. . . But didn't think about ( didn't know ) there where differences in the signal.

[nicknorman]

It looks like G terminal is for W connection on the alternator. I found this:

 

http://s275.photobucket.com/user/naskeet/media/VDO-Alt-W-Tach.jpg.html

 

But there should be a + terminal too. Can you post a photo of the back of your actual tacho, or is it identical to the catalogue link? Just wondering if the + terminal could be broken off.

 

The gauge is actually made by VDO, not thorneycroft, and quite likely that thorneycroft used different types of tacho at various times (though probably all made by VDO, it's a popular make).

Edited April 1, 2016 by nicknorman

[wrisberg]

it's identical.

[nicknorman]

it's identical.

 

Sorry, I'm stumped then. A tacho run from the w connector should always have a +, - and W(or G) connection I would have thought.

[wrisberg]

I presume it's from a engine with a signal from a source, let's say flywheel or somethings. Not the alternator. That must be the reason.

 

Any og you guys in here know of such a device?

Is it easy to mount? Is it direct bolt on, to maybe one of the belt drives? If it's mounted to the flywheel, it's a no go. Don't want to take it apart already ??? then I'll rather install a secondary tachometer, and leave it be, until winter.

[Tony Brooks]

Could it be a petrol Tacho driven by the coil SW (I think) terminal. If not SW the CD? Been years since I worked on petrol tachos so memory dim.

 

 

[wrisberg]

Just installed a secondary tachometer.

 

I am not up for taking the engine appart and installing a reader on the flywheel.

 

 

[wrisberg]

Now the time has come!!!!