Thornycroft 90 tachograph

[tamara]

Can anyone help, I have a Moody33 mk1 fitted with a thornycroft 90 (BL) engine, the taqcho failed a while ago. Can anyone out there tell me what size and type (ohms, amps) of out put should i be looking for from the sender or the head.
the sender is a mechanical type fitted just infront of the flywheel and connected to the head by a single wire.
/forums/images/graemlins/smirk.gif

 

[squidge]

Hi , If it's well and truly dead, have a look here http://www.netcomuk.co.uk/~marvelec/clearance.html

your thornycroft tacho is a VDO 80mm, the above link have new ones for around £90 or A.s.a.p supplies can supply a Faria? replacement for 1/2 the price.
First , b4 parting with the dosh id check the sender as i think it should have two wires out. The sender on the 90 is a small dc dynamo type with 2 pins + and - (if its the same as mine) .
hope this helps,

PS repack your rudder gland and grease to stop the leak.

 

[rogerthebodger]

Does the sender look like this

or this

more info here

 

[Gordonmc]

Now I am confused.
My T90 has, just in front of the flywheel, a disconnected mechanical drive.
The tacho is driven from the "W" terminal on the alternator.
The tacho itself is an Isspro programmable whose accuracy is questionable, over-reading if anything. To programme the thing I need to know the engine revs... which is what the tacho is for!
My question is where do the senders go... could I use one to engage on the dog for the old mechanical drive?

 

[chas]

My T90 has a mechanical drive to the tacho. I was once told by someone who supplied me with a new bowden cable that the BMC 1500 diesel was provided with one of two different tacho drives. Those going to the car market had an electronic drive and those destined for the tractor market (small 1960s BMC tractor - remember it?) had a mechanical drive. I therefore have a tractor engine!

 

[rogerthebodger]

There are generally 3 ways to drive a tachometer.

1) from the alternator "W" terminal. The tachometer needs to be calibrated to engine speed as the the speed of alternator varies with belt pulley sizes.

2) By a tacho generator (top pic). This can be driven from the crank or injector pump shaft and given an AC signal. No outside power is needed to the tacho generator or tachometer.

3) By a pulse generator (lower pic). This gives or pulses as a magnet or reflective strip passes the sender and can be driven from the flywheel or front pulley mounted magnet/ reflective strip.

The indicator may be the same as the alternator driven indicator and requires outside power normally from the ignition system

I have both 1 and 2 on my new yacht. The inside console is driven from the tacho generator and the outside instrument console is from one or the alternators on the main engine.

 

[tamara]

I think it looks like the top one, but I'll check this tomorrow when I'm on board and get back to you.

Thanks for the help.

 

[Andrew_Fanner]

If you mean the generator by "looks like the top one" you can test it with a meter set to AC. My generators shove out around 8-12V depending on engine speed. If the genny delivers then check the wiring continuity. If that is OK it gets harder, you need to get the gauge apart. If its a VDO one then very careful prising off of the bezel should get the glass off without breaking the siad glass, and being able to get it back together again. You need to get the circuitboard and galvanometer assembly from the case. Trace the input to the moving coil meter and put a standard 1.5V battery briefly across the direct coil inputs (not the meter terminals) It should deflect. If it doesn't, check continuity across the coil. If there is continuity the meter element works so try the bettery again!

Now for the fun bit. The remaining likely failure is the bridge rectifier that turns the AC from the generator into DC for a moving coil meter. Maplins will sell you a new one for about 26 pence. Unless you are skilled in soldering semiconductors buy several The pcb should have + - and AC or ~ indicators on it and the ectfier needs to be that way round. If all is well then bingo, a working rev counter. You are also £300 or so better off, looking at the price of a new gauge.

Also worth checking is any needle mevement at all, before any of this work. The scres holding the dial onto the mechanism frame can loosen and foul the needle! Had that one too. Again, dismantle the gauge by prising the bezel etc and tighten the offending screw. Note that all this dismantling will probably stop the gauge being waterproof so consider the use of a discreet bit of sealant on reassembly.

 

[tamara]

thanks for theindepth info, The sender or generator on my moody is the type in the top picture, i'll conduct the series of test an follow your advise.
many thanks /forums/images/graemlins/smirk.gif
I'm buying if it works