1988 JCB 3CX Grey Cab Electronic Tachometer Debug

[soulman]

Hi

On my JCB 1988 3CX the tachometer, fuel gauge and temp gauge are not working. I have decided to take on this as my next project to fix. On the tachometer side, as i understand it, the alternator has a send terminal (W terminal) that delivers an A/C voltage to the tachometer in the instrument cluster. I noticed that on my machine, there was no wire connected to the W terminal on the alternator and i spotted a loose screened cable not connected to any terminal in the loom feeding the alternator. I believe this screened cable is the tachometer cable. I checked the output voltage of the W terminal using a voltmeter and it read 7 Volts approx. I also did a resistance check between the screened cable and chassis ground and it is giving a very close to short circuit reading so i didn't connect the screened cable to the W terminal until i figure what is causing the potential short circuit.

It seems according to the wiring circuit diagram i have, that the alternator terminal is connected to the tachometer directly. Does anyone know if that is actually the case? btw item 33 in wiring diagram is the alternator and item 31 is the speedometer

i had a look at the back of the instrument cluster but i cant see straight off, a cable similar to the screened cable down at the alternator. see images attached.

 

[Lincsman]

I think... and only think, it needs to be on the terminal near that black lump thing. start it up and see what a voltmeter shows there.

 

[soulman]

I think... and only think, it needs to be on the terminal near that black lump thing. start it up and see what a voltmeter shows there.

yes its 7 Volts there on that W terminal. The issue i have is that the screened cable to ground check is showing close to a short circuit, so i need to figure what may be causing that but i cant see straight off where it is connected to the tachometer in the cluster?

 

[Vincent]

The fuel gauge is now an obsolete part, and the tachometer is probably shagged . I had the same problem with my 1990 grey cab and bought a new old stock dash panel from Vicary plant a good firm to deal with. You need to buy a adapter harness as the connectors are different . I have the wiring diagram to splice in the new harness saved some where. You have to cut off the originals.

 

[Vincent]

Dash part number is 704-19600 .

 

[Lincsman]

 

[Lincsman]

yes its 7 Volts there on that W terminal. The issue i have is that the screened cable to ground check is showing close to a short circuit, so i need to figure what may be causing that but i cant see straight off where it is connected to the tachometer in the cluster?

I think its in the top right connector, but pull them all out and check for short, if OK wire it on, as i think it will show short while connected.

 

[soulman]

that right top connector has a screened wire but its different from what is down at the alternator from what i can see, on the top right connector there is a core wire which is red colour and an outer braid wire, hmmm, maybe the wire down at the alternator has got meshed together such that the core wire and braid have become interconnected, hence maybe the short circuit? does anyone have a picture of what their jcb 3cx grey cab alternator looks like in terms of the wires connected to it?

 

[Combine Rescue]

Guys W terminal is not DC is AC.
Yes it will show 7V on multimeter but only because its trueRMS function of meter show you average figures. Output from W terminal is sinuous AC signal and is not about voltage but frequency.

 

[soulman]

so an update. I got the Hour meter working. I see it changing after a little while running. The issue was that the outside braid and inner red core wire of the tachometer cable were shorted together, i separated them and then i got 8vac up at the console connection. However the tachometer RPM counter is still not working. Is the RPM counter controlled from same feed from the alternator and also road speed ?? Perhaps there is another voltage feed needed for the RPM counter ?? .. see picts for alternator connection and hour meter updates

 

[Vincent]

The same shielded wire does rpm and running hrs.

 

[Lincsman]

You have the hour meter working, the RPM dial is a tachometer!... anyway they should both work together using same signal AFAIK

Road speed will be a cable from the transmission, probably not electric.

 

[soulman]

you are correct , i should have said Hour meter .. Now to try and get the Tachometer dial working. Since the Tachometer and the Road speed dial are combined, it must be a common signal? there are no physical cables attached to the console as you might note in my first post where i show the connection to the rear of the console.

from attached image of the JCB 3CX 1400B service manual, it looks like the alternator feed controls all??

 

[Lincsman]

you are correct , i should have said Hour meter .. Now to try and get the Tachometer dial working. Since the Tachometer and the Road speed dial are combined, it must be a common signal? there are no physical cables attached to the console as you might note in my first post where i show the connection to the rear of the console.

from attached image of the JCB 3CX 1400B service manual, it looks like the alternator feed controls all??

The alternator cannot know the road speed, it needs a signal from the transmission.

Edit.. looking at the display the road speed is only correct in top gear in relation to the rpm I think, someone will tell us what that resistor does on the back of the clock?

 

[soulman]

The alternator cannot know the road speed, it needs a signal from the transmission.

Since the RPM and Speedometer are in same instrument with a common dial (see picts), how does RPM and Road Speed work i wonder?

 

[Lincsman]

Since the RPM and Speedometer are in same instrument with a common dial (see picts), how does RPM and Road Speed work i wonder?

see my edit...

 

[Vincent]

The road speed is just gauged of the rpm as I said above the one shielded wire does all.

 

[Netherfield]

I think the resistor may be to stop any interference on the radio.

Road speed is just the reading for top gear, same as older tractors used to have a chart to tell you road speed.

 

[soulman]

ok cool.

someone will tell us what that resistor does on the back of the clock?

That is a suppressor for stop noise from the alternator coming over the radio as someone mentioned

 

[soulman]

ok so seems the issue may be that the Tachometer/Speedometer part is faulty.