Tachometer inductive wire burning up

[Ronan]

Hi all, I've got an issue with my tacho wiring and I'm not sure where to go next. I've got a 120y VB210 that had been running, but stalled at a set of lights (a leaking intake manifold gasket), and when I went to start it up again smoke started pouring out from behind the dash. I turned it off then put the ignition to On, the smoke started again. Pulled apart the dash and found the tacho inductive wire (the black and white wire that loops from ignition to the positive coil side) had totally fried on both sides of the loop.  The wiring looks fine from where it plugs into the harness back, just the loop that goes into the odometer is well cooked.

Has anyone seen this happen? I've replaced some of the usual ignition components but haven't touched the wiring or the coil or anything. I was thinking maybe the ballast resistor, but if that was open, wouldn't that stop any current from going through the circuit when the ignition was at 'On'?

Any insight much appreciated!
Cheers,
Ronan

 

[datzenmike]

The engine won't run if the loop of wire through the tach is broken. That loop is between the coil - (negative side) and the distributor.

 

I did have this happen on my 620 truck but the connection was bad. Once tightened the connection was restored and it fired up. 

 

I can't see why this wire would over heat right at the tach and not else where.

 

 

[Ronan]

Thanks for that mate! From what I can see on the car + what I've read the tach signal loop on these run from the ignition switch to the ballast resistor going into the positive side of the coil. I can actually see some signs of heat stress on the terminal at the ignition switch plug, and the wire is a bit melted where it contacts the terminal, so I'm guessing the whole wire is getting hot but maybe because of the magnetic field generated by the loop that part gets a lot hotter so it cooks? At the moment I think I'll have to replace the whole wire and all the terminals anyway to make sure nothing else is damaged, if this is a high resistance somewhere problem maybe that'll fix it, at least I'll be able to turn the ignition on without cooking the whole loom.

[datzenmike]

No.

 

On 1/31/2022 at 7:01 AM, datzenmike said:

 That loop is between the coil - (negative side) and the distributor.

 

 

The true signal for the tach is to place the tach between the coil negative terminal and the distributor, or in series. In other words...

coil negative ... wire ... tach... wire ... distributor.

[yenpit]

If his main engine ground failed, could the grounding have attempted to run thru that wire, cuz it runs from dizzy off the engine to ??

[Ronan]

Well, here is the diagram from the manual, it shows the wire running (in series) from the ignition switch to the Tacho then through to the positive side of the coil. 
 

https://imgur.com/a/nbUksuR

[datzenmike]

 

 

 

 

 

 Hummm Not what I expected.

 

This was running before and now is smoking???

[Ronan]

Yep, was driving it, it stalled, went to start it again and it filled the cabin with smoke in about 2 seconds. 

[datzenmike]

Then it's wired properly to begin with. That leaves something touching something else. Is this the factory wiring or did you have to run your own wiring?

 

Often when something out of the blue happens it's helpful the think back to the last things that were done by you to the car. Something that is unrelated but may have unintended consequences.

[Ronan]

Yep it’s all factory wiring, the only thing that looks like it’s been changed in this circuit is the ignition switch, but I checked the wiring and it’s the correct one. 
 

I checked the resistance of the whole circuit with the Tacho bypassed and it’s under 3 ohms, which I’m thinking means all connections are okay. 
 

Edit: worked out it has a short to earth somewhere between the Tacho and the coil, will just have to workout where.

[datzenmike]

You will have to disconnect the tach end and the coil end to measure the wire's resistance to ground. It should be infinite if it is indeed not shorted to ground somewhere.

[Ronan]

Figured it out, the anti dieseling solenoid valve is spliced into the Tacho wire just after the firewall, someone has cut off the anti dieseling solenoid and left the wire loose with the terminal exposed, which grounded out on the manifold and fried my Tacho wire! Luckily the wire gauge used on the Tacho is thinner than the loom wire so only the Tacho wire is fried. 
 

Spot on with something touching someone else and it being after I had done something unrelated, probably moved the wire when I took the air box off last. Thanks for the help!