Won't run if tach connect?

[chaseincats]

Hi guys,

Working on my friend's 1975 280z and when you connect the blue tach sense wire to the negative terminal on the ignition coil, the car will stall (and also won't start if already connected).  He has the Crane XR700 ignition system if that helps at all.

When I connect the blue tach sense wire and crank the car, the tach will move meaning it is connected but won't start until you remove the wire.

Any ideas what's going on here?

[SteveJ]

Is the resistor still in the circuit? Measure from the wire at the tach to the coil negative. It should be about 2.2K Ohm.

[chaseincats]

It isn’t, I’m guessing that’s part of the crane system.

[Zed Head]

Are you losing spark or injection?  The resistor is part of the 280Z tachometer circuit.

Here are the instructions for the Crane modules.  People often have problems when they use aftermarket ignition on the 280Z EFI systems.  Not sure why.   I have had odd problems with loss of injection if the ECU doesn't like the tach connection.

https://static.summitracing.com/global/images/instructions/xr700 instructions.pdf

 

[Zed Head]

 

[SteveJ]

2 hours ago, chaseincats said:

It isn’t, I’m guessing that’s part of the crane system.

That would indicate you're guessing wrong. If the resistor was removed from the tach circuit, there would probably be issues.

[chaseincats]

On 6/25/2022 at 4:30 PM, Zed Head said:

Are you losing spark or injection?  The resistor is part of the 280Z tachometer circuit.

Here are the instructions for the Crane modules.  People often have problems when they use aftermarket ignition on the 280Z EFI systems.  Not sure why.   I have had odd problems with loss of injection if the ECU doesn't like the tach connection.

https://static.summitracing.com/global/images/instructions/xr700 instructions.pdf

 

Finally got a chance to check and it's losing spark.

[Zed Head]

On 6/25/2022 at 12:50 PM, chaseincats said:

When I connect the blue tach sense wire and crank the car, the tach will move meaning it is connected but won't start until you remove the wire.

Any ideas what's going on here?

Probably messing with the make/break circuit to the coil.  Have to see how the whole system is wired to know what's really going on.  

[Zed Head]

On 6/25/2022 at 12:50 PM, chaseincats said:

blue tach sense wire

Just browsed thorugh the XR700 instructions.  There's actually a lot of good basic knowledge in there.  A whole section on adjusting the loop for the Smith type current sensing tachs.  Pretty cool.

But, the part you need is the basic setup diagram.  Nowhere in the instructions do they mention a blue tach sense wire.  There is a blue in the tach adapter section.  but it's a power supply wire.  If you're using the tach adapter you have blue connected to the wrong spot.

Start over and read the instructions, I'd say.  But maybe you don't need the adapter.

[chaseincats]

On 7/23/2022 at 4:25 PM, Zed Head said:

Just browsed thorugh the XR700 instructions.  There's actually a lot of good basic knowledge in there.  A whole section on adjusting the loop for the Smith type current sensing tachs.  Pretty cool.

But, the part you need is the basic setup diagram.  Nowhere in the instructions do they mention a blue tach sense wire.  There is a blue in the tach adapter section.  but it's a power supply wire.  If you're using the tach adapter you have blue connected to the wrong spot.

Start over and read the instructions, I'd say.  But maybe you don't need the adapter.

The tach adapter is used for an "erratic tach" only it seems.  Since the tach isn't working at all and is killing the car I don't think we'd need that.  I'll give the wire colors a look this weekend and report back.  Regarding the blue wire, it's on there in the FSM for '75 tho:

 

[Zed Head]

I screwed up.  I thought you were talking about connecting a wire from the Crane module.

Wild guess - the old ignition module is still connected, by the fuse box.  When you say "connect the blue tach sense wire" you're talking about connecting the wire at the coil?  That also branches off to the old module.  You have two modules connected when you connect the blue wire at the coil.

You can disconnect all of the wires at the old module and tape them off.  The blue tach wire branches off before it gets there.

On 6/25/2022 at 12:50 PM, chaseincats said:

When I connect the blue tach sense wire

 

[chaseincats]

3 minutes ago, Zed Head said:

I screwed up.  I thought you were talking about connecting a wire from the Crane module.

Wild guess - the old ignition module is still connected, by the fuse box.  When you say "connect the blue tach sense wire" you're talking about connecting the wire at the coil?  That also branches off to the old module.  You have two modules connected when you connect the blue wire at the coil.

You can disconnect all of the wires at the old module and tape them off.  The blue tach wire branches off before it gets there.

 

Hold on...what other module?  I thought the transistor ignition box was in the passenger footwell.  Is this something else?

EDIT: fuse box not fusible links lol sorry, brain fart.

 

Why would the car run with both connected but the tach wire being the final straw?  Is disconnecting it as simple as unplugging the box or are there other wiring changes I'd need to do for that to work?

Edited July 30, 2022 by chaseincats