Crane XR700 & Tachometer

[Zed Head]

That looks like electrical noise on the wire.  The tach just "counts" voltage pulses.  That tach looks like it reads right otherwise.  You might try adding a condenser to the negative post.  It worked for me, when I had a similar problem.  Might also be a sign of a failing XR700.  That's what the factory ignition module's do when they start to fail.  Weird tach behavior.

[SteveC-VA]

I dug around in some old tool boxes and all I could find was a Tecumseh mower condenser, I put it on and started up and there was still some jumping of the needle initially.  I took it for a short drive and as I went along the jumping stopped and seemed to be all good as I pulled it back in the garage. I let it idle for a short time when i got back and it stayed steady. Thanks Zed and Scotta for the input and sorry for hijacking your thread Scotta but appreciate the assistance in getting this tach fixed.

[scotta]

Really happy this post and the community could help you solve the problem. That is the whole point of the forum in my view. 

[Zed Head]

Glad it worked.  I was not kidding though about a $25 GM HEI module being better than the XR700 for producing a strong spark.  In its day the 700 was high tech, but now it's an antique.  Like a TI-99 compared to any of the cheapest modern computers.  Keep it in mind if things go back to weird.  Good luck. 

[SteveC-VA]

So I took the car for another drive Sunday morning and the erratic tach movement started back up. I have ordered a GM HEI 4 pin module to replace the Fireball, Zed mentioned this erratic needle movement could be an indication of the Crane module going bad. The question I have is this.  Can I just use the existing wiring for the Crane to tie in the GM module once it is mounted on a heat sink plate and grounded? Are there any other things I need to look at in order to do this swap? I am not good with carbs and distributors so need any insight the group might have for this swap.

[Zed Head]

Can you take a picture of what's under the distributor cap?  The XR700 used an optical trigger originally, but somebody might have kept the VR trigger.  You need to know what you have.  It might not be a straightforward swap to the GM HEI module.  Sorry, I would have added more detail if I'd known you were going to try it.

[SteveC-VA]

I will get a pic and post it when I get home this evening.

 

Pic attached.

Edited July 9, 2018 by SteveC-VA
Add pic

[SteveC-VA]

added the pic.

[Zed Head]

You have an interesting situation now.  You have the XR700 optical disc in the distributor, which uses a different trigger signal for  the ignition module.  It might work but we'd need to know what the XR700 optical unit puts out or controls.  It might take extra wiring, and electronics knowledge.  Some study is required.  

If you can find a 260Z or 280Z stock distributor that would be an easy wiring job.  The stock units create their own voltage.

If one of the electronics guys wants to take a stab at making an XR700 optical unit trigger a GM HEI module, feel free.  The GM HEI module is often used to replace the turbo 280ZX ECCS "ignitor" which uses a 0 and 5 volt square wave.

 

[SteveC-VA]

I do have a spare project car waiting to get started on that I could pull the distributor out of to put in the one I am currently working on. I can dig around to see what I can find for a replacement distributor later down the road. I'll go that route this weekend to see what I can come up with on replacing the distributor and install the GM HEI when it gets here.

[Zed Head]

Good luck.  Always good to have a spare setup waiting when you're working with old parts.  Is the spare distributor from a 280Z?  Check under the cap to be sure.

I think that a person could make that optical trigger work, with a separate power wire.  It's a project for nerdy electronics types though.  I'd probably try it if I had it in my garage.

[SteveC-VA]

I wanted to get back to this, I looked around and seeing the prices of the 280 distributors I opted to go with a new xr3000 box. That's in and seems to be working fine, still have some jumping of the tach but not like it was before.

 

Of course during the install I broke one of the brittle wires on the temp sending unit, that was a $100 dollar mistake. I did notice the temp sensor wire was sort of hanging out of the brass fitting. It wasn't a huge cost.