MSD Tach driver w/ Spridget tach

[Gerard]

Some of you may remember a couple months ago I was completing some items not done in the Miata powered '69 Sprite. I purchased the MSD Tach driver as Trevor had suggested, but the tach only responds for the first few seconds the engine is running and then drops to zero. I tested the MSD unit and it works fine. Two different units of the same tach were tried with the same results, so I'm guessing something needs to be wired differently in the tach itself to work with the MSD. The car is now residing in SoCal, so I'm not able to experiment with it. Does anyone know how this might be resolved short of sending the tach to someone like PAS?

 

[Trevor Jessie]

I do not remember ... what type tachometer?

 

[Trevor Jessie]

How did you test the MSD unit?
Where is it drawing its power?

 

[Gerard]

I contacted tech support and they gave me a couple procedures to follow to make sure the unit was functioning properly. I don't remember exactly now, but basically checking voltage and out the output to the tach. It checked out perfectly. The tach works when the car starts, but dies in under 15 seconds or so. It does this consistently.

I followed the instructions precisely for the coil pack install. (Figure 1)

MSD 8913 tach drive pdf


The tach is the factory tach found in any '69 Sprite.

 

[Trevor Jessie]

So it is an RVI tach?

 

[Gerard]

Yes. I pulled one of this type tach from stash and it's labeled RVI 1433.

 

[Trevor Jessie]

I believe you need an RVC tach. How did you wire the RVI to the coil signal wire?

 

[Gerard]

Not sure if that's the case or not. I'll call tech support again and see what they say, but the answer may be here: I need to review it more thoroughly.

https://www.msdignition.com/page.aspx?id=3300

I don't remember positively how I wired, but think I went to the spade terminal. As I said, no chance to review it, so looking to point the owner in the right direction.

 

[Trevor Jessie]

An RVI tach picks up the pulse going through the ignition wire. The ignition wire is looped through the sensor on the back of the unit. Sometimes if they do not work consistently you can loop the wire through an extra time and it will work. The later RVC tachs just work by reading the pulses of the coil. Easiest thing to do is install an RVC tach. Is this a 3-3/8" tach? If so, those are plentiful and cheap. I have one here that he can have for the cost of shipping (I'll have to test it to be sure it works).

 

[dklawson]

Trevor has already said most of what I was going to post. The RVI tachs as a category are called "impulse tachs" and they are generally incompatible with any electronic ignition. (Don't flame me.... Yes, there will be exceptions). RVC tachs are more like a modern tachometer and work by counting voltage pulses at the coil.

Find an RVC tach as Trevor suggested. If the RVC tach does not look correct, transfer the RVI face onto the RVC movement to create a working tach that also looks correct.

 

[Gerard]

Thank you Trevor and Doug.

Trevor, the is the tach you are offering a Midget tach? The complication in this has been that the owner want to keep the original tach, at least as far as looks. So I'm wondering if it's a matter of just swapping gauges or if what Doug is suggesting can be done if necessary. If so, he'll probably go for it.

Maybe you could send me a pic.

 

[Trevor Jessie]

Yes, it is a Midget tach. I thought they were the same in '69. I'll get a picture in just a moment.

 

[Trevor Jessie]

If this looks right, let me know and I'll test it. I'm pretty sure it works, but better safe than sorry.

 

[Gerard]

yep... his has the rounded edge bezel, but that's easy to change.

Let me know if it works and I'll check with him to be sure.

Thanks!

 

[Trevor Jessie]

I tested it and it seems to work fine.

 

[Gerard]

I tested it and it seems to work fine.

OK, let me finalize things with him and I'll get back to you as soon as I can. I think you can go ahead and find a box for it though.

Thanks Trevor!