Tachometer Recommendation Needed & Part Identification

[XC92]

I'm about to diagnose an idle surge issue with my '92 Talon TSi and will be thorough given the age of the car, and use the opportunity to replace aging parts even if they're not yet failing, e.g. gaskets, o-rings, hoses, etc. I intend to remove the throttle body to inspect it and the various sensors and actuators on it and clean everything properly.

I have a decent multimeter with various probes, I just ordered an auto stethoscope (Lisle, with both tube and rod to check for vacuum leaks and mechanical issues), and I'm going to rent a vacuum tester from Autozone.

But I'd also like to buy or rent a tachometer. Not the kind you mount on the dash, of course, but the kind you attach to the proper leads in the engine bay so you don't need a helper sitting in the driver's seat telling you what the RPMs are.

Can anyone recommend a decent and reasonably inexpensive one, preferably digital and with max, min, avg, etc.? Alternately, is the some sort of cable you can attach to a phone or tablet to use with an app to do that, or maybe bluetooth?

Also, while looking around the throttle and intake plenum I saw an unconnected connector and couldn't identify it, just behind the firewall side of the plenum attached to a wiring harness. Does anyone recognize it?

When I bought the car nearly 30 years ago I had a local alarm shop install the works, so perhaps this is from that? Or is it an OEM part and if so what does it do and why is there nothing connected to it? Is it for diagnosis?

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[1990TSIAWDTALON]

You would be looking for what is called a Tach/Dwell meter. I use mine on a lot of my old cars (domestic usually).
I found a cheap (which is all you need) one on FleaBay HERE. It is alot like mine except mine has more features.

 

[XC92]

You would be looking for what is called a Tach/Dwell meter. I use mine on a lot of my old cars (domestic usually).
I found a cheap (which is all you need) one on FleaBay HERE. It is alot like mine except mine has more features.

Do you know of digital ones? Ideally a dongle you attach to the car that connects to a phone app via bluetooth, like some OBD2 scan tools.

I assume that one can get RPM (and much more) from the OBD1 diagnostic port with something like ECMLink, but that's a bit much for my present needs and budget but something I'll look into eventually.

 

[1990TSIAWDTALON]

I've never used a digital tachometer, they don't read accurately enough for what I do. We use some small digital tachs on our buggies and gocarts but IDK how accurate they are, we just have them for a measure of speed, not really for diagnostic purposes.

 

[XC92]

Ah, I see. I guess the really accurate digital ones are too expensive to be worth it? Now I'm putting on my electronics tinkerer hat and wondering if I could build one with an Arduino and LED display, and if it can even handle as many RPMs as DSMs can put out. I assume that to be accurate it would have to be at least double the max RPM of the engine.

Do you know if the terminal you attach the tach to is 12V, and is the way it works that it cycles between 0V & 12V every RPM, a little like the error code output but much faster? I guess I could run some tests to find out.

 

[1990TSIAWDTALON]

It is a square sine wave I believe. You would be pulling the signal from the coil packs or the PTU.
@steve, can you add your .02¢ to this and make sure I am not feeding mis-information?

 

[XC92]

I "think" there's a terminal on the firewall to connect a tach to, that puts out the right signal, so long as you ground the other tach probe.

Although, I wonder if you could piggyback on the crank sensor to accomplish the same thing, possibly even using an inductive clamp.

Yeah, I know, I'm overthinking this. That's just me.

 

[1990TSIAWDTALON]

The small engine tachs that we use are inductive, I just don't know how accurate they are. Plus they are on a 1 cylinder engine (Predator 212's mostly).

 

[XC92]

I guess I have some homework to do!

 

[steve]

The engine speed check connector in the engine bay is a tap off one coil between it and the PTU. It's going to have a signal with all the spikes you would expect when looking at the coil primary and only half speed since it's one coil not both.

The output of the PTU for the cluster is the combined signals and likely cleaner 0-12v.

The ECU reports RPM via the DLC but it limited to 8 bits so there is a step size and limits on how fast you can query it over a 1952 bps serial link.

The factory spec for idle speed is 750 + - 100 RPM so there is the question of how precise the measurement needs to be. If you look at various DIY digital tachs they include some sort of averaging to make the displays readable.

Before I built a datalogger, I think I used an automotive multimeter on the check connector left over from the dark ages of mechanical ignitions and vacuum based control system.