Using Tach/Dwell Meter - need some help

[rgnprof]

I have bought 2 different types of tachs lately to try and measure my '72 250 rpms. I bought a digital one that didn't work and then I bought an older Sears Engine Analyzer - analog with the needle, measures volts, amps, dwell, and low/hi rpms.

Trouble is I can't get either of them to measure my rpms. I have an Equus Tach/Dwell Meter that works but it only measures up to 1600 rpms. It has a green clip which connects to the -coil terminal and a black ground clip that goes on the -battery terminal.

I tried connecting the digital one the same way and the numbers just jump all over the place and the Sears one doesn't measure anything. The Sears model has red and black (battery?) clamps and a green clip and a plate. I tried attaching the green clip to the -coil terminal and the black to ground and nothing (BTW, the Sears analyzer came with no manual).

One bit of information - I am using a Crane optical ignition, but the Equus model seems to work fine - it just just doesn't read high rpms.

Anyone have any suggestions?

ryan

[230/8]

Ryan:

Your Sears tachometer is most likely intended to sense the interrupted circuit between the coil and points, but with the Crane set-up there is probably not enough current flow in that wire to register the meter. Your Equus unit probably senses the collapse of the magnetic field in the coil so that is why it works.

My recommendation would be to get a tachometer that uses an inductive pick-up around the spark plug wire. I have a Sears unit that uses this method to gather the signal and it works fine with the Pertronix in my distributor. It has a power supply to the battery and two different plug-ins for tach and dwell. My Sears timing light also uses an inductive pickup so there is quite a tangle of wires off that #1 cylinder.

If you have the instructions for the Crane you might check there to see if they have a suggested pick-up point for dwell/tach hook-ups. Check into their website, too. I suspect you only need to score the right spot for the Sears unit to work, but I cannot suggest where that might be.

230/8

[jaimekop]

I have (I think) the same sears unit... the red and black clamps must goto the battery to power the unit. The green one goes to the coil.

It should work with any type of ignition (crane, pertronix, points, etc).

[rgnprof]

That's how I set up the Sears unit - black to neg battery post, red to positive and green to coil negative. Doesn't work.

Called Crane today and got a very helpful tech - but he didn't know why the Sears one wasn't working (I bought this thing on ebay, so I'm not sure it works, and like I said earlier, I didn't receive the manual with it). He said the digital one may have some problem with radio interference and suggested installing a 10,000 ohm 1/2 watt resistor (from Radio Shack) inline to eliminate this interference.

I don't know for sure...

ryan

[MunichTaxi]

black to neg battery post, red to positive and green to coil negative.

Try Green to coil +

[Mike D]

Coil positive is a constant feed. Coil negative is "switched" by the contact points (or optical sensor) making and breaking ground contact.

[rgnprof]

Mike,

Which means the negative coil terminal is the right one - right? I tried the positive anyway and it doesn't work either. I am going to try the both tachs on my van tomorrow.

ryan

[Mike D]

Yes, coil negative is the one to read. As the points/optical sensor/ magnetic switch closes or makes contact with engine "earth", the coil completes the circuit and discharges it's charge via the coil wire through the distributor center terminal, across the rotor to whatever sparkplug wire it is facing. Each time it grounds it is counted and divided by 4, 6, 8, etc. number of cylinders to reach the RPM count.

So much for engine firing ignition setup 101.

[rgnprof]

Mike,

So why are neither of these working (the digital one and the Sears engine analyzer)?

ryan

[Mike D]

Ummm, let me get this straight. You bought a digital meter which doesn't work and you bought an old Sears unit which you don't know if it works and you're wondering why you cant get them to work?

I'm not one who usually pumps products or stores but do yourself a favor. Go over to Autozoo and pick yourself up an Actron CP7677 AutoTroubleShooter. $59.99 and probably the best value for the money. It has tach, dwell, audible continuity tester, multi-range ohmmeter and voltage settings (AC and DC) and a light duty ammeter (up to 10 amps).

(offer not valid in Lower Republic of Estonia, prizes, shipping, handling, overnight stays not included, but wait! There's more.......)

[rgnprof]

I actually looked at Autozone - for that meter and they didn't have it. I ran across it online... I'll look around.

[rgnprof]

I decided to stop by Radio Shack and check out the 10000 ohm resistor approach, but I couldn't figure out how to wire it in. There was a time when Radio Shack employees knew a thing or, but that time has passed!

Anyway, I decided I would try and lengthen the distance between the meter and the coil and ground(battery) using longer leads. If I get the meter out of the engine compartment the digital meter works fine, so there is clearly something interfering with the signal if I have the meter too close to the coil.

ryan

[Mike D]

Keep us posted on the interference problem. I've never heard of it happening. Can't be the Crane because it's optical and not Hall Effect.

If you disconnect your alternator, does the interference go away?

[pmckechnie]

I don't know anything about the Crane system, but I do know that an MSD system WILL NOT work with a conventional tach or dwell meter. They use a capacitive discharge system and the voltage on the coil is much higher (200 to 300 volts if I remember correctly) and have up to 8 or 10 discharges per firing event at idle. The MSD does have a tach signal coming out of the unit for a tach and maybe the Crane has the same.
This is just food for thought.

[Blue 72 250]

You may want to check plug wires for cracks, frays, etc. The only thing I can imagine (other than alternater mentioned by Mike D)that would create that much interference is an un-insulated ht discharge to ground.