RPM Gauge Issue, Advice?

jordanZ · 09-29-2009, 10:17 PM

I have the 1965 Ford Mustang with the 200 CID 6 cylinder, automatic. For testing reasons I've installed an RPM gauge by Autometer, the "Short Sweep Retro" as the design works well with the car. It has three wires. One to a 12V ignition source, one ground, and the trigger wire to the negative (-) on the coil. For whatever reason, this brand new RPM gauge isn't accurately displaying the rpms. It'll idle a steady 800, normal enough, but acceleration makes little difference to the gauge as it only shows up to about 2K at most. I'm confident the gauge isn't bad. It's right out of the box. Any other possibilities?

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03svtcobra12 · 09-29-2009, 11:21 PM

maybe it is getting a weak signal for some reason? try hooking it somewhere else or checking the connections, and make sure it is grounded properly, on bare metal tightly.

kalli · 09-30-2009, 07:13 AM

just as a thought (not neccessarily) the problem. if you still use points, they might be misadjusted/worn or the condensor bad, so that the tach doesn't get a clear signal.
Further if you hooked it up to the 12V ignition being coil+, there's only 8v there on a stock car. So you might want to hunt for a different 12V source

MetalEd · 09-30-2009, 10:01 AM

Quote:

Originally Posted by jordanZ

I have the 1965 Ford Mustang with the 200 CID 6 cylinder, automatic. For testing reasons I've installed an RPM gauge by Autometer, the "Short Sweep Retro" as the design works well with the car. It has three wires. One to a 12V ignition source, one ground, and the trigger wire to the negative (-) on the coil. For whatever reason, this brand new RPM gauge isn't accurately displaying the rpms. It'll idle a steady 800, normal enough, but acceleration makes little difference to the gauge as it only shows up to about 2K at most. I'm confident the gauge isn't bad. It's right out of the box. Any other possibilities?

You can hard wire it temporarily to the battery for testing purposes, but I don't think electrical power is the problem.
First, you know that the tachometer (rally pac) for the 6 cylinder cars only goes to 6,000 rpms, the one for 8 Cylinder goes to 8,000 rpms. Several aftermarket/mustang Catalogs show the rally Pac for 6cyl Mustangs.
Now is your car is an automatic? Idle is supposed to be set at about 500-525 (650-725 is more realistic on my car) and then while normal driving the RPMs are usually up around 2,000 RPMs.. unless you are driving a manual and are real heavy on the gears.
I was watching them after I read your message, on my 2006 nissan Pickup truck (also a 6cyl since my Mustang is in parts for waiting for brake parts). The automatic transmission would shift at about 2000-2200 RPMs for normal city (L.A.) driving. About the only time it would go any higher is if I gunned it at red lights to accelerate to the point of really taking off (for these test purposes) or when I got on the freeway and floored it to pass someone. At those times it got to 2500-3000, the rest of the time it stays at about 2000-2200. I don't think I would ever see 4,000 RPMS in that truck.
If the tach you purchased goes to 8000 or even the newer ones that go to 12,000 RPMs, you are not going to see much in that range unless you are abusing your engine. It might not look like it even moves at that low of a resolution! You could test it by gunning the engine while parked, for short bursts only of course. Just completely floor it and let it climb, if it jumps to 3-4000, then there is no problem, your gauge and car are working right.
So my point is this, the rally Pac for 6 Cyl cars only goes to 6000 RPMs, you are holding out at about 2000, which is 1/3 of the max for those tachs. For your 6, I don't see a problem. It sounds like everything is about right. But if your new tach shows 8 or 12,000 rpms, you are going to see very little needle movement since you will never see 40-50% of the upper range unless you did some extreme mods to your engine and abuse your car. I would suggest looking for a smaller tach that only goes to 6k, then there will be lot more needle movement and greater resolution between each 1000 RPMs... (The numbers would be more spread out across the gauge)
I could be wrong in all of this, so it may not hurt to look it up and see what the normal operating RPMs and the MAX RPMs for your car would be and when it normally shifts gears. I think it all sounds right.

Piraya33 · 09-30-2009, 10:45 AM

Have you added an aftermarket ignition system like an MSD or anything?

JDraper · 09-30-2009, 06:04 PM

One other thing to check is to see if the gauge has a switch between 4-6-8 cylinders. If it does, you have to set it to the 6 setting to get the correct reading. If it doesn't, then the gauge needs to be specific to a 6 cylinder motor. The gauge is looking for a certain # of pulses to determine rpm's and if it's not set for a 6 cylinder the gauge will be off.

MetalEd · 09-30-2009, 08:01 PM

Quote:

Originally Posted by JDraper

The gauge is looking for a certain # of pulses to determine rpm's and if it's not set for a 6 cylinder the gauge will be off.

That's right, i didn't even think of that... The one I have also handles electronic ignitions differently (non-points). You are supposed to set it at 4cyl if you have an Electronic ignition. Each brand handles it differently and it does need a setting for 6cyl and used for a car with points (if you use them.)