You are so far off, it's not even funny

The tach signal is usually taken from either the points, or the output from the electronic ignition to the coil. Some had a direct-driven mechanical tach, using what could be called a speedometer cable, driving a gauge that is basically the mechanical equivalent of a speedometer.

I've never EVER heard of a car/truck/road-vehicle using the alternator as a tach signal. MOTORCYCLES on the other hand, DO sometimes (often?) use the output from the stator (charging circuit).

On the 7.3, the PCM will drive the tach, using the crank or cam sensor to determine engine RPM, just like gassers.

 

The 6.9 and 7.3 IDI used alternator driven tachs. Many other diesels used alternator driven tachs as well.

 

No. Its a magnetic pick up off of the IP timing gear.


Mine is dead on. says 88 when I am going 88. The cop did not give me a warning either since I was towing at the time.

 

Yes, I know some had direct driven mechanical, but I think most of those are pretty rare. I knew some one that ripped one off his '72 Roller on some road debris, but other than that I haven't seen many of those. Not that I'm a mechanic or anything...

On the point issue though... As was already pointed out to me, PSD's don't have those, nor ignition coils. Most cars that have ignition coils though, will probably just use the computer directly. If they are old enough to have points, they may use that. I know when I replaced gauges in my '73 Camaro, you just connected the tach to cigarette lighter where it read the AC frequency on the current through the car. I thought that was kind of hoaky and upgraded to one that measured from the points.

Here is an example of a cheap tach:
Sunpro ACP7901 - Sunpro Sun Super Tach II Tachometers - Overview - SummitRacing.com
install pdf, has you connecting to the battery to measure the AC current from the "charging circuit".
http://static.summitracing.com/globa...%20english.pdf

You could try to measure each time the points connected and the plugs fired causing a drop in voltage, but I figured that would be hard considering some cars used capacitors to help charge the condenser (I think Chryslers used to do this), and that give a very weird signal.

Thought I read the AC signal they measured was the result of the alternator, but perhaps it the points firing that caused the AC signal. In either case, it is an indirect measurement that can be alternated by many things.

In any case, the 7.3 was the only engine I wasn't sure if the tach was from the from the computer or not. Since it is, all PSD and V10's take from the computer. So the discusion is off topic.

 

Then yours is off compared to everyone elses

Honestly, I haven't check the one my F250. I know my F150's were pretty close, maybe 1 mph off at 70, closer than my Honda's and my current Saab's though. Maybe foreign cars use the wrong conversion factors to screw with us Americans.

 

Both times I got pulled over (91 Mustang and 99 Oldsmobile) I was doing exactly what the cop said I was doing too. Cops on I-81 between Bristol and Abingdon went through a phase about 10 years ago where they would give you a ticket if you just thought about going too fast. Both of my tickets were 70 in a 65 and one of my friends got one going 66 in a 65.

 

The 5 tickets I've had, all when I was younger, 4 of them I thought I was going faster than the cop said. Interestingly I had a 91 Mustang too, you could actually slam on the brakes and come to a stop and the speed would still read 15 mph the instant you stopped. It would then fall down to 0, but the speedo in that was a joke. But the car was beaten up pretty badly by the time I got it. So it doesn't surprise me it was off.

 

I busy in the morning and there are 5 pages to catch up
To clarify my previous statement, I rounded the rpm to 3000 at 75 mph, when it is about 2900. My rear is 4.3 from what I remember and I have 6-speeds.
Still pretty annoying when you are burning fuel unnecessary turning high rpm driving with small or no load.
I posted the picture before, but even with this load I was driving on 6th gear even on slight inclines.

 

Many different things can affect/interfere with a radar gun. Chances are the error was on the guns side and not your speedo.

 

In those instructions you posted a link to, READ ABOUT THE GREEN WIRE.

Capacitor to help charge the condensor?

Sheesh. I dunno where people get this stuff

 

Stupid green wire

Maybe my old car had the "green" wire integrated into the gauge cluster or something...

I need to learn to spell... nothing more lol. they used a capacitor to help build the charge the condenser (aka coil). The capacitor allowed for the ignition system to "charge" all the time instead of only between contacts from the points. They did it because they ran a little higher voltage than most other cars. I'm not sure if they did it in all their cars, but they did it in the 440 6-pack. besides, Dodge always does weird things, specially with their electrical stuff

 

that is true. But for the Saab, someone over at Saabcentral had their speedo verified on a dyno, and it was off. Anyway, I used to have the papers from that lawsuit (actually class action) with Honda, not sure if I moved them yet though since I got rid of that car about 5 years ago.

 

it looks like your rear springs are a rubber band ready to snap

 

The "condenser" in points-style ignition systems is to keep the points from arcing and burning out.

OK, way off topic.

 

I was thinking the same thing...