1968 Ford 4000 alternator conversion tach pegged high?

I have a 1968 Ford 4000 3cyl gas burner. I replaced the generator with the alternator conversion kit the other day and hooked up the tachometer. Initially when I turned the engine on it showed it was idling around 1,000RPM. I turned it off to finish the install and when I fired it back up the tach read 1,000 but then it gradually started sweeping higher and eventually pegged out. This is while the engine is at idle. It was late last night so I quit messing with it and went to bed.
I know the tach cable is spinning. Is my gage messed up somehow? All I did was replace the generator with the alternator conversion. Is it possible the gear drive is wrong on the alternator and spinning it too fast?
Thanks for the help.
-RC

Same thing happened with my buddies 5000. Contacted the supplier, they replaced the tach drive, same result. The comment from the
manufacturer was "Maybe the cable is slipping". It registers app. twice actual rpm. wish I could find an old 2-1 speedometer reducer, I
think that would cure the problem.

(quoted from post at 10:55:29 01/20/20) I have a 1968 Ford 4000 3cyl gas burner. I replaced the generator with the alternator conversion kit the other day and hooked up the tachometer. Initially when I turned the engine on it showed it was idling around 1,000RPM. I turned it off to finish the install and when I fired it back up the tach read 1,000 but then it gradually started sweeping higher and eventually pegged out. This is while the engine is at idle. It was late last night so I quit messing with it and went to bed.
I know the tach cable is spinning. Is my gage messed up somehow? All I did was replace the generator with the alternator conversion. Is it possible the gear drive is wrong on the alternator and spinning it too fast?
Thanks for the help.
-RC

Click to expand...

eems simple enough to compare gen to alt gear ratios?

Generator has about a 4" pulley on it. The tach drive is a worm gear off the back of it.
Alternator is about a 2" pulley on it. The tach drive on it is a couple of gears stacked off the back of the axle.
It's not really easy to compare ratios...

(quoted from post at 13:16:27 01/20/20)
Generator has about a 4" pulley on it. The tach drive is a worm gear off the back of it.
Alternator is about a 2" pulley on it. The tach drive on it is a couple of gears stacked off the back of the axle.
It's not really easy to compare ratios...

Click to expand...

ust count number of pully rotations needed to make tach output drive turn one revolution. For pulleys, measure diameter at shinny point of belt contact. Then myself or others will help you from there.

Make sure the tach cables square drive end fits easily into the alternator tach adapter, if not it can push on the cable causing pressure on the gauge making it's internal components to drag and peg the needle.
Try loosening the nut on the cable housing a few turns and see if the tack starts working again.

I've done 3 alternator conversions, all of my tach's work but they read fast 1-200 rpm at idle and 3-400 rpm fast at pto speed, I bored the Lucas 4" pullet to fit the Delco but that only helped about 100 rpm at pto speed.
I used a digital tach to find 540 pto speed and marked the tach face where the needle was pointing.
On my 5000 at 540 pto rpm, even thought the needle is at 2300 rpm I know it's actually only running 1900, I've gotten use to it, if anyone else drives the tractor I tell them where to set the rpms on the tach

OK I messed around with it last night and you are correct. Something's wrong with the tach cable install to the gauges.

I counted the rev's on the generator and compared them to the alternator tach drive. Both generator and alternator are the same ratio-- roughly 3 turns of the drive pulley to 1 turn of the tach drive.
When I turn on the tractor about 10 seconds later the tach is pegged high. It stays pegged even after turning it off. If I unscrew the tach cable on the back of the gauge it comes back to 0. So I have come to the conclusion I'm installing the cable wrong. I did notice it's really tight going in to the back of the gauge. I'll file it down maybe so it fits better. Hopefully it solves the problem and I don't have to buy another gauge cluster.
Thanks for the help guys.
-RC

I counted the rev's on the generator and compared them to the alternator tach drive. Both generator and alternator are the same ratio-- roughly 3 turns of the drive pulley to 1 turn of the tach drive.

Click to expand...

Are the pulleys the same size? If not, then there will still be a difference between them for the ratio from the crankshaft pulley to the tach cable drive.

(quoted from post at 13:03:09 01/21/20)

I counted the rev's on the generator and compared them to the alternator tach drive. Both generator and alternator are the same ratio-- roughly 3 turns of the drive pulley to 1 turn of the tach drive.

Click to expand...

Are the pulleys the same size? If not, then there will still be a difference between them for the ratio from the crankshaft pulley to the tach cable drive.

Click to expand...

Generator has about a 4" pulley on it. The tach drive is a worm gear off the back of it.
Alternator is about a 2" pulley on it.", he said.

I was wrong the other day-- pulleys are roughly the same size. Since the ratios are similar, I think my problem is mounting the tach cable. The new one I got was a little snug going in, and I think it's binding the needle.
I'll try re-fitting it tonight to see if it helps.
Thanks guys.
-RC