Alternator question(s)

[Iceni John]

Does anyone here know what AC voltage should be from the signal terminal of a Leece-Neville 160A alternator?   I measure mine, and it's giving only about 0.8VAC at 190hz when the engine's at low idle, but the voltage does rise at fast idle.   Is this signal produced by the voltage regulator, or is it from one of the windings?   The reason I'm faffing about with this is because my original Teleflex tachometer stopped working a while ago:  sometimes it doesn't read at all and sometimes the needle just bounces about.   A bought a VDO 333-952 tach, but it's only managed a brief twitch of the needle when starting the engine, and nothing otherwise.   Two weeks ago I emailed VDO Tech Support, but they've not replied.   My alternator's technical info doesn't state what its signal voltage should be, and neither does VDO's info state what signal voltage the tach needs, so I'm in the dark here.   The alternator turns about 2.5 times faster than the engine  -  at low idle it's at 1510 RPM according to my handheld optical tachometer.

Anybody have any ideas on this?

Thanks, John   

[freds]

Generally alternators have built in diode arrays; so you might just be seeing ripple voltage. Generally you see greater variation if some of the diodes are bad.

Is this engine attached or belt driven? If belt driven I would just yank it off and take it to a local alternator rebuilt place...

I did that with the alternator on my airplane and they didn't even blink, just fixed it.

[richard5933]

The VDO should be reading the pulse rate, not the voltage. On that AC feed the pulse rate should equal the Hz. The manual on the VDO I have shows it can be calibrate for a pulse rate up to 200.

Are you sure that you're using the proper terminal on your alternator to get the signal?

[Iceni John]

The VDO should be reading the pulse rate, not the voltage. On that AC feed the pulse rate should equal the Hz. The manual on the VDO I have shows it can be calibrate for a pulse rate up to 200.

Are you sure that you're using the proper terminal on your alternator to get the signal?

I've tried setting all possible combinations of the tach's three DIP switches, but nothing happens.   I'm wondering if the 0.8VAC signal (measured at the tach) is sufficient to drive the tach.

In addition to the two big terminals for Pos and Neg there are just three other smaller ones, two for the regulator and one for the tach.   The old Teleflex tach was connected to that one, and it worked fine for many years until recently.

Strangely, I also have a separate signal generator driven from one of the camshafts, but I've never traced what it feeds.   Maybe the transmission, but it's an all-mechanical HT740, so who knows?   Perhaps I can drive the tach from it instead?

I don't know why VDO hasn't responded to my enquiry after two weeks;  even if they're all working from home they should still be able to answer emails.   This is such a bummer  -  the VDO looks good and is a 3000RPM tach, so I'd get better resolution than the 4000RPM Teleflex.   Maybe I have to buy some cheapo Indian or Chinese tach for now?

Thanks, John

[richard5933]

Is your VDO programmable? That's the one I have (but never installed). Instructions look like it would pick up most any signal out there.

That signal generator might have fed a speedo at one time. If there is nothing on it right now, why not give it a try?