Thoughts on exciter line diodes (for engine runon when ignition switch is off)

[hcpookie]

My A4 setup had the original Motorola alternator when I bought it. I found out "the hard way" that the alternator is non-functional (now THAT was a fun day! )

I have replaced the alternator with a Delco 10SI. I chose this because the 10SI is more familiar to me and apparently more robust than the Motorola.

I have two questions about the installation:


1.
My engine exhibits the runon problem after the engine shuts off, which I've already read is due to residual charge on the exciter wire. This is, apparently, to be expected with this configuration.

I've read the few threads in this forum and understand the need to install a diode on the exciter line, with the mark pointing toward the alternator. This solution is also recommended in a few automotive forums as well.

QUESTION: What kind of diode? Is there a certain amperage level I need to work with? I've never been able to determine what is too big or too small. Would you need a spare? I intend to run the wire with connectors and make it long enough that the diode "link" could removed from the exciter wire, allowing it to be directly wired to the alternator as it is right now.


2.
I've read on some different automotive forums that some people have resolved the issue without a diode by using a switched power source instead of wiring the exciter wire to the coil. For example, wiring the exciter to a relay or sending unit that is powered through the switch and not directly from the battery.

This actually sounds like a better solution to me than the diode, since it would eliminate the need to include "yet another" piece (the diode) to the alternator circuit.

There is reportedly no ill effect from using this switched solution for the exciter. However this was always for automotive applications, even for a tractor application!


QUESTION: What are your thoughts/recommendations about a switched power source instead of to the coil on an A4? Where would you wire it... directly to the switch? ... the oil pressure switch?


What are people's experience with the 10SI and the A4? Any other hidden problems I should consider?

TIA
- Jerry

[ndutton]

Jerry,
I found your post from a couple of years ago and wondered how your alternator installation worked out. I'm planning on installing a 10SI as well and have run into some conflicting information.

For instance, mine is referred to as a "single wire" hookup but it still has the exciter and sensing terminals 1 & 2 present, although they are capped off with a rubber insulator. I have continuity between the exciter and the output terminals suggesting a potential engine run-on problem which led me to your diode post. There is also continuity between the sensing and output terminals, in fact, ALL the terminals have continuity between each other.

As much as I would like to have the exciter and sensing connections, it looks like I'd better stay with the single output connection only (as designed), but then I'm wondering how it senses battery condition to adjust the charging output.

All thoughts appreciated.

[ndutton]

100 amp Delco 10 SI Single Wire Alternator

I half thought the electric guys would have been all over this one. Here's how it went:

The higher output alternator is physically larger than the 35 amp Motorola but only a little. I had to modify the lifting eye/alternator mount slightly to allow for the new alternator to pivot sufficiently for belt adjustment. Other than that, it bolted in easily and still fit in the very tight confines under the Catalina 30 dinette seat - overall height was an early concern.

Regarding the connection, I finally stopped myself from rethinking and over-analyzing the original design and connected it with the single output wire as specified by the manufacturer. It worked beautifully and I was pleased that it produced well at low RPMs. I don't have a tach but at what could be considered a high idle the output was clearly evident on the voltmeter.