D17 ballast resistor
Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=174223
Printed Date: 10 May 2025 at 6:46pm
Software Version: Web Wiz Forums 11.10 - http://www.webwizforums.com
Topic: D17 ballast resistor
Posted By: Ryan Renko
Subject: D17 ballast resistor
Date Posted: 08 Sep 2020 at 8:29pm
| They say there is not such thing as a stupid question. I am wiring my D17 IV gas tractor using a harness I bought. I am no expert but I thought the resistor went between the starter solenoid and the coil to reduce the voltage but the wiring diagram shows it getting placed as the wire splits and returns back to the key switch. I know this resistor can be avoided by buying a coil with one internal but I already have a Agco resistor and I just want to use it with my coil. Maybe I am over thinking this. Ryan |
Replies:
Posted By: DrAllis
Date Posted: 08 Sep 2020 at 9:17pm
| Starter relay sees to it that 12V momentarily gets to the coil for hotter starting spark. Resister from the key drops the voltage down to save the points. |
Posted By: john(MI)
Date Posted: 08 Sep 2020 at 9:38pm
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It doesn't matter where the wire comes from. The wire going to the coil should be cut and the resistor connected in this wire. ------------- D14, D17, 5020, 612H, CASE 446 |
Posted By: FREEDGUY
Date Posted: 09 Sep 2020 at 6:52pm
Did all gas engines after the "mag" ignition have this resistor ? I know our 180 has one, but I am too young to recall one on the S2 '17 we ran  .
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Posted By: Ryan Renko
Date Posted: 09 Sep 2020 at 8:01pm
| I mounted and wired it up tonight like the book and Dr.Allis verified. Hopefully I can get it assembled and ready for a start up in the next 2 weeks. Ryan |
Posted By: DaveKamp
Date Posted: 09 Sep 2020 at 8:24pm
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Some coils are wound specifically for full-voltage, some are wound for partial, and some are wound for partial, and have an internal resistor. The important thing to understand here, is the resistor's PURPOSE. Let's say you have a 12v coil, on a 12v battery, with a 12v starter, on a big, cold engine full of thick oil. When you turn on the ignition, battery voltage (around 13.8) will appear at the coil... and when the points are closed, you'll get about 15A of current flowing through the coil's primary winding. When you engage the starter, a whole lotta current runs through it's windings. The battery, being a chemical storage device, has a certain amount of 'limitation' to providing current, and since the starter is a 'more hungry' device, the battery's voltage will 'sag' to about 10v or so. This means the coil will only be getting around 10v, it's corresponding current flow will be lower, thus the spark will be a bit weaker as a result. So a clever guy came up with a dandy idea- re-wind the coil primary for 9V. Now, when the engine cranks, the 10V or so, is still more than enough to fire the coil. The problem is, that once cranking is done, system voltage jumps back up to 13.8, and once generating, closer to 14.2 or so, which causes our 9v coil to overheat and fail. SO... what they do, is power a 9v coil, but limit it's current with a ballast resistor so that 14.2 won't kill it. To take advantage of the coil's 9v-ability, there's a BYPASS wire from the starter motor. When you engage the starter, the coil is powered DIRECTLY from battery, so you get full ignition output under cold-cranking load, and once started, it's back to resistor-feeding. I'm skipping past lots of other finite details (like dwell time, duty cycle, HF suppression, frequency limit, etc.) as they're just too deep for this thread, no need to delve into that., but suffice to say there's a whole mess of other things that come into play in ignition systems that you really don't have to worry about, particularly from 400-1800rpm. Read your coil, if it is new, it will probably say 'no resistor required', or 'has internal resistor', or 'use external resistor'. If it doesn't say any of that, looking for a part number, and doing some web-searching is in order. If it's an OEM looking coil, then consult the oem wiring diagram. ------------- Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest. |
Posted By: FREEDGUY
Date Posted: 10 Sep 2020 at 6:09pm
This in EXCELLENT description of how the ignition components "mesh" during starting a gas engine, THANK YOU very much Dave for your time typing the "series of events"  
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