Need Ballast Resistor Education for a 78
04-30-2016, 12:33 AM
#31
Intermediate
Thread Starter
yeah, I don't know what the heck happened with that post. I couldn't get it to post right so I gave up. Sorry bout that.
What I was trying to say is that my MM does
What I was trying to say is that my MM does
04-30-2016, 12:41 AM
#33
Intermediate
Thread Starter
Don't know the brad and model off the top of my head but it is by no means anything special. I have no reason to spend big money on a MM. I'll shoot the info over to you over the weekend if you want. I have 2 old radio shack ones that do
04-30-2016, 09:21 AM
#34
Rennlist Member
Could not find the ballast resisters on the Porsche parts catalogue. Looked everywhere for them.
04-30-2016, 10:33 AM
#35
Rennlist Member
04-30-2016, 11:08 AM
#36
Rennlist Member
That's what I thought, but mine has 2, and they are cylindrical, like the WSM picture linked to in #19 above. The PET part is only one 4 ohm, and not cylindrical. I know those pictures are just for general reference, but they usually portray the part fairly close.
04-30-2016, 11:17 AM
#37
Rennlist Member
http://forums.pelicanparts.com/uploa...1372835243.jpg
04-30-2016, 11:23 AM
#38
Chronic Tool Dropper
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Catching up on my reading after a couple months of focused project work.
The resistors:
The normal running voltage in the car is between 12.4 (no load, charged battery), maybe 13.7 or so with minimum electrical load and engine 1500 RPM or greater. While the starter is cranking, system voltage drops to less than 11 for the brief period while the starter is in service. For all these situations, you want a constant voltage available to the coil so you can reliably make a spark.
The "normal" voltage that the coil likes is somewhere a little less than the available cranking voltage, but not by much. Resistors reduce the amount of current to the coil, effectively reducing the voltage at the coil. When the starter is not engaged, current flows from the ignition switch via circuit 15, and on through both of the the resistors in series to supply the coil. Per the wiring diagrams for the 1978 car, there is a 0.4 Ohm resistor, followed by a 0.6 Ohm resistor between circuit 15 (on with key in RUN position 2 or START position 3) and the positive terminal on the coil.
When you turn the key to START position and the starter motor is engaged, a new additional current flow path is added to boost the voltage the coil sees. The starter motor is activated by a solenoid and switch. Turn the key to START position, current flows from the ignition switch through circuit 50 (on with key in START position 3 only) to the starter solenoid, causing first the starter drive to slide to engage the ring gear on the engine. Only when that gear is engaged, switch contacts send direct battery current through the starter motor. Another set of parallel contacts sends battery current to the ignition coil, through the 0.6 Ohm resistor only, using connection between the two resistors. So the coil sees the benefit of the current flow directly from the battery cable at the starter motor, and also effectively bypasses the 0.4 Ohm resistor. In actual service, this doesn't "boost" the voltage available at the coil, but instead it compensates for the normal voltage drop seen when the high-current starter motor is engaged. There's a bit of a fringe benefit to this method, as it also supplies the spark box "backwards" through the 0.4 Ohm resistor to reduce the effects of the starter current on voltage there.
As Jim points out, the coil is not intended to operate continuously on direct battery voltage without the current-limiting effect of the resistors. Bypassing the resistors briefly for troubleshooting purposes is OK, but... Continuous operation of the coils without the current-limiting resistors will cause the coil to heat up, expand, and eventually rupture the coil case. Oil is used to transfer heat from the windings to the shell in the coil, so that oil expands and finds its way out. Once the oil is no longer contained, the windings inside will quickly overheat and fuse.
The resistors themselves are wire wound on a small ceramic form. Current flowing through them causing them to heat, but unless the heat becomes excessive they virtually never fail completely. The connections at the ends do get dirty/corroded over time, and deserve the same level of care that other open/exposed electrical connections get. As you do to your ground connections, these get cleaned to remove oxide layers that increase resistance in the loop. DeOxit would not be a bad way to follow up on mild mechanical (small brass detailing brush) cleaning of the disassembled connections. The cost of new replacements is low enough that they could be put on a replacement schedule of, say, 20 years or so. If you have the originals in there, plan on replacement the next time you do your Annual Electrical Maintenance and are disassembling and cleaning the ground points.
One other thing to note is that the connection from the starter solenoid to the resistors starts under the car, where conditions aren't always ideal for electrical stuff. With the battery isolated, the connections at the starter motor should be periodically disassembled, cleaned, and possibly protected some with Vaseline, or maybe with a spray-on battery terminal protector. I'm not normally a fan of the spray-on sealer/protector in the battery well, but the connections at the starter are good candidates for some real protection due to road-crud exposure.
Hope this helps!
The resistors:
The normal running voltage in the car is between 12.4 (no load, charged battery), maybe 13.7 or so with minimum electrical load and engine 1500 RPM or greater. While the starter is cranking, system voltage drops to less than 11 for the brief period while the starter is in service. For all these situations, you want a constant voltage available to the coil so you can reliably make a spark.
The "normal" voltage that the coil likes is somewhere a little less than the available cranking voltage, but not by much. Resistors reduce the amount of current to the coil, effectively reducing the voltage at the coil. When the starter is not engaged, current flows from the ignition switch via circuit 15, and on through both of the the resistors in series to supply the coil. Per the wiring diagrams for the 1978 car, there is a 0.4 Ohm resistor, followed by a 0.6 Ohm resistor between circuit 15 (on with key in RUN position 2 or START position 3) and the positive terminal on the coil.
When you turn the key to START position and the starter motor is engaged, a new additional current flow path is added to boost the voltage the coil sees. The starter motor is activated by a solenoid and switch. Turn the key to START position, current flows from the ignition switch through circuit 50 (on with key in START position 3 only) to the starter solenoid, causing first the starter drive to slide to engage the ring gear on the engine. Only when that gear is engaged, switch contacts send direct battery current through the starter motor. Another set of parallel contacts sends battery current to the ignition coil, through the 0.6 Ohm resistor only, using connection between the two resistors. So the coil sees the benefit of the current flow directly from the battery cable at the starter motor, and also effectively bypasses the 0.4 Ohm resistor. In actual service, this doesn't "boost" the voltage available at the coil, but instead it compensates for the normal voltage drop seen when the high-current starter motor is engaged. There's a bit of a fringe benefit to this method, as it also supplies the spark box "backwards" through the 0.4 Ohm resistor to reduce the effects of the starter current on voltage there.
As Jim points out, the coil is not intended to operate continuously on direct battery voltage without the current-limiting effect of the resistors. Bypassing the resistors briefly for troubleshooting purposes is OK, but... Continuous operation of the coils without the current-limiting resistors will cause the coil to heat up, expand, and eventually rupture the coil case. Oil is used to transfer heat from the windings to the shell in the coil, so that oil expands and finds its way out. Once the oil is no longer contained, the windings inside will quickly overheat and fuse.
The resistors themselves are wire wound on a small ceramic form. Current flowing through them causing them to heat, but unless the heat becomes excessive they virtually never fail completely. The connections at the ends do get dirty/corroded over time, and deserve the same level of care that other open/exposed electrical connections get. As you do to your ground connections, these get cleaned to remove oxide layers that increase resistance in the loop. DeOxit would not be a bad way to follow up on mild mechanical (small brass detailing brush) cleaning of the disassembled connections. The cost of new replacements is low enough that they could be put on a replacement schedule of, say, 20 years or so. If you have the originals in there, plan on replacement the next time you do your Annual Electrical Maintenance and are disassembling and cleaning the ground points.
One other thing to note is that the connection from the starter solenoid to the resistors starts under the car, where conditions aren't always ideal for electrical stuff. With the battery isolated, the connections at the starter motor should be periodically disassembled, cleaned, and possibly protected some with Vaseline, or maybe with a spray-on battery terminal protector. I'm not normally a fan of the spray-on sealer/protector in the battery well, but the connections at the starter are good candidates for some real protection due to road-crud exposure.
Hope this helps!
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belgiumbarry (03-22-2020)
04-30-2016, 01:36 PM
#39
Rennlist Member
Excellent summation Dr Bob 
Thank you
Thank you
04-30-2016, 02:08 PM
#40
Also, to add to Dr. Bob's post and one that i've been encountering a bit the past few years. The 2nd power source on the starter, that he references sending power to the resistors and coils, fails in the starter. No amount of changing parts on the car is going to work, except a new starter. Oddly enough, I don't think the people doing the starter rebuilds for Bosch (unless Bosch is doing the rebuilds) even know to check that switched starter circuit. It was frustrating trying to find that fault the first time I encountered it.
04-30-2016, 02:18 PM
#41
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Also, to add to Dr. Bob's post and one that i've been encountering a bit the past few years. The 2nd power source on the starter, that he references sending power to the resistors and coils, fails in the starter. No amount of changing parts on the car is going to work, except a new starter. Oddly enough, I don't think the people doing the starter rebuilds for Bosch (unless Bosch is doing the rebuilds) even know to check that switched starter circuit. It was frustrating trying to find that fault the first time I encountered it.
04-30-2016, 03:37 PM
#42
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Also, to add to Dr. Bob's post and one that i've been encountering a bit the past few years. The 2nd power source on the starter, that he references sending power to the resistors and coils, fails in the starter. No amount of changing parts on the car is going to work, except a new starter. Oddly enough, I don't think the people doing the starter rebuilds for Bosch (unless Bosch is doing the rebuilds) even know to check that switched starter circuit. It was frustrating trying to find that fault the first time I encountered it.
If it's like most others...
Inside the solenoid housing is a sliding "bolt", that is drawn back by the current in the solenoid, attached at the business end to a little lever and fork that move the drive gear. At the other end of that bolt is a copper disk that does the high-current switching for the starter motor. The "contacts" for the battery cable and the motor are typically "T"-shaped bolts, with the head of the T facing the copper disk. There's a coil spring in the housing that retracts the bolt and the disk, causing it to rotate slightly with each actuation. Purpose of that is to "distribute" any arcing/flashing/damage to the T-bolt contacts around the disk itself. Meanwhile, the contact for the ignition resistor bypass is just a smaller version of the big ones. It sees battery voltage exactly in parallel with the starter motor since it is connected to the battery (and starter motor) contact by the same copper disk when solenoid coil draws it back. But that arcing/flashing damage to the disk is important, since what might be a few ohms of resistance is a lot more critical to the coil than it is to the motor. Resistance in the starter motor contacts gets 'burned through' pretty quickly, but that damage is what keeps the current from flowing through the relatively high-impedance ignition coil.
So what to do? The solenoid can be disassembled from the starter, disengaged from the fork for the gear, and "serviced". You can "clean up" the copper disk, or more likely just turn it over on the end of the bolt if it's screwed on. You can also carefully "dress" the ends of the T bolts that are the contacts. With everything out, the bolts come out of the cap on the solenoid. Resist (sorry...) the urge to go to town on them with a file, as they all need to protrude to the same plane when they are reinstalled.
Many "starter" failures are actually solenoid failures, where current is no longer carried to the starter motor through those bolt contacts and the disk. Many hard-start problems begin with the little ignition contact, then quickly get worse when the you end up cranking longer trying to get the car to start, doing more arc flash damage to the disk, and on down the slippery slope to whole system failure.
Like many things you can find if you are looking, a DMM can help you identify a DUFO ignition connection through the solenoid before you mount a new or "rebuilt" starter on the car. Most "rebuilds" get tested on a bench for motor run and drive gear engagement. Just MANUALLY engage the starter solenoid without battery attached to the battery terminal, and use your DMM to verify continuity between the battery cable post and the ignition bypass contact with the solenoid engaged.
05-02-2016, 01:37 AM
#44
Intermediate
Thread Starter
Dr. Bob,
I have read through your solenoid explanation a number of times now as well as spending some time studying the Engine Ignition System in the spec book for a 78 and think I may have a bit of a grasp on all of this and need to thank you immensely. Before I get too carried away though I want to confirm I am getting this right--
Bad contacts and a copper disk getting progressively more beat up on the solenoid can be playing a very significant role in the no-start. It sounds like it could be in mine for sure. The power to the .6ohm resistor comes directly from the circuit on the solenoid and goes directly to the 15 term on the coil from there. My resistors are 38 years old and both out of spec. Too much resistance=poor voltage to the coil debilitates it's ability to give the distributor what it needs for decent spark. If that isn't happening then the pulsing through the green wire either isn't happening or isn't providing the ignition box with what it needs to supply the FP relay with the voltage it needs to keep the pumps running. That's also what is happening on my car. Do I have this right?
With a timing light on my coil wire I am getting one pulse then dead. The car fires of that one spark and the spray from the CSV. Then I get neither spark nor fuel.
My issue has been getting progressively worse but I have been progressively replacing things on the other side of all this and never knew to look towards the starter. I just figured since it was turning then the starter was good to go. With all of the times I have tried starting, re-starting, killing batteries in the process, I can only imagine what has been happening to the disk. This also started a few days after a 26 hour round trip drive from Atlanta to Philly and back at the tail end of one of those snows in february. The underside of my car was transformed to just plain nasty after all of that road treatment for snow and ice that I drove through. I can only imagine what it did to my starter terminals. I can't believe I never even looked yet. I will tomorrow
Thanks again. Will keep you posted.
Robbie
78 5spd
PS. JP-no clue on my DMM. It's nothing special but it does do
I have read through your solenoid explanation a number of times now as well as spending some time studying the Engine Ignition System in the spec book for a 78 and think I may have a bit of a grasp on all of this and need to thank you immensely. Before I get too carried away though I want to confirm I am getting this right--
Bad contacts and a copper disk getting progressively more beat up on the solenoid can be playing a very significant role in the no-start. It sounds like it could be in mine for sure. The power to the .6ohm resistor comes directly from the circuit on the solenoid and goes directly to the 15 term on the coil from there. My resistors are 38 years old and both out of spec. Too much resistance=poor voltage to the coil debilitates it's ability to give the distributor what it needs for decent spark. If that isn't happening then the pulsing through the green wire either isn't happening or isn't providing the ignition box with what it needs to supply the FP relay with the voltage it needs to keep the pumps running. That's also what is happening on my car. Do I have this right?
With a timing light on my coil wire I am getting one pulse then dead. The car fires of that one spark and the spray from the CSV. Then I get neither spark nor fuel.
My issue has been getting progressively worse but I have been progressively replacing things on the other side of all this and never knew to look towards the starter. I just figured since it was turning then the starter was good to go. With all of the times I have tried starting, re-starting, killing batteries in the process, I can only imagine what has been happening to the disk. This also started a few days after a 26 hour round trip drive from Atlanta to Philly and back at the tail end of one of those snows in february. The underside of my car was transformed to just plain nasty after all of that road treatment for snow and ice that I drove through. I can only imagine what it did to my starter terminals. I can't believe I never even looked yet. I will tomorrow
Thanks again. Will keep you posted.
Robbie
78 5spd
PS. JP-no clue on my DMM. It's nothing special but it does do
