What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

starter selonoid and diode

Christopher Murphy

Well Known Member
Today my starter stayed engaged after I released the starter switch. I checked the power lead to the solenoid and the switch was functioning properly. I isolated the solenoid and mechanically jumped the solenoid and the starter continued to turn after I opened the jumper. I then removed the
didode jumper and jumped again and after I opened the jumper the starter motor stopped.

Can failure of the diode jumper cause this? Anyone care to elaborate on the diode jumper that goes from the power coil term to the I term on the solenoid.

thanks

cm
 
simple

pretty standard set up

battery wire in and out to starter

starter switch wire to coil terminal..( the one that energizes the relay)

the diode wire was from the above term to the I terminal... according the the early vans instructions that came with the solenoid. the black anoid term on the coil term and the cathode end ( red) on the I terminal..


Seems it should be to ground to me.

cm
 
Diodes do fail

I have seen many diodes fail short. My opinion is if it is critical, use 2 in series. Most people assume diodes are 0.7 v drop, but in reality my experience is that they are typically 0.3 v, so two in series wont hurt anything.JMHO.
 
It sounds like it's wired incorrectly... the "I" terminal has power anytime the starter is engaged. If the diode is between the "S" and "I" terminals then I'd expect exactly what you're seeing. The banded end of the diode should be toward the "S" terminal and the other end should be grounded.
 
+1 to what Bob said. A diode on a starter contactor (actually any of the contactors) should be wired across the coil of the contactor. That means from the control terminal (the one that's made hot by the start button) to ground. The banded end goes to the control terminal. Many starter contactors have the ground side of the coil tied to the case of the contactor, which means the other end of the diode would be tied to a mounting screw.
 
Thanks guys

Bob and Charlie

I thought the same thing. I think its been wrong forever and when the diode failed it energized the coil..

I put it on as directed by the instructions in the packet it came in an ive always thought that was wrong..

Thanks
Cm
 
if you have a 4 post MASTER contactor, the 2 small posts are the coil and and it would be appropriate to put the diode across those terminals (band toward the side connected to the battery).

Starter contactors with 4 posts however are different animals; the coil will have one end grounded through the case and the other end connected to the "S" terminal. You apply power to that terminal to energize (internally connect the large terminals). Some starter contractors have a 4th terminal labeled "I" and it also gets power also when the contactor is energized. Purpose of the optional "I" terminal is to drive a starter mounted solenoid and/or a "starter engaged" light.

It might be that you were reading the instructions for a 4 terminal master contactor rather than a starter contactor...
 
Wellll, the I terminal has been repurposed by some aviators for those uses, but the original use was to bypass the ballast resistor in the coil circuit of old style Kettering (breaker points) ignition systems, while the starter operates ("I" terminal...). Kinda tangential to the current discussion, so I'll shut up.
 
Bob and Charlie

I thought the same thing. I think its been wrong forever and when the diode failed it energized the coil..

I put it on as directed by the instructions in the packet it came in an ive always thought that was wrong..

Thanks
Cm

The instructions specify installing it on the "S" terminal.
 
I think I understand the diode better

So basically the I term completes the circuit to ground..so putting the the banded end of the diode on the terminal is the same as grounding it. Alot of videos and articles show the diode going directly to ground on the frame..correct? Im thinking that would be better because as in my case that diode failed and allowed the I term to hold voltage on the s term keeping the starter energized.


Am I correct about this?
Cm

Cm
 
So basically the I term completes the circuit to ground..so putting the the banded end of the diode on the terminal is the same as grounding it. Alot of videos and articles show the diode going directly to ground on the frame..correct? Im thinking that would be better because as in my case that diode failed and allowed the I term to hold voltage on the s term keeping the starter energized.


Am I correct about this?
Cm

Cm

No. The "I" Terminal is energized positive when the solenoid engages. Putting the diode here to ground would basically do nothing, unless you put it in backwards, then it would blow itself up in quick order, or in your case taking a bit of time to fail as you connected it.
The "I" Terminal is like a second lower current pole to the solenoid. If you tear one apart you will see a copper strap connected to the "I" terminal such that when the solenoid pulls in, the strap makes the connection.
The diode is there on the "S" terminal as a rudimentary "filter". The starter and solenoid can fly back "dirty" spikes of power when their coils collapse. The diode shunts spikes to ground to protect the Bus power.
 
Last edited:
Failure to cummicate

I guess I am not being clear..the diode was installed exactly per the instructions for the 4 pole solenoid.
The diode was from the S term to the I term.

I understand the purpose of the diode and the way the solenoid works.. my point is..basically the diode became a wire connecting the S term and the I term. When I turned the start switch the starter engaged. When i released the switch the starter remained engaged. If the diode had been on the S term to a frame ground this would not have happened. So why connect it from the S term to the I term..? If it wete a 3 pole solenoid it would go to ground..

Cm
 
I guess I am not being clear..the diode was installed exactly per the instructions for the 4 pole solenoid.
The diode was from the S term to the I term.

I understand the purpose of the diode and the way the solenoid works.. my point is..basically the diode became a wire connecting the S term and the I term. When I turned the start switch the starter engaged. When i released the switch the starter remained engaged. If the diode had been on the S term to a frame ground this would not have happened. So why connect it from the S term to the I term..? If it wete a 3 pole solenoid it would go to ground..

Cm

You must have a wiring diagram for a different application, or a non-standard solenoid. Vans wiring diagram, and those in the previous links show it correctly.
Take a pic of the wiring diagram or instructions you are referencing. Maybe we can figure out what is amiss.
 
Last edited:
I guess I am not being clear..the diode was installed exactly per the instructions for the 4 pole solenoid.
The diode was from the S term to the I term.

I understand the purpose of the diode and the way the solenoid works.. my point is..basically the diode became a wire connecting the S term and the I term. When I turned the start switch the starter engaged. When i released the switch the starter remained engaged. If the diode had been on the S term to a frame ground this would not have happened. So why connect it from the S term to the I term..? If it wete a 3 pole solenoid it would go to ground..

Cm

I suspect you are using parts sourced from Van's but instruction documents sourced from somewhere else (aeroelectric connection?)

It sounds like you don't understand how the solenoid works. There are two types that have 2 small studs. On one, each stud is the opposite end of the activation coil. On the other type, the S stud is one end of the activation coil (the other end is tied to the mounting foot for a ground path), and the I stud is a second switched output like Jon mentioned. The solenoid sold by Van's as a start solenoid is the second type.
If you want to learn more detail of the operating specifics and internal configuration read through the discussion/debate in THIS THREAD
 
Last edited:
So basically the I term completes the circuit to ground..

I guess I am not being clear..the diode was installed exactly per the instructions for the 4 pole solenoid.
The diode was from the S term to the I term.

Ok, but based on your previous posts it seems you don't understand how it works. We are all just trying to help....

I am not aware of any document that shows the diode connected between the S and I terminals for the type of start solenoid that Van's sells.
Here is one (from the old thread I previously linked you to) that clearly shows diodes connected from the terminals to ground. The standard install using Vans plans does not install a diode on the I terminal (it is not necessary). There is only one on the S terminal.

"You also wrote that the I terminal completes the circuit to ground."
Only on the type of solenoid where the two terminals are connected to each end of the activation coil, and you obviously don't have that type because if you did, the terminals would not be labeled I and S.
It is not the solenoid type we typically use for the start circuit on RV's. We use the second type mentioned in my previous post and like the diagram linked to in this post.
 
No. The "I" Terminal is energized positive when the solenoid engages. Putting the diode here to ground would basically do nothing, unless you put it in backwards, then it would blow itself up in quick order, or in your case taking a bit of time to fail as you connected it.
The "I" Terminal is like a second lower current pole to the solenoid. If you tear one apart you will see a copper strap connected to the "I" terminal such that when the solenoid pulls in, the strap makes the connection.
The diode is there on the "S" terminal as a rudimentary "filter". The starter and solenoid can fly back "dirty" spikes of power when their coils colthlapse. The diode shunts spikes to ground to protect the Bus power.

The diode does not protect the buss, it protects the contacts in the relay/contactor. Without the diode, the collapsing field will jump the contacts to dissipate. This is how your sparkplugs fire BTW and they diode is acting like the grounded P lead. This jumping of the contacts causes arcing and pitting. Also, it does not really shunt the energy to ground, it simply creates a path (same one that the contacts providerd when closed) for the energy to flow so it doesn't arc.
 
Last edited:
The diode does not protect the buss, it protects the contacts in the relay/contactor.

A diode on a high power relay/contactor does not protect the contacts in the relay.

It is in place to protect the contacts in the switch that is controlling the relay. In the case of an airplane, the battery master and starter motor switches

Do a google search on relay coil protection diode - there is a lot of info and detailed explanations available.

One simple answer found -

Since an inductor (the relay coil) cannot change it's current instantly, the flyback diode provides a path for the current when the coil is switched off. Otherwise, a voltage spike will occur causing arcing on switch contacts or possibly destroying switching transistors.
 
That makes sense Scott. I was also told it keeps the spikes from damaging other equipment in the electrical system. Considering how it is connected, your explanation makes the most sense.
 
A diode... is in place to protect the contacts in the switch that is controlling the relay.
Thank you for pointing that out. In fact, Service Bulletin SB 92-01 was issued on the Aircraft Spruce starter switches to add a diode to keep the contacts from arcing like this (on the left):

m679nB.jpg


My starter solenoid is the "four-lug" version with the "S" and "I" terminals. The "I" terminal is connected with a flexible braid to the contact bar (or disc) that completes the connection between the two large lugs when engaged. I added a large 18V MOV between the "I" terminal and each of the big lugs to (hopefully) protect the main contacts in the starter solenoid.
 
My starter solenoid is the "four-lug" version with the "S" and "I" terminals. The "I" terminal is connected with a flexible braid to the contact bar (or disc) that completes the connection between the two large lugs when engaged. I added a large 18V MOV between the "I" terminal and each of the big lugs to (hopefully) protect the main contacts in the starter solenoid.

Interesting. Vans standard solenoid uses a copper strap instead of a flexible braid. I had the strap fail, which doesn't matter unless you are using the "I" Terminal to energize the on board starter solenoid, which was recommended for light weight starters early on. Those instructions now state to do this only if the starter won't disengage.
I like the MOV "Quench Arc" idea. However, those contacts are very robust and the "disc" is designed to rotate making fresh contact points. Mine looked brand new with no pitting after 750 hours. Still not a bad idea.
 
The fascinating thing to me (that I still haven't quite wrapped my head around) is that starter solenoids have dual windings (See aeroelectric link above). When the starter switch is first energized, the two windings work in tandem to give the starter solenoid a "kick". When the starter switch is released, the current in the two windings is equal and opposite, thus canceling out any switch contact arcing due to the collapsing solenoid field. That's the theory, anyway. I'm wondering if that arcing seen in my photo above was from inrush, not release. Someone needs to put a digital oscilloscope on a starting system to see what's really happening.
 
A diode on a high power relay/contactor does not protect the contacts in the relay.

It is in place to protect the contacts in the switch that is controlling the relay. In the case of an airplane, the battery master and starter motor switches

Do a google search on relay coil protection diode - there is a lot of info and detailed explanations available.

One simple answer found -

Since an inductor (the relay coil) cannot change it's current instantly, the flyback diode provides a path for the current when the coil is switched off. Otherwise, a voltage spike will occur causing arcing on switch contacts or possibly destroying switching transistors.

Thanks, I stand corrected here. It is protecting the contacts that are activating the coil, not the contacts driven by the coil. In my background, it was usually another relay switching the coil and didn't think through my explanation.

Larry
 
In the interest of science

In the interest of science, today I sacrificed a spare starter solenoid to see what was really inside it. This was one of the $10 ones that ACS sells Starter Relay and is what I have in my plane. I'm going to have to take back some of the things I posted above. First off, the "I" terminal was connected to the moving disc with a piece of corrugated brass. Secondly, the coil had only a single winding, with one end connected to the frame of the unit and the other end connected to the "S" terminal. The coil resistance was 3.7 ohms. Here are the photos (may it rest in peace):

Flexible brass jumper between "I" terminal and disc:
2OSSYz.jpg


"I" and "S" terminals:
IdbbKG.jpg


With the jumper cut, the plunger can be removed:
yItzSD.jpg


The plunger:
nB8988.jpg


The coil:
pHdssL.jpg


The metal shroud and caps around the coil:
kieFAH.jpg


The terminals can't rotate because they are sitting flat on an insulator on top of the coil:
MUgLsv.jpg
 
I don't know where the recent thread went, but in that one I mentioned that I connected two MOVs (ZNRs, Varistors, TVSs) to my starter solenoid to prevent arcing at the main contacts. I used something similar to the Panaxonic ERZV20D270 : Varistor Type: D Series: V between each stud and terminal "I" which is connected to the disc. I also used the required diode between terminal "S" (the coil) and ground to protect the starter switch. Maybe there's a better way to do it, but this is what I did.
 
Back
Top