Reversing Brake Release Solenoid Back EMF Problem

[Oskar 26M]

Hi all. I'm new to this site, having only purchased my 26M in Australia a few months ago. I thought I'd describe a towing electrical problem that can arise with newer cars.
My 26M has the painted steel trailer which is fitted with UFP's TrailerBuddy disk brake and their A60 Actuator which includes the solenoid to release the brakes when reversing.
The other part of my "escape package" was a new Toyota Kluger, fitted with its Toyota Tow Pack, which includes a solid-state relay box to "protect the Toyota's electronics" from the trailer's electrics. In the Aussie configuration, pin 2 of my 7 pin coupler is wired to the trailer's reversing solenoid and in turn connected to the Toyota's reversing lights via the relay box
PROBLEM: The trailer's electrics seemed to work ok initially but after engaging reverse a few times, ALL of the trailer's lights would be permanently lit (regardless of which car lights were operating) and soon after NONE of the trailer's electrics would work via the towing connection. Although the 30 amp towing fuse was intact, the Toyota's relay box was dead (AU$130!!) .
The Toyota dealer was totally disinterested and couldnt even supply specs for their relay box. UFP were more responsive but suggested a wiring or grounding problem on the trailer. Direct connection to a battery showed that all of the trailer's electrics were functioning correctly, including the brake solenoid.
Initial attempts to get an auto-electrician involved were unsuccessful because of their fear of Toyota's warranty, but I eventually found a brave heart who recognised that the problem was a high back EMF generated by the collapsing magnetic field when the brake solenoid was switched off. This fed back to the Toyota's relay box and fried its electronics.
SOLUTION: fit a mechanical relay to physically isolate the solenoid when it is switched off.
It all works fine now.
Of course the back EMF would never have been an issue with older mechanical towing relay systems, but might be problem with other solid state relay systems in newer cars.

[Hardcrab]

Oskar26M,
A clever answer to your problem!

Perhaps if others have the same issue, they may also consider a simple diode wired across the solenoid to provide a shunting path for the counter EMF.

A common 1N-4004 would work perfectly fine, with no moving parts or contacts to wear out.

[Hamin' X]

Oskar26M,
A clever answer to your problem!

Perhaps if others have the same issue, they may also consider a simple diode wired across the solenoid to provide a shunting path for the counter EMF.

A common 1N-4004 would work perfectly fine, with no moving parts or contacts to wear out.

This should be SOP for any relay, or solenoid. Be sure to place the "banded" end of the diode toward the positive lead and the "non-banded" end to negative.

Rich---Hamin' X---N7ZH

[Oskar 26M]

Thanks Hardcrab and Hamin X.
A diode does sounds like a much more elegant and reliable solution. I tried one when I was in no man's land trying to find a willing auto electrician, but being electrically illiterate in my ignorance I guess I wired it wrong (it worked on the battery but blew the second relay box on the car).
I put the diode in the positive feed line to the solenoid but I note Hardcrab said "across" the solenoid and Hamin said "with the non-banded end to negative". Does this mean I should have connected the non-banded end to ground?

[Hamin' X]

The diode should go across the terminals on the solenoid coil, not to the positive wire just anywhere and definitely not to trailer ground. On the coil terminals directly. This will eliminate any chance of stray capacitance or inductance in the wiring, or resistance in the connections from letting the diode do it's job of shorting out the reverse flow due to the collapsing magnetic field.

Rich---Hamin' X---N7ZH

[Oskar 26M]

Ahhh.. Thanks for the further explanation Hamin' X. It appears that back emf problems can be the bane of model train enthusiasts (solenoids used for switching railroadtrack points ) and some circuit diagram solutions I've seen look fairly similar to what you describe. Unfortunately, the solenoid valve unit I have is hard to get at and looks like a sealed unit. It has only a positive lead in and according to UFP it grounds through its frame). I dont think I'd be game to dismantle it enough to get directly at the coil... yet!. So for the time being I'll stick with the relay solution.
Many thanks for the suggestions though - I'll try to put them into effect at re-fit time.

[baldbaby2000]

The diode should work. This is common protection for electronics that switch relay coils. I'm surprised that the vehicle manufacturer didn't include simple protection! What happens is like a spark coil effect. When the inductor is disconnnected from the power the magnetic field starts to collapse inducing a high voltage across the coil because the coil current will in theory remain the same, but since there's no load the voltage will try to go to infinity and beyond! Basically current in an inductor can't change instantaneously and the diode provides a path for the current.

[Hardcrab]

IMHO, I personally/technically have no doubts or problems with the anode ( "non-banded") of the diode wired directly to ground. After all, the negative side of the battery is ground, both in the vehicle and the trailer.
The solenoid mfgr evidently holds a simular view, as seen by their one wire solution.

Hamin'X is certainly most correct with his at the coil answer and should be done with any design. But in your "gray" case, you have to clean up the "black and white" mistake of the mfg in a way that will work for you. Therefore, mechanically place the diode where you can, and wire it to ground and the solenoids positive lead at the most convenient point. A shunt path for the counter EMF has now been established. It will not be text book perfect, but it will be an effective tradeoff.

By the way, does your clever relay fix have a diode across it's coil? It should, for the best long term AUS$130 solution.

[Oskar 26M]

Thanks for the extra input Hardcrab. I shall try your suggestion and rewire the diode with its ringed end connected to the positive feed going to the solenoid and the other end to a ground on the trailer frame.
The answer to your other question about a diode in the "relay fix" is I don't know for sure., but as far as I can figure out, the relay my electrician fitted does not have a diode across its coil. The relay is a Bosch 12v NO 30A Mini Relay (Part number 0 332 019 150). Bosch's online catalogue (I haven't been a member long enough to post the url), contains huge amounts of specification information about this and other relays they sell. I wish I could have obtained the equivalent information from UFP about the solenoid they use! It appears that this particular relay has Bosch's S1 circuit diagram which does not show a diode across the coil, in contrast some of their other circuit diagrams that do show one so I hope I'm interpreting this correctly.
Maybe the Toyota Kluger's relay box can tolerate a relatively small back emf from this mechanical relay coil.
This is a whole new world for me (and a fascinating one at that) and my rather hazy recollections of very ancient high school physics is being sorely taxed. But, despite the being at the very bottom of a steep learning curve, and feeling woefully ignorant, its been good to try to understand what is going on and I greatly appreciate your assistance

[Hamin' X]

Osakar,

According to the Bosch catalogue, your relay does not have a diode. You would need part #0 332 019 155.

If you do forgo the relay and use a diode at the solenoid, you should ground it to the solenoid case and not the trailer frame. Clean connections and short leads are mandatory. Seal all connections with liquid electrical tape, or similar product. DO NOT use silicone RTV.

IMHO your setup is just bad engineering, Especially for a marine environment. Manufacturers often confuse earth ground, chassis ground and negative for the same thing. They are not. Each can have different levels of potential and may not remain constant. In my personal and professional experience, 99% of electrical problems where something is working, but acting weird, can be traced to improper grounding.

Rich---Hamin' X---N7ZH

[Boblee]

Not being into the newer auto electrics but why wouldn't you just put the diode in the positive line to the solenoid? ie in series with the solenoid.
Maybe the Hilux circuits are different but have never had a problem (yet),
maybe I should do something?
Perhaps the easiest (read safest) way is to just disconnect the solenoid and use the lockout key when backing?
Have several relays the same as Oskars and have not had a problem so maybe it is another price of technology making it impossible for the handyman to fix his own vehicle.

[Oskar 26M]

Thanks Harmin' X and all others. I got beaten by the time difference hence the delay in reply.
Trying to put all the advice together, here is my current plan:

Although my 26M and trailer were only 9 months old when I bought them, corrosion was already evident in all of the trailer's "waterproof" lights as well as in the wiring so that has all been replaced in a totally removable configuration that wont go near sea water again.
The brake solenoid is the only thing I cant make removable. I'll connect the diode's ringed side to the positive feed wire for the solenoid (it will be about 5 cm from the actual solenoid) and will connect the other end of the diode to a screw hole I've found in the base of solenoid's brass casing. I have already added a short 10cm wire that leads from the casing to a common ground bolt welded on the trailer's frame, which in turn is cable-connected to the car's battery negative via the 7-pin towing coupling.
To be on the safe side, I'll still use a mechanical relay to power the solenoid but will change it to the model number Harmin' X recommended so it has its own diode protection built in. I'll ensure that it too is properly grounded by connecting it to the common ground wire from the car.
The mechanical relay will get its positive from the car's relay box reversing light outlet via the towing coupling, and I'll have to parallel this source to the relay's normally open terminal to connect power to the brake solenoid when the relay is energised.
I'll seal everything with liquid electrical tape once I've been able to track some down. Maybe when I've done enough posts to allow links, I'll post some pictures and diagrams of how it finally worked out.
Thanks once again to everyone for all the helpful advice. I have to admit that trailer electrics were not where I expected problems when I brought the M.
Meanwhile, since my M is kept in a mast up storage yard where I don't need any electrical trailer connections, I might even be able to get some sailing in

[Hardcrab]

Boblee,
Placing a diode in series with the solenoid would not be an effective answer.
The problem needing correction is that the de-energizing solenoid becomes a "source" for voltage and current. This "source" exists for a short period of time. This voltage and current can become quite high, which can cause all kinds of grief when it's seen back in the operating circuit. Further, and importantly, it reverses in polarity from whatever the orginal voltage applied was.

If you attempt to keep this "counter voltage" contained in the solenoid with a series wired diode, that same installation will not allow the solenoid to operate from the control circuit in the first place! Conversly, if you series wired the diode to allow for controled operation, the diode will act like a straight piece of wire when the polarity reverses at de-energizing!
A true lose-lose outcome.

Providing a harmless path to ground with a correctly wired parallel diode is the standard engineering "fix".

I hope my explaination was mostly clear.

[Boblee]

G'day Hardcrab
Yeah mostly clear mate, I presume the negatively charged emf is too high a voltage for the diode and so could get back past it into the vehicle circuits looking for a path (higher blocking diode?).
The correct diode to run parralel with the solenoid would then need to be voltage sensitive so that it didn't divert working power to or away from the solenoid.
It just seemed logical to block a high negative seeking voltage and probably more importantly current source going back up the line instead of diverting it with a surge arrester when the source is known or even using both if the emf is that high in case the earth connection fails especially in a marine environment.
Of course having a surge outlet could be handy for any other spikes as well.
My electronics has been neglected for the last thirty years and even then it was fairly basic but there were no diodes to earth on our relays then? although the circuitry was quite a bit more robust too.
Thanks for taking time for the explanation proffessor really appreciate it
Have about six other relays scattered about the vehicle that may neeed an escape to earth although have never seen it done by auto elecs here which really doesn't say much, although with more sensitive electronics perhaps old fogeys like myself have had their day at being able to muck in and get out of trouble.
As before though the unprotected solenoid has caused no damage to date but will now disconnect it in case.

[baldbaby2000]

quote]Not being into the newer auto electrics but why wouldn't you just put the diode in the positive line to the solenoid? ie in series with the solenoid.[/quote]
In theory that sounds good but in practice the voltage generated when the circuit is deenergized could exceed the reverse voltage rating on the diode and cause it to fail. Another disadvantage in series diodes is that there's a voltage drop and also some heat generated in the diode.