fused OD solenoid

[tahoe healey]

I've just replaced the OD solenoid for the second time in 6 months. (Poorly adjusted the first time.) Looking at the wiring diagrams on a BJ8 it appears an in line fuse will only protect the relay and not the solenoid. Is there a way to protect the system? ( I am looking at Norman Nock's Tech Talk.)
Thanks
Rich

 

[RAC68]

Hi Rich,

There was a thread a short while back where placing a fuse to protect the OD was discussed. I placed a 10A fuse on the white wire entering the Relay because it seemed to me that that was the source of power to the full circuit. You could place a fuse on the C2 line leading to the OD 3/4 gear switch that feeds power to the Solenoid or take the time to remove the transmission cover and place a fuse on the direct line to the solenoid. The I believe the latter will require a larger fuse or a slow blow fuse due to the voltage created by the solenoid. I had considered using a Transient-Voltage Suppression (TVS) diode to diminish the high voltage generated by the active coil in the solenoid but was not sure and, since all was working, let it alone.

For my part, I have stayed with the 10A fuse on the white main input to the circuit between the fuse box and relay.

All the best,
Ray(64BJ8P1)

 

[Keoke]

Hi TH:
I recommend a 10 amp slow blow fuse off the C2 line.
A std 10 Amp fuse that limits the transient suggests that the wiring associated with the fuse is limiting the current to the fuse.
Consequently,under hard short conditions the wire resistance can increase due to heating to the point where the fuse will not blow.
Summary the wire becomes the protector for the fuse

.( I am looking at Norman Nock's Tech Talk.)
He and I had a discussion on this years ago and he finally agreed Lucas was out of date.--LOL

 

[Michael Oritt]

My OD (and solenoids) went away along with my 3-speed transmission in 2002 so I am speaking from old memory, but as I recall the draw of the pull-in solenoid is something like 18 amps and the hold-in solenoid is less than 1 amp.

The trick, I believe, is to use a low amperage slow-blow fuse that will not blow while the circuit is being made (high amperage) and yet will protect the circuit when all that is necessary is enough power to hold the relay in. Putting too high a fuse into the system runs the risk of the wire, and not the fuse, becomes the "sacrificial element".

I don't know what will burn up first--a 10 amp fuse or the wire--but if it were me I would spend a few bucks and work my way up to the slow-blow fuse that did the job without blowing, and so have the circuit protection that the fuse is supposed to offer.

 

[RAC68]

Hi Michael,

I think I understand your thinking but I would think if it takes around 18 amps to set the solenoid, the wiring (connecting wires and internal to the solenoid) would have been selected to address that amperage as a minimum and the lower amperage used to maintain the set solenoid would also be carried by the same wiring. If I am correct, then using a slow blow 10 amp (if it doesn't blow) or a standard 20 amp would be just as protective of the solenoid. Am I missing something?

Ray(64BJ8P1)

 

[Keoke]

Hi Michael,

I think I understand your thinking but I would think if it takes around 18 amps to set the solenoid, the wiring (connecting wires and internal to the solenoid) would have been selected to address that amperage as a minimum and the lower amperage used to maintain the set solenoid would also be carried by the same wiring. If I am correct, then using a slow blow 10 amp (if it doesn't blow) or a standard 20 amp would be just as protective of the solenoid. Am I missing something?

Ray(64BJ8P1)

No:Ray

The 18-to-19 Amp current we are talking about here is not a steady state current created by a resistive type load, it is a very short lived transient that occurs only with inductive loads like the solenoid.

 

[TimK]

I had a real life proof of the value and effectiveness of a 10-amp in-line fuse. I had installed an in-line fuse with a 10-amp standard fuse. The OD worked fine. Later the solenoid failed and I replaced it with a new one. The solenoid comes with a separate pin. I stupidly decided not to replace the old pin with the new one that came with the solenoid and re-used the old one. It turned out to be too short in combination with the new solenoid to deactivate the hi-amp pull circuit. Consequently the fuse blew and saved the new solenoid. I realized my error and replaced the old pin with the new one and the OD has performed flawlessly ever since.
[h=1][/h]

 

[Keoke]

I had a real life proof of the value and effectiveness of a 10-amp in-line fuse. I had installed an in-line fuse with a 10-amp standard fuse. The OD worked fine. Later the solenoid failed and I replaced it with a new one. The solenoid comes with a separate pin. I stupidly decided not to replace the old pin with the new one that came with the solenoid and re-used the old one. It turned out to be too short in combination with the new solenoid to deactivate the hi-amp pull circuit. Consequently the fuse blew and saved the new solenoid. I realized my error and replaced the old pin with the new one and the OD has performed flawlessly ever since.

That is what it should have done under those circumstances.

However:
A fuse is a link between circuit points that will melt and open up the circuit if more current flows between those points than the circuit was designed to safely withstand. 99% of the time, if a fuse blows there was a good reason for it. Fuses are rated in amps (the unit of current measurement), and, in addition to that current rating, also have a voltage rating and other parameters such as time constant or delay. A simplified explanation breaks fuse types into two categories; fast blowing and slow blowing. A fast blow fuse will usually open up on an instantaneous current peak[This we do not want to happen when the solenoid functions normally and exceeds its steady state current rating. A slow blow fuse will withstand this over current for a specific time, but open up if the higher current continues beyond the fuse's time constant.

ON the other hand, for your case either fuse type would have blown!

 

[RAC68]

Hi Keoke,

Well, I now understand the Slow Blow Fuse much better after your explanation and some additional research. As I understand, in normal operation, the OD Solenoid will have an initial activation surge amperage of around 18-19 amps for around 1 second and a steady state amperage of 1-2 amps to maintain the set state. A Slow Blow Fuse allows an unimpeded initial amperage surge for a short term before falling into its max amperage limitation. Since the most common faulot experienced by the Solenoid not to complete its initial setting and continue to draw high amperage, the Slow Blow Fuse would exceed it set time to allow an initial surge amperage and, since its Max steady state amperage has been exceeded, the fuse will fail (blow).

You had recommended a 10 amp Slow Blow Fuse to be attached to the C2 circuit. Although I appreciate that this is a reasonable selection, as Michael indicated, an amperage closer to steady state could be more effective in protecting the Solenoid. Since this is a type of fuse I am not familiar with, do you have a source and what to ask for to get the proper time pill needed for the initial surge?

Keoke, thank you for your focus and your instruction. It is appreciated and I have learned,

Ray(64BJ8P1)

 

[Keoke]

Ray:
Simply check Fuse sizes By little fuse.
1 normal/fastblow
OR
2 delayed/slowblow

Pick a group of items from 2 Amps-to- 10

 

[Michael Oritt]

Hi Michael,

I think I understand your thinking but I would think if it takes around 18 amps to set the solenoid, the wiring (connecting wires and internal to the solenoid) would have been selected to address that amperage as a minimum and the lower amperage used to maintain the set solenoid would also be carried by the same wiring. If I am correct, then using a slow blow 10 amp (if it doesn't blow) or a standard 20 amp would be just as protective of the solenoid. Am I missing something?

Ray(64BJ8P1)

Ray--

I simply do not like to "overfuse" a circuit as, in an overcurrent/short circuit situation if the fuse is more resistant to blowing than the wire is to melting you have not protected either the appliance or the wires.

My thinking is that a low-amp slo-blow fuse could withstand the very momentary 18-amp pull-in draw and serve as a fuse for the hold-in draw of, I believe 3 amps. If a 3 amp slo-blow fuse is not sufficient to handle the momentary high amperage then stepping up in 1-amp increments could suffice and give real fuse protection to the wiring. If I had an OD--which I don't and don't plan to--this is what I would do, or at least give it a try.

 

[Keoke]

Well:FWIW
Sometimes if the fuse is improperly located but of the proper size.
The initial thermal leakage out of the fuse into the wire whose resistance is increasing and consequently reducing the current to the fuse below its blow value,can allow the wire to protect the fuse. So it is not just a matter of over fusing.Certainly that is undesirable.

Most Transient times are in the Millisecond range.
Additionally as this transient time approaches zero the transient voltage amplitude approaches infinity.
So we can see that the solenoid does not put out a constant peak voltage ,when it is cold and a little slow we get one characteristic when it is warm and operating faster we can see a higher peak voltage this spread is not real wide but must be accounted for.

 

[Michael Oritt]

Well:FWIW
Sometimes if the fuse is improperly located but of the proper size.
The initial thermal leakage out of the fuse into the wire whose resistance is increasing and consequently reducing the current to the fuse below its blow value,can allow the wire to protect the fuse. So it is not just a matter of over fusing.Certainly that is undesirable.

Most Transient times are in the Millisecond range.
Additionally as this transient time approaches zero the transient voltage amplitude approaches infinity.
So we can see that the solenoid does not put out a constant peak voltage ,when it is cold and a little slow we get one characteristic when it is warm and operating faster we can see a higher peak voltage this spread is not real wide but must be accounted for.

Yeah, that's what I meant to say....

 

[andrea]

just to inform about causes than can destroy the wiring of the OD
during the Test drive(some year ago) I switch ON my OD all works good but after about 20 min I noted some white smoke from engine bay- opening
smoke arrived by the OD harness- switched the OD OFF with no result-I must switch OFF the car and the burning finished
dismantling the gearbox cover for investigation -resulted that Mechanic haven't mounted the safety stop (never in place on the original barn find car)
and the sliding part of the solenoid was completely OUT just rest on the stop bracket - I have assumed that if this type of bug are present the solenoid
stay in the maximum absorbition 18A condition and wire burning occur if fuse aren't in the correct position

 

[Keoke]

I have assumed that if this type of bug-[ Sliding part of Solenoid completely out] -are present the solenoid
stay in the maximum absorbition 18A condition and wire burning occur if fuse aren't in the correct position

YEP Andrea:
That will do it . Good Point