Solenoid bench test

[daverclasper]

Hi.  just took a punt on an old Lucas one (square type) as a spare for £1.00, that was apparently working when removed !. (copies are crap I understand?).

Could I test if this is switching by 12v to the smaller connection prong that's on it's own (rather than the 2 that are part of the starter cable bridge) and earth to solenoid body?, please.

I'm just listening for a click I guess?.

Thanks, Dave

[JohnD]

Yes!

If you have a multimeter (so cheap, so useful!) connect that across the main terminals in resistance mode.    It should flick from Infinite, or no contnuity, to a low figure of Ohms when it clicks.     If it doesn't do either, then experience is always expensive and usually more than a quid.    

John

[dougbgt6]

What John said, some on here report hearing the click, but the solenoid's still not working. 

Doug

[NonMember]

I've had the "clicks happily but makes no contact" more than once. I've also had a solenoid that would click, and would run the starter if I pressed the button, but didn't actually work as a solenoid.

[daverclasper]

Thanks folks.  My multi-meter has a resistance range of 200 ,2k, 20k, 200k, 2m. What setting please.

Also, will + and - probe be ok on either of the threaded main contact stubs.

Cheers, Dave

  

[dougbgt6]

Dave,

+ & - polarity doesn't matter, reading should be infinity switching to zero. I would start on 2M and then on 200, you don't want ANY resistance at all. 

Doug

[NonMember]

No point checking on the 2M range. Go straight for the lowest resistance range you have. A starter solenoid needs to drop well below 0.1 ohm to be any use. The multimeter test will only give you a gross failure check, it won't actually tell you whether it's as good as it needs to be.

[dougbgt6]

I only suggested 2M because the circuit should be open or closed, infinity or zero. Nearest you can get to infinity on a multimeter is 2M.  I have found in electronics things that were supposed to be open to have a high resistance and not fit for purpose, but unlikely with a solinoid I know.

Doug

[NonMember]

Sensitive electronics might be worried about a 100K drain when it's supposed to be open but your starter motor certainly won't. Maybe you might want to check on a higher range just to be sure that there isn't a 400ohm leakage, which would (eventually) run the battery down. So OK, there is some point testing on 2M just to look for the (unlikely) scenario of a small leakage current, but you probably (also?) want to do that test from the battery terminal to the case (ground) as that's a more likely place to see such a thing.

[dougbgt6]

I also suspect Dave needs some multimeter familiarization training, or maybe I'm doing him a disservice. 

[Pete Lewis]

what ever you do on the bench with a meter this will prove it basically operates but it wont guaranty the contactor will pass and 300amps

lets face it its only two nuts and a lucar and screw it down to to go yippee or ive bought a dud

and all these  units , unless you have one with earthed returns ,   do need a good earth to the body , a point missed by many

when its bolted to a painted rusty baulkhead,  adding an earth bond to the hold down screws always helps 

Pete

[rlubikey]

Use a light bulb. This puts a bit of current through the contacts - called wetting current - which breaks through and surface oxide layer. This is the disadvantage of a DVM as the test current is next to nothing.

(The only way to get around wetting current is to plate contacts with gold or silver - like your high quality audio connectors.)

Cheers, Richard

[NonMember]

17 minutes ago, rlubikey said:

Use a light bulb. This puts a bit of current through the contacts - called wetting current - which breaks through and surface oxide layer.

My understanding is that this is not quite right. Wetting current doesn't "break through" anything - if the surface has corroded to the point of not conducting then you're just not going to get that current - but rather helps to discourage that oxidisation while operating. The quick test with a multimeter is not going to take long enough to worry about that.

On the other hand, a light bulb with a 12V battery will be a more representative scenario for the contact voltage, which is what determines how capable the circuit is of breaking through any surface layer.

Mind you, if you want to know whether the thing is OK, wouldn't it be better to test for the harder condition, knowing that if it meets that it will meet the real world needs? A multimeter provides much the harder test of wetting ability, and a light bulb is a long way short of testing the current capacity you need.

[dave.vitesse]

Most multi-meter cannot measure a low enough resistance where the contacts have to pass a high current. Even a small resistance can cause a drop in voltage, this will generate heat which can burn-out or oxidise the contacts. 

Special low resistance test meters are made, but there is no point in buying one for just one test. 

I agree with Richard use a (headlight) bulb as it has a higher current and should help find any weakness.

We use a bulb to check when the points open and close, and not a multi-meter as the higher current of the bulb gives a truer test and also provides a wetting current to clean the contacts.

Dave  

[NonMember]

36 minutes ago, dave.vitesse said:

I agree with Richard use a (headlight) bulb as it has a higher current and should help find any weakness.

Headlight bulbs are typically 50-80W, so 4-7A draw. A starter solenoid is no good if it doesn't do 100A. Like I said, the bulb test is only a little better than the multimeter here, it falls well short of being "proper". Either will give a "general feel" or detect a totally dead solenoid. Neither will tell you whether it's going to actually work. As Pete said, for that you need to actually fit it where it belongs.

[dave.vitesse]

Sorry Rob, it does work testing with a bulb. True the greater you can get to the operational conditions the better the test. However, you have too remember that current from a multi-meter is only micro-amps hence the problems using it to test a switch which is intended handle 100 amps. 

The different between the bulb current and the starter motor current is much smaller.

Dave 

[dougbgt6]

Or, you could attach it to the starter motor and battery on your car and see if it works. That almost replicates the operating conditions. 

Doug

[Pete Lewis]

Hey I said that 

You need to  screw it down to earth the case

Pete

[NonMember]

Sorry, Dave, you're wrong. Pete and Doug are right. A bulb is giving you a false sense of security.

[JohnD]

I get a strong whiff of a discussion on how many amps can dance on the head of a solenoid.

If it conducts at all, fit it and see!   

Or else try a bench test with a starter held in the vice.

John (AKA Aziraphale)

[dave.vitesse]

I will stick with 50 + years of experience and past training.

Dave

[Clive]

15 hours ago, dougbgt6 said:

Or, you could attach it to the starter motor and battery on your car and see if it works. That almost replicates the operating conditions. 

Doug

Got to be about the easiest test......and takes less than 10 mins.

(ok, a simple continuity test first to make sure it isn't totally duff. If you have the means. And can find the meter hiding in the garage)

[rlubikey]

23 hours ago, NonMember said:

My understanding is that this is not quite right. Wetting current doesn't "break through" anything - if the surface has corroded to the point of not conducting then you're just not going to get that current - but rather helps to discourage that oxidisation while operating.

I quite agree NonMember - putting current through isn't going to magically "cure" any badly corroded contacts - perhaps I should have stated that. However, contacts will usually be plated with something more stable than the underlying metal in order to give reliable switching or connection characteristics. So steel contacts might be plated with nickel for example. However, unless plated with a noble metal - usually gold - there may be some corrosion in the form of a thin stable layer on the surface. The wetting current helps break through this layer. Many switch and relay manufacturers will specify the minimum current while others just assume that you know this.

Connectors have a wiping action which cleans the contact surface but, to get good life, switches and relays may not have this feature as a wiping contact eventually wears away the plating. Even so, after a while, who hasn't had to wiggle cheap nickel plated phono connectors on their HiFi to bring a channel back to life? And have you ever rotated the batteries in, for example, a remote control to eke out the life of those batteries? Nickel suffers surface corrosion, albeit quite slowly. This is where gold comes in - it's noble so doesn't corrode. However, it's expensive and softer so wears away quickly. It's not likely to be used on high-current contacts due to cost and lifetime. Cheaper nickel plated contacts are kept fresh by the current.

Even if the contacts show good with a DVM or the light bulb test, that doesn't mean to say they will withstand the current drawn by a starter. Usually, this is just for a few seconds which limits heat build up. I seem to recall measuring the current of a Lucas starter - can't remember if it was a 4- or 6-cylinder engine - as being 180A. This is one heck of a current to go through tiny little contacts in one of Joe Lucas's solenoids - especially as originals are 40-years old or more.

The bottom line; try a DVM by all means. If it doesn't work, don't write it off but try for example a light bulb. Even this isn't going to tell you how long it will last on a starter motor!

Oh, and one more thing, light bulbs have a very low resistance when cold, so they take a high current for half a second or so when contact is made - perfect for a contact test!

Here endeth today's lecture. Is anyone still awake?

Cheers, Richard

[dave.vitesse]

I agree with your comments.

As interest the basic teaching for testing high current relays was -

Bench test of contacts with bulb. The bulb start current being key to the test.

Load test by installing into the car or system.

Pre DVM an AVO 8 or VVM were deemed not suitable for testing high current relays for reasons you have stated. 

Dave

[SpitFire6]

Hi,

 a bit late.

If you are wanting to test the switching contacts for low resistance you can use a 4 wire mill-ohmmeter as mentioned above. If you don't need to know the exact resistance value, but would like to know it is low and OK:

Power up the solenoid with a car halogen lamp connected as a load for the battery to illuminate thru the solenoid switch.

Measure the DC voltage on the millivolts range across the switch contacts.

I would want to see tens on millivolts. I would be sad if I saw 100s of millivolts. If you can measure the current thru the lamp, you will be able to calculate resistance and then calculate how many volts are lost on the contacts and whether it has got much life left.

Never add a diode across the solenoid thinking it will help the contacts; it will destroy them. A capacitor across the contacts is good for long life.

low-level audio & RF are the only contacts that need wetting I/V as mentioned on a previous post.

Cheers,

Iain

 

 

 

Edited January 16, 2019 by Spitfire6
PS. If SM in car; Measure voltage across contacts while cranking; you want less than 0.5V