voltage drop across solenoid

ArtS

I need some advice from the electrical experts. I want to check out the old solenoid on my Hornet. I plan to measure voltage drop during cranking. Do I put the black lead (neg) from voltmeter on neg bat terminal and red lead (pos) to battery side terminal of solenoid? Then I will measure across the solenoid. Again, black lead to bat terminal on solenoid and red lead to starter terminal on solenoid, correct? After removing coil wire and during cranking, I should get 0.2V or less on my voltmeter, right? Is this plan OK? Thanks!

Park_W

You want to put the red lead where it will be "closer to positive" than the black lead. So to check across the solenoid, you'd do it as you describe (the second case above). With the solenoid not energized you'll read full battery voltage, since the meter sees the starter motor as pretty much a solid circuit to ground (i.e., the starter motor has very low resistance). So if you hook up the leads before energizing the solenoid, you usually have to have the meter on a 10v scale to keep it from pegging out, then when the solenoid is energized you can switch to a lower scale to read the (hopefully) small voltage drop across the solenoid. Touch your meter leads to the studs coming out of the solenoid to get the most accurate reading. That eliminates any loss in the cable connections. And speaking of those, be sure NOT to use stainless steel washers or nuts anywhere in this high-current circuit, including battery cables to frame, etc. Using stainless will cause some power loss.

ArtS

Thanks, Park, for your detailed response and tips about stainless washers. I'll be cleaning up and repairing old connections from frame to battery tray on ground-side and then toward solenoid and starter on the other side. I'll avoid stainless washers at both sides. Best wishes. Art.

Park_W

Art, when I put my '36 together it just didn't want to crank that well, even though the engine wasn't tight and the starter was good. Went through the cranking circuit as you're doing, and found that when cranking, I was losing 2 volts at the connection between battery ground strap and frame! Saw that I'd used a stainless bolt, nut and washers (I knew better, but forgot), so replaced them with standard ones and the problem was gone. I've never talked with a chemist or metallurgist about why stainless introduces resistance as it does, but have seen enough cases to know that it happens. (But it doooo look purty! LOL.). Of course with a car that's been restored, you also have the issue of the parts being freshly painted, and they don't make good electrical contact that way. Kind of painful to scratch brand new paint off something, but you have to do it to ensure a good connection.

ArtS

Park W wrote:

Art, when I put my '36 together it just didn't want to crank that well, even though the engine wasn't tight and the starter was good. Went through the cranking circuit as you're doing, and found that when cranking, I was losing 2 volts at the connection between battery ground strap and frame! Saw that I'd used a stainless bolt, nut and washers (I knew better, but forgot), so replaced them with standard ones and the problem was gone. I've never talked with a chemist or metallurgist about why stainless introduces resistance as it does, but have seen enough cases to know that it happens. (But it doooo look purty! LOL.). Of course with a car that's been restored, you also have the issue of the parts being freshly painted, and they don't make good electrical contact that way. Kind of painful to scratch brand new paint off something, but you have to do it to ensure a good connection.

Park, I also am interested in the reason for the higher electrical resistance of stainless. Perhaps you have already checked this out, but the Stainless Steel Industry of North American (SSINA) has a really fascinating web page with PDF downloads of technical info, fabricating with stainless, fastener design, and tutorials on the mechanical and chemical properties of SS (www.ssina.com). Don't know for sure but nickel and chromium are relatively poor electrical conductors compared to high carbon steel or plain old Fe. There are LOTS of stainless blends out there and just what we get at Sears Hardware probably varies from month-to-month. ArtS