I don't think you understand what I mean by a Voltage Drop test. The idea is not to check voltage at various points as referenced to ground. The idea is to check the voltage ACROSS individual components and connnections.

Example, if you have a bad battery terminal connection there will be a voltage difference between the terminal and the battery post. If you put one lead on the battery post and one lead on the battery terminal, there should be no voltage measured between them because they are the same electrical point. A bad connection, however, means resistance. Resistance causes a voltage drop that would cause you to read voltage across what SHOULD BE a dead short.

Doing a Voltage Drop test is like testing each link in the chain one by one. Go ahead and put your leads across the battery connection and have someone try to crank it. If there is more than zero volts then there is resistance. Resistance means a bad connection.

Now, let's say you did this and there was zero volts between the battery post and the battery terminal. You know that this is a good connection so leave the lead on the battery terminal and put your other lead at the other end of that cable on the solenoid and check again. There should be only a small fraction of a volt dropped across that cable. If there is more than a few tenths of a volt, then that means the cable has resistance, but that still would not be your problem.

Now, assuming that the solenoid is clicking in, put your leads across the solenoid itself while someone tries to start it. When that solenoid clicks in, there should be a virtual short across the terminals. If there are several volts dropped across the solenoid terminals (the big cable to the solenoid measured against the big connection that goes to the starter) then that means there is too much resistance across the solenoid terminals.

SO.. You are putting your volt meter only across one small link in the chain at a time until you find the bad link. The only component in a properly working secondary starter circuit that should be dropping significant voltage is the starter motor itself. It is the load in the circuit. It has to have voltage across it in order to run. Everything else in the circuit should not be dropping significan voltage.

Remember, the starter motor draws HIGH CURRENT. All secondary connections must be STELLAR! If you have even a little resistance it robs the starter motor of needed energy.

Hope this helps

I totally do understand what you mean. I am curious about what the specific connection points you would use to do it, for my car. again, the solenoid for example, would use the solenoid lead, and the starter housing, and get the voltage while someone is cranking.

I dont know where they would be, if there are any, for the nss and the ignition switch.


Maybe this is where the confusion is...its the lead that activates the solenoid that I have been refering to. the smaller (I guess 7A load carrying) terminal. In your terms, its the primary circuit I have issues with. I am getting good starting, with the seconday.

If you were going to do a voltage drop test for the nss and ignition switch, what connection point would you use?

You were kinda doing it when you figured out that you dropped 0.7 volts across the solenoid wire. You can measure all your voltages from a reference point like the negative battery post, or you can measure from one end of a wire to the other end. Whenever you have current flowing through a resistance you'll see a voltage drop which can be measured.

If you want to measure across the NSS and the start switch look in the schematic you should be able to do that at that junction box on the fender. You may have a few connection points thrown in the path, but you'll get a good idea of what's going on. With the solenoid connected, put one lead from your meter on the right hand pair of connections and put the other lead on the left hand connection where the solenoid connects. When you crank the engine, the voltage drop will tell you if you need to look at those switches.

__________________
mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

Okay, the solenoid activation circuit can be tested in the same manner as the high current portion of the circuit. Just put your voltmeter leads across the item under test and see if it is dropping voltage. If it is open, then virtually all of the battery voltage will be dropped across that point.

It's DIFFICULT to describe things electronic without a schematic or black board, but let's say that you have B+ going to an ignition switch, then a neutral safety switch and then to the coil of a solenoid. If everything is correct, when the ignition switch is turned to start you will have virtually all (99+%) of B+ ACROSS the solenoid. That is with one lead on ground and the other on the solenoid power wire you will have B+, say 12 Volts.

NOW, let's say the resistance across the coil (yes I know it is an inductive load, but I'm trying to keep it simple) is 2 Ohms, then 12 Volts across 2 Ohms gives you 6 Amps although exactly how much current is in the circuit doesn't matter in what I'm trying to explain.

So NOW, let's say the NSS is open. Put your voltmeter leads across the NSS and turn on the ignition switch to start and you will see 12Volts across the switch. WHY? Because the resistance across the switch is infinite, while the resistance at the solenoid remains 2 Ohms. So you have BEAUCOUP resistance across the switch and relatively nothing across the solenoid. Forget the exact calculations, just remember that 99% or more of the circuit resistance is at the NSS, SO.. the Voltage is dropped across the NSS in your test.

Now take that same thinking to a situation where there is a dirty connection in the circuit causing say 2 Ohms resistance when carrying full current. That would mean that this dirty connection is dropping the same amount of voltage as the solenoid. So, the dirty connection would be dropping 6 Volts leaving only 6 Volts across the solenoid, which probably wouldn't pick it.

Does this help?

I appreciate the time and detail that you are providing. It is apparent you are trying to teach me over a very difficult medium (a forum) with the tools available. I really am thankful.

I just want to make it clear: I understand the theory (V=IR, In a series circuit the summation of Voltage drop at individual elements is equal to voltage applied to the entire circuit)
I understand the general methodology (apply dmm leads in parralel i.e. across element you want to determine Voltage drop on, looking for voltage draw at element while cranking, this is done sequentially on each element ).

I don't know the specifics of application for an '83 300sd, primary ignition circuit (e.g. nss, and ignition switch). I don't know where connect my leads to execute the test. Thats all I dont know about this test. Thats what would be helpful.

From my post #35 in this thread...

Measuring at this point will measure both the NSS and the ignition switch and the wiring to them. If you see very low voltage drop, then you've eliminated both switches as potential problems. If you see a relatively high voltage drop, then you'll know you need to look into this part of the circuit further. If this test eliminates the two switches, then you can kick the "need to know" about their locations and how to test individually down the road a bit.

Also, the schematic I uploaded has pictures of component locations. See page 215 as noted at the top of the Start/Stop schematic (page 107) According to the manual, the NSS (aka Starter Lockout/Backup Light Switch) is on the left hand side of the tranny (see page 210). The connector for it is in the driver's footwell, and you probably have to remove the carpeting or at least the center console to access it. These are in much more prohibitive locations than the fender, so you're best off testing at the fender well first.

__________________
mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

very good sir

WOW. 7A. That is 84W for a solenoid. I do not expect that. That is still very small wattage compare to the total energy a battery can hold.

Additional info:

It was my mistake, I worked with switching relay before and normally the current is small. The solenoid needs to push the pinion gear in position, may be that is why it needs more energy. When I read the Wikipedia, it says

"A small electric current flowed through the starter relay coil, closing the contacts and sending a large current to the starter motor assembly." May be it is not that small after all.

Thanks for the info.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Okay, great. You're more than half way there. Understanding what you're testing and how to test it is the hard part.

NOW, what you're looking for is the location of the switches. I wish I could be more help with that. It seems like someone earlier in this thread or another thread recently described where to find the NSS. My 123 cars have been manual transmissions except one that my wife had and I never had to deal with this particular problem.

I would expect that if you get underneath and look at the transmission shift linkage you can find the switch. Maybe someone on here has an FSM for your car.

I am assuming that you have already checked for a voltage drop at the starter solenoid. That one is relatively easy to get at.

Probing the test points can be a PITA if they are hard to access.

You said jumping the termimal block (small screw to big screw) nothing haopens. Did you try jumping small screw to battery + terminal? put a 20 A ammeter between the small screw and battery + terminal. The should pull in the startier solenoid and draw 7A..

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

One comment - high current DC ammeter is very expensive as ammeter is inherently intrusive to the circuit. It needs a very small value and precise resister to do the measurement. Most DIYers do not have one. Need plan B.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Most DVMs have a 20A scale that will do the job. I have a HF and a Craftsman at $15 and $25 respectively both have 20A scale. I wouldn't call that expensive..

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

So where did using an ammeter come into the discussion?

Since I measured my starter solenoid drawing 7 amps, I suggested Hinu do the same (if he has an ammeter). Real easy to do. Jump the terminal block with the ammeter. He should see around 7 amps and it should crank unless........

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Most DVM with high current are not full duty cycle ones. Just make sure one knows the limitation of the meter and do not bust it. If one thinks it can resolve the problem then go for it.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.