Electrical not starting

Tiny
DWIGHT F
  • MEMBER
  • 1984 MERCEDES BENZ 300SD
  • 5 CYL
  • 2WD
  • AUTOMATIC
  • 280,000 MILES
I just recently replaced my starter motor in my car and after two weeks the car refused to start. So, I am confused as to what is the issue.
Before the car stopped running I noticed a cascading series of electrical failures:
Electric windows stopped working, and other electrical systems began failing.

I thought the culprit was a bad rebuilt starter motor.
Then started doing a little research into all the electrical relay boxes in the fuse box.

Before the complete failure I noticed that after running the car for a while, when I would turn it off the car would not restart immediately it would take time for "something" to cool down then eventually I could restart the engine. Eventually that "something" has completely failed not allowing the car to be started at all. Does anybody know what might be the problem? I do not think it is the rebuilt starter. Could it be a relay that I do not know about? How do I trouble shoot this?

Thanks for the help!

Best,
Dwight L
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Wednesday, April 19th, 2017 AT 4:05 PM

6 Replies

Tiny
CARADIODOC
  • EXPERT
Everything you described can be caused by a failing generator that is not fully charging the battery. As system voltage drops, more and more computers become confused and start to shut down or do weird things. The charging system is the first thing we check when you list a multitude of electrical problems. Have the system tested by your mechanic, not at an auto parts store. Post the results, and I will interpret the numbers for you.
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Wednesday, April 19th, 2017 AT 4:11 PM
Tiny
DWIGHT F
  • MEMBER
Only problem with this that theory is the battery is fully charged with 14.75 volts D.C. Before load.

I think it is a burned out OVP relay. "Over volt protection" in the fuse box.

Thanks for your help! I do appreciate it!

Will seek a mechanic.

DL
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Wednesday, April 19th, 2017 AT 5:47 PM
Tiny
CARADIODOC
  • EXPERT
A three-cylinder water pump with one dead cylinder can still fill a municipal water tower. A generator with one bad diode of the six will only be able to develop a maximum of exactly one third of that generator's rated output current. The standard generator for your car is a 55 amp unit which is very small by current standards, and was not much back in 1984, but it was sufficient when cars did not have a lot of unnecessary electronics. If your generator has a bad diode, the most you will be able to get from it during a full-load output current test is 15 to 20 amps. If the electrical system needs more than that, the battery will have to make up the difference until it slowly runs down over days or weeks.

All AC generators put out three-phase output current which provides a very stable and smooth voltage. The voltage regulator maintains that voltage between 13.75 and 14.75 volts. That is just the right amount to force electrons into the lead in the battery's plates, so they can be stored there. Just like you would get little pulses of varying water pressure from the three-cylinder water pump, you get little pulses of voltage variation from the generator. That is "ripple voltage". That cannot be measured directly. It can only be measured with a professional load tester found in repair shops. The testers found at some auto parts stores do not test for ripple voltage. That is why I send everyone to their mechanic for this test.

When a diode has failed, (those are one-way valves for electrical current flow), you lose one phase of output current, and during the time that missing phase is supposed to be there, output voltage drops a lot. That larger difference between the highs and lows is that high ripple voltage. My tester is sitting in the back of my minivan right now. I just performed this test on a friend's minivan, with the same results. The most we could get from the 90-amp generator was 28 amps, and ripple voltage was off the chart. Charging voltage was perfect at 14.4 volts, but the battery had to be charged with a portable charger about every other week.

The voltage regulator monitors system voltage, and most of them respond instantly to the low end of the ripple voltage by bumping up the target charging voltage. The regulator does not know when a diode has failed. It just assumes you're running a lot of electrical loads, and it needs to bump up the voltage to produce more current to meet the demand. The problem is when the next two working phases show up, output current is momentarily normal, plus the regulator is running the generator harder. It takes longer for it to decide to bring the target voltage back down. Before it does that, the next missing phase is detected, and the regulator continues to bump up the desired voltage. That is what you are seeing with the 14.75 volts.

This was one of my prepared "bugged" cars I built for my students to diagnose. The first time they saw the "battery" warning light on the dash, they became confused because charging voltage appeared to be fine. At the end of the learning experience they saw that charging voltage actually went up about 0.4 volts when the defect was switched in, and had they checked initially, they would have found full-load output current dropped from 65 amps to around 20 amps. Everything on the car worked fine, except for the "battery" light being on, because that car had very few electrical toys.

GM owners have a real lot of diode trouble with their 1987 and newer generators, and there are special considerations for those cars. For the rest of us, a diode can fail in any brand of generator, and while it isn't very common, that is the first thing we test for when we run into the symptoms you described. There is no value in searching for an elusive cause to a problem when a simple solution could be right in front of us.
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Wednesday, April 19th, 2017 AT 11:43 PM
Tiny
DWIGHT F
  • MEMBER
Thank you! You are most probably correct.
How can I just hire you?

My limited understanding of electricity.
This makes sense.

Thank you!
DL
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Thursday, April 20th, 2017 AT 4:15 AM
Tiny
DWIGHT F
  • MEMBER
Thank you!
Now this makes sense.
So, if I can get the car running I will have a mechanic check the generator.

At least now I have a place to start the diagnosis.

Thanks very much!

Best!

Dwight F.
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Thursday, April 20th, 2017 AT 10:55 AM
Tiny
CARADIODOC
  • EXPERT
Thank you. The only observation that could be not related is needing the engine to cool down before it would restart. The is a real comment complaint on newer cars and is usually caused by the crankshaft position sensor or the camshaft position sensor failing by becoming heat-sensitive. The engine cranks just fine, but there is no spark and no fuel injector pulses until those sensors cool down for about an hour. Those are on gas engines. Natural air flow over the engine while driving keeps the sensors cool. It is when a hot engine is stopped briefly, as in when stopping for gas, that the heat from the engine migrates up to the sensors and can cause them to fail temporarily.

Given the multitude of additional, related symptoms you are experiencing, you could also have a failure to restart due to a run-down battery. A hot engine is harder to crank, so the starter motor has to work harder, and that means it will draw more current from the battery. If the battery is run down to the point it can't supply that current, the engine will crank too slowly to start. Once the engine cools down, it will spin more freely, so it spins faster, and the starter needs less current to do that, so it is easier for the battery to supply sufficient current. The bottom line is a mostly-discharged battery that cannot crank a hot engine might be able to crank that engine when it has cooled down.

If this is all related to a charging problem, charge the battery with a portable charger, at a slow rate, for a couple of hours. If the charging system is the cause of the trouble, the engine will crank normally after charging the battery. Remember too that your engine uses glow plugs for starting, and those draw a real lot of current. Those can kill a discharged battery, so even if the starter is able to crank the engine yet, the air in the cylinders may not be hot enough to support firing the fuel.
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Thursday, April 20th, 2017 AT 3:03 PM

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