Battery or cables or K40? 96 e300 - PeachParts Mercedes-Benz Forum

DieselJim · #1 10-24-2016, 12:31 PM

I found a thread on this subject early this morning but it was a bit old so going to start a new one. Its been very dry this summer in metro Atlanta and I don't see any corrosion issues under the hood with a quick inspection. However I've had a few mysterious computer resets and the next time I go to start it I may have to jump. This battery is a little over 2 years old but it has been abused. So I think it is safe to replace the battery and then verify cables and alternator later?

After driving 15 minutes this morning and filling up with fuel it barely started. Bad cell or just inadequate juice to charge?

DieselJim · #2 10-24-2016, 12:39 PM

That K40 is a fuel pump relay? That might explain several problems

DieselJim · #3 10-24-2016, 12:59 PM

Sluggishness starting has been gradual over the last few weeks.

tjts1 · #4 10-24-2016, 01:23 PM

K40 is not for fuel pump. There is no fuel pump. When you turn the key the K40 switches on every major circuit in the car. A bad k40 can cause random stalling or no start issue but not sluggish start. I would put a Volt meter on the battery and watch it as you start the engine. You should see it drop to about 10v as you glow and crank the engine then rise to 13.5+ V within a minute of idling. And stay there throughout your trip.

If it's slow to crank it could be a bad battery. If it's slow to reach idle speed I would look for air bubbles in the fuel system.

DieselJim · #5 10-24-2016, 02:33 PM

Quote:

Originally Posted by tjts1

K40 is not for fuel pump. There is no fuel pump. When you turn the key the K40 switches on every major circuit in the car. A bad k40 can cause random stalling or no start issue but not sluggish start. I would put a Volt meter on the battery and watch it as you start the engine. You should see it drop to about 10v as you glow and crank the engine then rise to 13.5+ V within a minute of idling. And stay there throughout your trip.

If it's slow to crank it could be a bad battery. If it's slow to reach idle speed I would look for air bubbles in the fuel system.

Cool I saw on the diagram all fuel pumps are mechanical like my old w124. I agree the battery is probably bad. The alternator may be contributing. I suspect I have a K40 issue but that is seperate.

seo · #6 10-24-2016, 03:18 PM

I have a somewhat similar problem with my 85 300d (123 chassis): Battery is only a couple of months old. Battery voltage after sitting overnight is 12.6V, which I think is the voltage of a fully charged battery. But it won't start, won't even engage the starter. Jump the battery, voltage goes up to 13.8, car starts right up. But there's a funny delay between when I turn the key from "on" to "start," maybe two seconds, and then the starter cranks and the engine starts. Hmmm.
It's a nasty cold day for crawling under a car, so I'm wondering if there might be low voltage at the starter caused by corroded contacts between the battery cable and the solenoid?
I assume from the look of the starter that the solenoid acts on a lever to engage the starter gear with the ring gear, and a high power contact to power the starter motor. Is this right? Does the contactor ever get corroded and fail?
This might be relevant to the question of whether DieselJim should replace the battery. My advice would be to disconnect one of the battery cables (to eliminate a slow dead short), and charge the battery at 13.6V-14.2V. After it's fully charged, check the voltage with a good digial Voltmeter. Write the number down. Check is again the next morning. If the voltage is above 12.5, my guess is that the battery isn't the problem, and while the battery is disconnected I'd slither underneath, remove the nut attaching the cable to the solenoid, and scrub everything with a copper or brass brush. Just for good measure clean up the lighter gauge solenoid wire. If that takes care of the problem, you've saved some money over replacing the battery.
ALSO:
I have three 123 Mercedes: a) 1983 240d with 4-speed manual. Parts car. b) 1984 grey market car with 5-speed manual and natural aspirated 5 cyl. engine. It's been hit on the nose, and is a parts car. The driver is a 1985 US market 300d turbo, automatic trans.
Question: Is the starter out of one of the manual transmission cars compatible with the Ottomatic car?
Thanks for any responses.

tjts1 · #7 10-24-2016, 04:03 PM

Quote:

Originally Posted by DieselJim

Cool I saw on the diagram all fuel pumps are mechanical like my old w124. I agree the battery is probably bad. The alternator may be contributing. I suspect I have a K40 issue but that is seperate.

Something to keep in mind, the 90a alternator in the 96-97 is really undersized if you do lots of very short trips. These cars have afterglow which means the GPs keep running for up to a minute after you start the engine and they consume a lot more than the alternator can output at idle. GPs consume 12a x 6 while your alternator can only output about 45a at idle. If you end up replacing the alternator, get something bigger, 120a or 150a with an OAP pulley.

DieselJim · #8 10-24-2016, 05:57 PM

So now it won't start at all after a brief chug and it doesn't seem to want to jump. Could the starter be grounding out?

tjts1 · #9 10-24-2016, 07:46 PM

Check the battery voltage.

jay_bob · #10 10-24-2016, 09:08 PM

If you have a dead cell in your battery the car will light up fine but be unable to handle the surge when you hit the starter. Even if you jump it you will have a hard time starting it when you are trying to parallel with a battery with a dead cell. The same thing can happen with a bad connection.

Remember you have to overcome the 22:1 compression ratio on a diesel, so your battery has to be stronger than a gas car.

Have you checked the cable connections at the battery terminals, and also the ground disconnect bolt in the rear foot well? This is covered up by a plastic plate in the carpeted wall where the right rear passenger's left heel would be if they were sitting in the seat. Also the jumper strap from engine to body, this is under the car about where your feet would be in the drivers seat.

DieselJim · #11 10-24-2016, 09:38 PM

Quote:

Originally Posted by jay_bob

If you have a dead cell in your battery the car will light up fine but be unable to handle the surge when you hit the starter. Even if you jump it you will have a hard time starting it when you are trying to parallel with a battery with a dead cell. The same thing can happen with a bad connection.

Remember you have to overcome the 22:1 compression ratio on a diesel, so your battery has to be stronger than a gas car.

Have you checked the cable connections at the battery terminals, and also the ground disconnect bolt in the rear foot well? This is covered up by a plastic plate in the carpeted wall where the right rear passenger's left heel would be if they were sitting in the seat. Also the jumper strap from engine to body, this is under the car about where your feet would be in the drivers seat.

I know a small battery cannot work if a cell is bad but should still have enough juice to crank it a little? Had the jump car caused the engine to turn I would have used the heavy duty diesel that towed me to crank it up. The starter wouldn't even try to turn and I was sure it was drawing. I will take the battery out and get it tested and do some other debugging before I go starter.

DieselJim · #12 10-24-2016, 09:46 PM

All right it briefly tried to turn just a few minutes ago. I'm back to battery or connection. I should have let the tow guy try to jump me off

DieselJim · #13 10-24-2016, 10:07 PM

If a cell is dead you think its a bad battery or the alternator not regulating properly? I would have thought I would see lights flicker and surges if the alternator was running to high a voltage?

jay_bob · #14 10-25-2016, 06:17 AM

The battery is six 2-volt cells in series. All the current delivered by the battery has to flow through all 6 cells through the load and back again. If one of the cells "dies" its internal resistance goes way up. This prevents large currents from flowing but small currents still get through.

A little EE school here. V=IR (voltage = current times resistance). Small current times large resistance equals small V. Large current times large resistance equals large V. This V is a loss that is subtracted from the total string voltage. If you have one bad cell it prevents the entire battery from delivering full load current but your dash lights will work just fine.

And if you have a weak connection at the terminals or bond strap it is the same effect. All the R's in the circuit pass some I and develop a corresponding V that subtracts from the battery voltage (known as voltage drop). One weak connection or bad cell develops enough drop that the applied voltage across the starter gets too low for the solenoid to pick up or the motor to turn.

DieselJim · #15 10-25-2016, 12:21 PM

Quote:

Originally Posted by jay_bob

The battery is six 2-volt cells in series. All the current delivered by the battery has to flow through all 6 cells through the load and back again. If one of the cells "dies" its internal resistance goes way up. This prevents large currents from flowing but small currents still get through.

A little EE school here. V=IR (voltage = current times resistance). Small current times large resistance equals small V. Large current times large resistance equals large V. This V is a loss that is subtracted from the total string voltage. If you have one bad cell it prevents the entire battery from delivering full load current but your dash lights will work just fine.

And if you have a weak connection at the terminals or bond strap it is the same effect. All the R's in the circuit pass some I and develop a corresponding V that subtracts from the battery voltage (known as voltage drop). One weak connection or bad cell develops enough drop that the applied voltage across the starter gets too low for the solenoid to pick up or the motor to turn.

Yeah. I know V=IR wasn't thinking about how that applies to a 6 cell battery but that makes a lot of sense. I don't see any major issues at the connection points. I'll have the battery tested and take it from there. My main concern is an bad alternator taking out the next battery but I would think I would have component failures all around if the alternator was failing