Wont start when temp is hot - PeachParts Mercedes-Benz Forum

pshibly · #1 08-30-2002, 02:58 PM

Ok. So my 1989 260E wont start when its extremely hot outside. It will be sitting in a parking lot all day and why i try to start it in the middle of the day when its blistering hot outside, it wont even turn over. You can hear the initial "spray" sound coming from the engine, but it wont turn over, or even try to turn over. It just sits there. But when i go back when its cool outside, like later in the evening, it will start fine with no problems. Im thinking its a wiring problem and it somehow shorts when its hot out. I know this is probably useless to post this because it could be a million things, and i have no other symptoms to look at. So if anyone would take the time out to suggest some things to look at, that would be great. Thanks for tryin to solve a problem that could be a million things.....dont worry, i dont have any hope as to find the solution for whats going on. this was just a shot in the dark.

#2 08-30-2002, 05:31 PM

Probably not much help, but I have the same problem with my Jaguar, when it runs enough to get the engine hot, then is parked in the heat it will not restart until it cools way down, and just like you everything happens correctly but the engine just fails to even try to turn over. The first time, I thought it was a weak battery, but it turns over and starts right up every time when it can be cooled. I have resorted to placing a fan under it when I pull into my garage, outside the workaround is to open the hood.

Have been successful enough with the workaround I have not tried to troubleshoot. It must either be something shorted or open circuited in the heat, or else something dependent on engine temperature, such as the aux fan control circuit. Good luck, and if you stumble onto anything, let us know!

pshibly · #3 08-30-2002, 07:09 PM

well its independant of the engine temperature. its just dependant on the outside temp...how hot is is during the day. so i dont know. looks like ill be wasting more money on the mechanic.

EricSilver · #4 08-31-2002, 11:38 AM

I have been having this problem for about three weeks, becoming progressively worse each time. The problem is almost certainly in the starter or battery.

1.) It could be a short in your starter. It's basically saying "leave me alone and let me die in peace" when you have hot-start problems. (Heat breaks down the insulation.) However, I would expect this after the *engine* got very hot, not the warm outside air.

2.) Your battery is weak, and is not being properly charged by the alternator. Check the Voltage Regulator brushes which you can unscrew very easily from the back of your alternator. This solved my problem entirely. (See post titled, "My 260E is Running Great") The brushes should be evenly worn, and about an inch long (when new). If they are about half that, replace them. (About $30 for parts.)

stevebfl · #5 08-31-2002, 12:00 PM

You have the ultimate in a simple diagnostic problem. You do have a symptom and since it is predictable, what else do you need.

A problem exists when you don't have a repeatable symptom. Your problem is easily testable because it exists. Don't look for it when it doesn't exist. You have two circuits to verify. The first is the basic power circuit to the starter (the large positive battery terminal). The test is simple; it must stay the same as battery voltage all the way to the starter. If its is 12v at the center of the battery post and 12v at the large connector on the starter, it has done its job. It must pass this test while the problem exists!!!

The second test is the activation circuit (#50 on the wiring diagram). This circuit should be the same voltage as the battery when the starting attempt is made.

A third observation can also help; what is the change in battery voltage while an attempt is being made. If the battery voltage stays high and both power and term# 50 are the same during the starting attempt, then an internal OPEN circuit exists in the solenoid or its grounding circuit through the armature. If the battery voltage drops then a load has been applied. This would be a short. If the voltage drops below 9v without any movement you have a dead battery (or a huge wire burning short - shorts on power circuits, burn wires; unless they burn a fuse).

There are a few switches involved that shouldn't be temp dependant, but when the problem exists it is the most basic of circuits. You will learn to evaluate this circuit or you should forget dealing with such.

pshibly · #6 08-31-2002, 12:06 PM

well i highly doubt its a dead battery, or its not charging all the way. i will be checking the wiring for voltage drops. i think im going to find a short if anything. im amazed my Princip. of E.E. class comes in handy in the real world!

EricSilver · #7 08-31-2002, 12:19 PM

Don't be surprised if it is indeed the battery. I never would have believed mine was weak -- since everything else electrical was performing normally.

A strong voltage should overcome the heat-related resistance you are apparently encountering, unless the starter is truly shot.