280SE 3.5 W108 still playing up...

[chazola]

trying to trouble shoot rough running/starting on my 280SE still... seems like it acts differently every time but here's the latest symptoms:

on start-up, alternator warning light will stay on, brightness varies, for around 10 mins. Car will run ok, but fairly rough when at standstill. Once the light goes off will run good, but still miss occassionally. If you pull the air-temp plug out of the intake it will start running better, and when put back in continues to run well, like it's been re-set.

Tried swapping batteries, still did it with fully-charged new one.
Alternator belt is new and adjusted correctly.
Tried disconecting radio, glove box/trunk lights etc for drain, but after drive or leaving overnight etc battery only shows charge of around 12.25v, and less than 12v when engine is first running (an alternator light is on).

so i've tried 2 batteries, both known to be good, and the system is fine after 10mins or so of engine running...

[Mike D]

If the car is idling too low the alternator will not charge. 12.25V after sitting all night is about right. I think the formula is about 1.04V per cell. So, 12 cells times 1.04 volts is 12.48V for a perfect battery, at sea level at 70 degrees F.

The alt light stays on above idle?

You've checked the obvious? Ground cable, loose plug at the alternator, positive battery cable where it runs by the exhaust, position of Mars and Saturn in alignment with Capricorn?

[chazola]

yea, light stays on while driving for about 10 mins.

The idle is actually quite high in P/N at the moment.

I thought 12.25 seemed a bit low as the battery in my 124 always shows a charge of around 12.60

Will double-check earth points etc...

[Mike D]

I guess I should have looked at your signature in order to know what year of car and your location.

You have a voltage regulator either on the fender or located under the battery box. It is a mechanical relay and the contact points are probably either corroded or burnt away. Have you experienced a large coolant leak recently? Say, on the heater core return piping between the firewall and the lower radiator hose?

Do you have the transistorized ignition box? Low voltages and surges will wreak havoc on the control unit.

[chazola]

no coolant loss at all. I think I have the transistorised ignintion- have the blue bosch coil and 2 ballast resistors and then control box located next to the radiator. i noticed one of the ballast resistors is cracked at one end.

is the voltage regulator obvious to spot?

[Mike D]

The voltage regulator looks like a large Zippo lighter shaped can. It will have a wiring harness on the bottom and a flange which is connected to the fender well or the mounting plate over by the transistor control for the ignition.

The ballast resistor might be part of your running problem but not the charging issue.

[Tomguy]

The voltage regulator on the 3.5/4.5 108/109's is on the board the ECU is mounted to (to the right of the rad as looking from driver's position - your driver's side). They can fail intermittently like any electronic, but have your alt tested when cold first at a parts place that does free testing (in the US, AutoZone does free testing, and I think PepBoys and Advance do as well)