Troubleshooting teaser - w126

[Lurch]

1986 560 sel

On a trip this past weekend, I was floating along @ 120km/hr with the cruise on when i encountered the problem. My observations were as follows:

Son had been adjusting the rear seat
A/C began to lose cool
Speed began to slowly drop (120 to 105)
Revs climbed higher (slow rise to ~ 3500rpm)
Economy guage was nailed to the right.

I took it off cruise and the tranny shifted dropping the revs back to normal (near 2000 for just over 100 kph)
Economy guage was still showing full to the right as I feathered the pedal to keep speed to the nearest exit.
Pulled into a gas station and shifted into park.
Idle was fine, revved with no probs
Checked tranny fluid - okay.
Shut down to check oil - that was okay.
Went to start up again and the starter kept turning even when i pulled the key out of the ignition.... I had to lift the terminal off the battery to kill it.

Towed to shop... (Canadian Tire of all places)

[psfred]

Low voltage does all sorts of strange things in the W126. Did your wipers start up too? This has been known to happen. The cruise will not work properly at low voltage, the gauges will go nuts, and the compressor will drop off line, ditto for the climate control -- it defaults to full heat defrost.

Get the battery checked, it may have a bad cell so the voltage drops. Get the alternator checked too, you may have blown a diode.

Check your fuses too, in case something got hot and melted, shorting things out.

Could also be a fried ignition switch, completely unrelated to the rear seat adjustments

My brother didn't show the kids the rear seat adjuster on purpose.......

Peter

Peter

[MikeTangas]

I know that feeling. When I was resurrecting the 560 I was sitting there idling the engine and decided to cycle the sunroof. Everytime I would try to close the roof the RPM would drop, almost to the point of dying, then pick up and idle fine as soon as the switch was released. Replacing the regulator cured that.

I have a bum seat adjustment switch in the back and purposely am not replacing it. It keeps the kids from jacking with the seat.

[ejsharp]

I second what Mike has to say. My guess is that because the car has been on the road for a few years... the brushes of the voltage regulator are worn and no longer producing sufficient voltage to keep the battery at peak charge. The seat games simply set things into motion.

[Lurch]

Problem turned out to be a corroded wire connection at the fuel pump.... go figure.

Billed 2 hours of time to diagnose & fix, no parts... bargain city IMO.

[mithra]

Considering the reputation that some of the Canadian tire service centres have, it sounds like you did extremely well.

[hedpe]

I love troubleshooting the W126 so much better than the 500E, when the 500E has a problem, it SERIOUSLY has a problem, but the W126 is fun to fix

[ejsharp]

I have reservations about the fuel pump wire being the cause of the symtoms you first described. Before exhaling I would check the output voltage of the altenator and also make sure the battery does not have a bad cell.

[Lurch]

Seems to be fine now. Just did 350 km with a couple of short stops and the "feel" is as good as it was when I bought it.... IOW pretty decent. Noticed a bit more punch when I hit the giddyup pedal which may have slowly deteriorated prior to the problem slowly enough that I didn't clue in.

Perhaps one of the techs on the board can shed some light on why the starter would not disengage even when the key was turned off and pulled out. The CT mechanic speculated that perhaps the ECU was not getting a signal that it was firing and would not disengage the starter until it received a firing signal.

As part of his troubleshooting, he poured a small amount of gas into the air intake and cranked it... once it caught a spark the starter disengaged.

His somewhat sheepish comment was that he moved away from Toronto to get away from cars like that.

I'll keep an eye on it for a bit anyway and have it looked over later this week by my usual tech for any electrical heebee-jeebees.