Red Ignition Light ON with Ign Switch OFF

[hands_aus]

I started the car and took it to a garage for fuel.

When I turned the engine off the Red ignition light came on. I then started the car again and the light went off. I shut the engine off and there was the red ignition light ON.

I have done these things so far :-
1  removed all fuses, one at a time, and checked …..the light was ON for each

2  removed the connectors for the start relays and checked…the light was ON for each

With Ign Switch in OFF position I
1  tested the voltage at the wire from the Ign Sw to the Ballast resistor….. 11.2 Volts
2  disconnected the wire to the ballast resistor from the ignition switch….. OFF
3  Disconnected the Pertronix unit wire…. OFF

The 11.2 Volts was passing through the Pertronix unit to earth.

By mistake I left the battery isolating link connected and the alternator felt hot after a while. I think the electrical magnet of the rotor is activated too.

I remember reading a discussion about something similar.
It was stated that if a diode in the alternator was blown, a positive feed could leak through the diode  to the rotor magnet and the ignition system and eventually drain the battery.

I checked the wiring diagram. It seems to me that diode failure and battery leak is possible.

A blown diode in the B+ circuit could leak its way back through the dash red light to fuse No2  (Ign Switch side) then down to the Ignition Circuit.

Initially I was blaming the Ignition Switch shorting internally.

Another question :- Will the alternator charge if the red blub is blown?
I ask because the red bulb is in series with the rotor magnet of the alternator.

I am thinking of having the alternator checked.

Any ideas please


Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[hands_aus]

lurkers with nothing to say?

Very Unusual.

OK, The auto electrician said it is definitely one of the diodes blown.

I asked him about maybe replacing the 35 amp unit with a 55amp unit.

He asked me what I was planning to put in the car that would require the upgrade.

He then said that the existing unit was the one designed for the car and that even adding additional lighting, the existing 35 amp unit is more than adequate.

As I couldn't think of anything additional to install ( air-con is definitely not going to happen) I am going to have the unit rebuilt at about $160.00 AUD.

I will remove it next week.

This weekend I am planning a short trip to the country to a huge (by our standards) swap meet.

He said it would be ok to use the car as long as I remembered to disconnect the battery +ve terminal when I turn the engine off.
He also said that the battery will still be charged but at a lower rate because of the reduced alternator output.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Naj ✝︎]

The advantage of going for a later alternator (in my view) is that you can have one with an internal regulator which is electronic with no moving parts. The current regulator/wiring can be left in place, undisturbed.
Need to add a bit of wire (regulator plug to alternator D+) to operate the warning light.
Just my $0.02 AUD.

naj

65 230SL
68 280SL

[ja17]

Hello Bob,
Make sure your brake fluid level is up. All kinds of strange things can happen if the level goes down enough to activate the warning light circuit.

Yes a bad diode in the alternator can also cause this problem (easy fix for most automobile elec.  shops).

Joe Alexander
Blacklick, Ohio

[A Dalton]

quote:Originally posted by ja17

Hello Bob,
Make sure your brake fluid level is up. All kinds of strange things can happen if the level goes down enough to activate the warning light circuit.

Yes a bad diode in the alternator can also cause this problem (easy fix for most automobile elec.  shops).

Joe Alexander
Blacklick, Ohio

 Sure can...
 http://113.mbz.org/hints/brakelite.htm

[hands_aus]

Thanks for the advice about the brake fluid warning light but my EARLY 250sl doesn't have those mod cons.

Naj, I asked about the alternator charging if the red dash light bulb blows.
Does the rotor of the alternator have a residual permanent magnet with enough magnetism to start the self propagating affect for charging?

The wiring diagram shows a path for such a current with a blown bulb.



Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Naj ✝︎]

Hi,Bob,
My understanding is that the alternator will still charge with the warning bulb blown.
The light works in a very clever manner. With ign. on, it gets 12v from the switch. With engine running and alternator charging, it gets voltage from the D+ terminal of the alternator and cancels the voltage from the switch, turning the light off.  :|
Not sure if only one blown diode will charge the battery but not shut off the light. A voltage check would tell  

I stand to be corrected of course.
naj

65 230SL
68 280SL

[hands_aus]

Hey naj,

Thanks for that info. I was wondering how it worked.

I tried removing the alternator today.... there is very little room.

Do I remove the alternator from the top or bottom?

The belt tightening strut NUT is restricted by the clamp on the short water hose that comes off the thermostat housing.

I managed to disconnect the one plug, but I can't get my big hands in there to disconnect the cables.

Can I remove them from the terminal block on the oil pan?

If I do squeeze my hand in there I can't SEE anything.

Wish I had more room in my garage.

Looks like a trip to an auto electricians workshop.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[ja17]

He;llo Bob,
On factory AC equipped cars the alternators are right on top. All the rest are down below.

Don't forget to unhook the battery. Removal of the horizontal 13mm  head lock bolt on the alternator at the adjuster arm will allow you to remove the alternator when you remove the long lower bolt.

Yes you can remove the electrical block or lower radiator hose to gain more room. working from above and below is best.

Joe Alexander
Blacklick, Ohio

[Naj ✝︎]

Yes, Bob,
Taking the alternator off on a RHD car is much more difficult as the exhaust down pipes are in the way. It may have to drop out of the bottom. I did mine with the rad off so can't be much help here.
May be easier to take the wires off after the mounting bolt / adjuster are out. Early alternators had all wires stud mounted while later alternators have a 3 pin plug and just the main 'B' wires stud mounted. You probably have the later type if you've got the terminal block ??

naj

65 230SL
68 280SL

[hands_aus]

Well, I am persistent if nothing else.
I went back to my garage yesterday afternoon and tried again to remove my alternator.
This time I managed to get a 10mm spanner to the B+ terminal nut and then I squeezed a couple of fingers in behind the water pipe to remove the nut. The plug for the wires to the regulator was easily removed.

The up shot of it all is that with some manouvering of the alternator I pulled it out from the TOP between the fan blades and radiator hose.

I had to wipe it down because it had a lots of engine oil all over it. (There is a front crankshaft seal leak that I have learned to live with).

I intend to connect the wire to the B+ terminal and the suppressor before re-mounting the alternator.

At the moment part of the wiring harness (B+ wire) is below the cooling pipe and the
regulator-to-alternator wire is over the cooling pipe.

QUESTION What is the correct layout of the wires?

NAJ, What do you mean by later type alternator?

The auto-electrician asked if I wanted it all polished and painted like new. I said I wanted it cleaned and well presented but not restored because I am tryng to keep my car original looking but not restored.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Naj ✝︎]

Hi, Bob,
You said you had a plug for the regulator wires, so that makes it a later type alternator  
naj

65 230SL
68 280SL

[hands_aus]

55amp with a separate regulator?

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Naj ✝︎]

Probly not!!
They were fitted to later 280s. My '68 280 still had a 35 amp.
Now fitted with a 55 amp with internal reg.

Does your plug have round or flat pins at the alternator end?
p.s: Check the mounting bracket to block bolt(s). Oil could be leaking from there.

naj

65 230SL
68 280SL

[hands_aus]

The pins are round in a T configuration

Do you mean the alternator mounting bracket bolts?

What about the alternator strut bolt onto the block?

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Naj ✝︎]

Yes, Bob,
I meant the alternator mounting bracket. I believe Joe Alexander discussed this. The threads run thru the block into the oil cavity. Not sure about the adjuster strut. Use of sealant was suggested?

BTW, the Bosch tag on my 280 alternator was 0 120 400 504. Do you have that?

naj

65 230SL
68 280SL

[n/a]

instant well-based cause for panic! Either diodes in alternator or if an early alternator with separate regulator not unlike a generator regulator, can be there too. Either way the drain current can be huge. Current equals flat battery at best, damaged components on average, and heat/fire at worst.

A warning light indicates any of the above - an ammeter narrows down the likely outcomes. Either way, either keep the engine running or pull off a battery lead!

Neil Rote, Melbourne Australia
Unrestored '64

[hands_aus]

Follow up,

I had the alternator rebuilt by a local Bosch auto electrican who replaced the 'half moon' diode unit.

After I re-installed the alternator and reconnected the wires the car started right up and the red dash light was out when I turned the engine off.

The idle charging voltage was about 13.75 Volts and increased when I increased the engine speed. Previously the idle charging voltage was about 12.92 Volts.

I should have recognised that as a possible problem.

Thanks for all the replies and help.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto