1990 240, just put in a new battery and a new alternator. Background: Old (5-7 years old) battery was continuaply drained by a failing alternator (whose belts were also a bit looser than probably should have been). Replaced the battery, same issue. Old one wasn't holding a charge, so it was time for it to go anyways. Removed the alternator and had it tested at Autozone, of course it failed. It was the original one. So, I bought a new one. After getting everything put back together, I notice the battery light on the instrument cluster was now on, but only when the car was off. I hadn't even started it yet at this point. The spade connector was loose on its terminal, so I replacd it with a fresh one...now it's nice and snug. Same for the ground wire that attaches to the block. Still, the light was on. I pulled off the spade connector...and light goes off. Started the car (spade still disconnected), and measured 11.8V on the battery.
My questions are, is it vital to the proper operation of the starting/charging system for that spade connector to be connected? Seems the battery light being continually on would drain the battery quickly. And, would the low voltage when the car is running be related to the spade connector being off? I should have left spade connected with car running, but the low voltage discouraged me from working on it any more for today.

 

the alternator(battery) light on the dashboard is connected to the ignition switch and the alternator's D+ terminal (thinner red wire). if it does not turn off with the key, that implies there's a short in your dash, or the ignition switch isn't switching that circuit off properly.

the D+ signal to the alternator provides the bootstrap power required to 'start' the alternator. once the alternator is running, D+ should be the same voltage as B+ which is connected to the battery, and if this is true, the light goes out. if D+ is disconnected, the alternator won't charge at all.

 

Thanks! I'll check that when I get off work tomorrow. It's interesting to me that the light never came on with the original alternator, only the new one.

 

then maybe you're not connecting D+ to the right place.

Is this a Bosch alternator or a AC Delco or something?

frankly, I bet all your old one needed was new brushes, and maybe the regulator

 

The car had a Bosch alternator, and all I've ever had to do to those was replace the regulator, which costs all of $25 on autohausaz. My '84 BMW 533i uses the same alternator. But the old one was was really haggard and dirty, so I just swapped it out for a new one entirely. The car must have lived in a cornfield before I bought it last month. The new alternator has a spade terminal labelled D+, and that's where I put the connector. I'm almost reluctant to say that the new alternator....is an Autozone rebuild of a Bosch. FWIW, I've never had issues with their parts before though.

 

most of those rebuilds consist strictly of cleaning it up, and putting new regulator/brushes on it and testing it. I doubt they replace the bearings, or refurbish the rings

 

Sure seems that way. I made a trip to the salvage yard near me, which had three 240's to choose from this week. I took a clean-looking oem alternator from an '85, but the wiring looked slightly different..the case of the alternator was grounded to a bolt near the bottom of the engine block. Mine isn't. I'm wondering if it's supposed to be, and that's my problem. Alldata said my light indicated a shorted diode inside tbe alternator. Not sure about that either.

 

the shorted diode doesn't explain why the light stays on when the key is off.

battery + -----(ignition switch)--------(alt light)-----D+ (alternator) B+ ---\
. . . . . \---------------------------------------------------------------/

so... if the switch is open (key off), no current can flow through the lamp.
if the switch is on, and the D+ is at +12V because the alternator is working, then no current flows through the lamp.
if the switch is on, but the alternator isn't working, then D+ is at ground, and the light is ON.

there's no scenario where the light can be on when the key is switched off.

 

Hey Pierce,

I'm having a similar issue with my car, taking the key out of the ignition does not turn off the ignition itself. This started just today.

If I'm understanding you correctly, its either a short in the wiring in the dash or the ignition switch has failed. I've seen the extraction process for an ignition switch, not sure I'm up to removing my dash right at the moment. *edit* This isn't the case for the switch itself, only for the whole assembly. I'll just go to the wreckers and extract the darn thing and stop complaining so much *edit* This particular 240 is my only mode of transportation while the turbo gets replaced in my 850.

Is there a method of verifying that the ignition switch is bad with the switch in the vehicle?

 

weell, the ignition switch has 1 or 2 inputs (red wires on pins 30), and a bunch of outputs, labeled typically 15 (power on in run or start), 50 (starter only), 15X (power in run or accessory) and 15P (power in run only). On the typical 240 (I'm looking at an 87/88 wiring diagram), the 15X goes to fuses 1/2/3, so if they have power when its switched off, I'd guess something is wrong.

ignition and fuel injection itself is on the 15 signal, this goes to the ECU, and fuel injection+pump relay on red/black wires, and on a blue-yellow wire to a blue wire to the ignition coil pin 15 and the ICU (this blue-yellow wire also goes to fuses 11-12-13).

so I do believe I'd get in there with a volt meter and start checking voltages at the fuse panel with the switch on and off. note that fuses 6-7-8-9-10 are wired directly to the battery via a red wire, so they are always powered, key on or off.

now, if your 240 is an early version, like late 70s, or up to 82 or 83, their may be significant differences. and the 1990s models may have some differences too.

 

Pierce, sorry for not be specific enough. The car I'm working on is a 1990 240 DL.

Last week I replaced the ignition switch, with no change.

Fuses 6-10 show 12.5 volts, as you have said. Fuses 1-5 also show voltage, between 0.2 and 1.6V.

I had an opportunity to go over the car in a little more detail again today, and found that the alternator ground was pretty well destroyed. That explains the charging issue. Replaced the ground and at first it appeared this had also fixed the battery light. After starting the car, the light stays on again though.

I'll have a little more time tomorrow to try replacing the thin +ive wire from the alternator as mentioned. Failing that, I guess I'll be digging into the dash.

For now I continue disconnecting the battery haha, its my favourite new game.

 

Alright, after a little more digging I have come to a few conclusions and a lot better understanding of how the charging system works on this car.

Hunting back and forth between the alternator and fuse 13, which drives the battery light, I was not able to find a short in any of the sections of wire. Everything appears to be intact, free of fraying and operating correctly. The multimeter showed no ground fault in any of the sections either.

I was able to find something strange, however, within the alternator. The connection on the alternator that drives the thin red wire shows 12V when the car is off. This must mean that something within the alternator is shorted, correct? Perhaps the stress of having such a badly frayed ground on the alternator caused something inside to burn up?

I'm off to the wreckers tomorrow to pull an alternator and put my theory to test.

 

the D+ input (which is the light on the dash) has its own diodes, seperate from the main power diodes on the B+ output (which is the main battery charging cable output)

 

The alternator from the wreckers has fixed the problem, along with a few other small problems that I'm kind of surprised by. I'll probably return the old alt for the core deposit, but out of curiosity where are the diodes located for D+? As well as B+ for that matter, are the both within the alt?

 

yeah, there's a diode board inside the alternator. these are big high current power diodes, not the little things you more typically see.