240 charging problems (exciter wire)

I've got an 88 240 that had a bosch 80amp until about a month ago I killed it with a big mud puddle. It would charge at idle sometimes but the more rpm I gave it the weaker the voltage it put out. 13 at idle, down to 11.9-12.3 at higher rpm. I had the alternator tested and it was found to be bad so I replaced it with a 740 100 amp. I also replaced the battery, b+ and d+ wires and got good voltage around the block on her first run but then I lost power again. I've tracked down the problem to the exciter wire, but I'm stuck. It only puts out ground, I know (er think I know) its supposed to be ground when the key is off but 7-9 volts with the key on with the battery bulb illuminated until the motor is running and alt charging. Problem is I only get ground. The bulb does not illuminate but it is not burned out, I've swapped bulbs and checked voltage at the gauge circuit board and it's still ground. As a dumb test I disconnected the exciter wire from the alternator and fed it 12 v to see if it would illuminate the bulb but it didnt. When I swapped ends and hard wired the alternator d+ exciter wire to the positive battery terminal my voltage jumped up to 14.08 telling me the alternator is working like its supposed to. (When redneck rigged together).

Dumb questions:
Is it in the ignition switch? This is a gambler 500 car and the switch works fine otherwise so I'd rather not replace it, could I just run a new exciter wire from a keyed hot source off the fuse box or does the exciter wire need to go to ground when it's off like factory to not drain the battery?

The studious reader can tell I have just enough electrical knowledge to be dangerous and that's exactly why I'm in the pickle I find myself in.

I'm a bit a confused. your exciter wire - d+ wire - should get battery voltage when ignition is ON. it is not a ground wire. you don't need to feed it voltage. the alternator should have a separate ground wire at the bottom of the case that grounds to the engine block.

Exciter wire test= will a test lamp illuminate on it? Unplugged from alt. Key on.

is the problem that your charge light isn't turning on in the dash with ignition on?

Sorry I read a thread somewhere that said the d+ is grounded until the key is on.
I dont have a test lamp but I get 0.0 volts from my multimeter when I disconnected it from the alt with the key on.

Charge light stopped working somewhere along these repairs but the bulb is okay and I get ground at the bulb socket on the cluster board.

It only puts out ground, I know (er think I know) its supposed to be ground when the key is off but 7-9 volts with the key on with the battery bulb illuminated until the motor is running and alt charging. Problem is I only get ground.

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To be clear, you are talking about the wire to the D+ terminal on the alternator? If so, are you measuring 0 ohms to ground with the wire disconnected from the alternator or with the wire connected to the alternator? If the wire is disconnected from the alternator and you are measuring 0 ohms to ground then there is a problem with the following statement.

As a dumb test I disconnected the exciter wire from the alternator and fed it 12 v to see if it would illuminate the bulb but it didnt.

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If you measured 0 ohms to ground on the exciter wire and then applied 12 v to it you should have a direct short accompanied with some sparks and things getting hot / melting.

The charge wire circuit is simple. One side of the charge indicator light bulb is connected through the ignition switch to the battery + terminal. When you switch the ignition on (before starting the engine) current flows from the battery through the charge light and into the alternator excitation circuit (D+ terminal). This causes the charge light to illuminate. Once you start the engine the D+ terminal rises to 12 V (or the charging system operating voltage). With 12 v on both sides of the light bulb there is no voltage difference across the bulb and illumination stops - indicating the charging system is working. If the alternator is dead no voltage appears on the D+ terminal and the charge light stays illuminated indicating you have a problem. The charge light / D+ circuit does double duty in that the tiny current flowing through the charge light bulb also boot straps the alternator on start up by providing a small amount of current to its field circuit to kick-start operation of the alternator. Once the alternator is bootstrapped it generates its own field current internally and no longer needs the bootstrap current through the charge light. Your redneck jumper D+ to 12v just bootstrapped the alternator.

If you are sure that the charge light bulb is working I suggest you check the socket for the charge light to make sure the bulb is making contact. If that isn't the problem then you have a problem with the instrument cluster. Do all the other warning light in the cluster work correctly? If so, it might be an external wiring problem.

My 1987 745 T had Volvo's infamous bio degradable engine wiring harness. Does the 1988 still have that style wiring or was my 1987 the last year? If you have the original wiring and it is the bio degradable harness you could have a wiring harness problem that is causing your flakey operation; but, I would have expected the wiring harness problem to emerge 15 - 20 years ago.

could I just run a new exciter wire from a keyed hot source off the fuse box or does the exciter wire need to go to ground when it's off like factory to not drain the battery?

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Yes; but, not the way you want to do it. If you don't want to deal with an instrument cluster problem or dodgy wiring in the harness run a wire from the switched +12v to the instrument cluster to one terminal of a light bulb of the same wattage as your charge light and from the other terminal of the light bulb to the D+ terminal. The reason you want the light bulb is that it is a variable resistance that limits the bootstrap current on start up. You can hide the bulb somewhere under the dash if you want to. Your redneck trick of applying +12v to D+ will work on a spinning alternator ; but, it is not a good idea to apply +12v to D+ when the alternator is not spinning ( i.e. you turn the key on; but, don't start the engine right away or have trouble starting the engine). The resulting bootstrap current will be very much higher than normal and will likely pop fuses if it doesn't damage the alternator field flashing circuit.

Canadan said:

I dont have a test lamp but I get 0.0 volts from my multimeter when I disconnected it from the alt with the key on.

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you probably have a wiring harness fault then. main engine harness. your battery isn't pushing voltage to the alternator.

just to echo 142 guy. i only recently recently replaced the harness in my 85 240 that was the original harness. I had no wiring faults to/from my alternator when i put in my new harness.... but they looked on their last leg. in any event, I say this just to say, if you have the original harness, I'd bet that's your fault.

in fact, reading your account more, I bet it's the harness. you were having intermittent issues with the old alternator that are consistent with faulty wiring.

look at dave barton's website for the new harnesses. not a complicated install.

'87.5 is the first year of GXL wire/updated harness. Shouldn't rot, but that wire might be broken, it's small & flimsy and gets cooked/vibrated & fatigued in that location.
Extra fine wire strands & thicker gauge than necessary & thick super toxic insulation don't come cheap 1978-1979+ on the volvos!

Doesn't rot, but that weenie thin little red wire on an often vibrating alt on those hooptie little bushings that are all shriveled up on the average neglected oil leaky redblock/wiring harness vinyl sheathing full of hot oil doesn't do them any favors.

Should have 0 ohms between the alt D+ spade and the bullet on the red wire at the firewall connector by the engine lifting eye at the middle of the firewall there & D+ on the alt should be near 0 ohms to ground/alt case with the engine off.

Check your alt ground for corrosion & fraying as well as cables on the engine.

Good luck unplugging them at the firewall there, the female bullet clear coating/flange usually goes away/requires that they be plugged in individually at the firewall plug after years/heat even if the wire insulation isn't wasted on an '87.5+ 240.
Typical (of many) vol-void annoyance. Usually stay in place carefully re-connected individually in the plug with care/not worth 100-point resto to fix.
The weird volvoid 3.5mm bullets are sorta their corrosion resistant euro-trash version/answer en lieu of the GM water-tight bullet weather pack, rarely see them in that sizing on anything but Volvos and SAABs, not sure why that is.
Are they made by VDO originally? IDK...

Fuse 13 should power the batt light IIRC, printed circuit board on cluster has a fuse-able link, though IIRC ribbon fuse failure is rare and IDK if it is used as a power source for the alt lamp. People cook the cluster fuse dropping the clock power on ground IIRC?

Sometimes the oil pressure lamp wire (black) and alt exciter/D+ wire (red) rub together, but in that case the symptom is usually perpetually glowing pair of lights (oil & alt) with the engine off, as opposed to bad alt diode (lone alt/batt light glowing, ignition off IIRC), or alt lamp glows perpetually (dead short to ground on red wire).

No alt lamp at any time is often broken red wire or corroded fuse 13 from a leaky windshield &/or cheap quality aluminum replacement fuse (over the original german copper) in my experience.

Grounding red wire should illuminate the lamp with the key on/engine off if there's continuity to the bulb or power to the bulb from fuse 13 (assuming it has battery voltage on output end of fuse spade at fusebox). If no, something went wrong between alt spade and fuse 13. Trace accordingly.

Good luck, alt *should* self excite at some point.
Silently slipping belt?

Thanks for all the info guys,
So some more background, yes it has the gxl wires, the engine harness was from a 740 to get ezk but it's not rotten like my old 85 240. I replaced that grey connector on the firewall with butt connectors probably 10 years ago when I did the ezk swap. About 5 or 6 years ago I bottomed out the front cross member which severed all the wires that go under the crank (oil pressure, d+ and b+? Whatever the big alt cable is called. I soldered it back with a bit of heat shrink until about a month ago when all these charging problems started. Along with an old battery, a bad alternator, the b+ also grounded through its 5/6 year old heat shrink that's why I said screw it and replaced the b+ and d+ entirely and ran the d+ all the way to where that grey plug on the firewall used to be. I fired it up and had power for a bit but after 5 min nothing. Which lead me to start this thread.

Today I went out and checked the resistance between d+ and ground as 142 guy suggested and I didnt get anything. Tried continuity cause I swore I was getting ground the other day and again nothing so I said screw it again and ran a new d+ from the fuse block (with a dash light in line) to the d+ and shes good to go now. Not perfect but nothing on this car is. No telling what's wrong but everything else works so I dont care, it's either something in the gauge or theres a short somewhere in the cabin.

Thanks again guys.

if it works, good. if you are cool with rolling with it, do it! but, the d+ wire is designed to come off the battery as part of the main engine wiring harness. if you were not getting voltage at it with ignition on, that means your main engine wiring harness had at least a fault on that wire. other faults may follow quickly behind. and long term, I am unsure about the reliability of running the d+ wire off the fuse block.

all that said - if she runs, let her ride.