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SOLVED** Strange electrical problem - Alan or electrical gurus, please chime in
1984 16V L-Jet, US model
After 8 years of trouble shooting, I thought I knew this car pretty well, but this one has me stumped.
I had first noticed a couple weeks ago that my volt meter was reading low. I first assumed it was the battery (its about 3 years old) or alternator was going out. The battery charge was low. I put it on a charger for a couple days and it fully charged. Load tested at Advance Auto Parts and showed 955 amps. Seemed good so I deduced the alternator must be going out.
However, when I apply a multi-meter at the battery I see this reading:
Symptoms:
Car off, voltage at battery is 14.37.
Ignition on, voltage at battery 14.1
Car running, battery voltage 15.4
So the alternator is definitely charging - and if anything over charging.
This is also consistent (within a couple tenths) with what I'm seeing at the jump post under the hood. So the connection from at least the battery to the jump post appears good.
However, at the cigarette lighter:
Ignition on, voltage 10-10.5V
Car running, 11.5-12V
It's almost like the alternator "thinks" the battery is low and is overcharging to compensate. And the car seems to "struggle" turning over the engine, almost like a weak battery. But the battery tests fine and the alternator is obviously putting out a charge. So there seems to be some extra load or increased resistance somewhere between the battery and the rest of the car.
I will get up under the car this weekend and check for loose connections, clean the grounds, all they typical stuff. But any additional input would be appreciated.
Last edited by z driver 88t; 08-07-2017 at 09:16 AM.
Reason: Solved problem
Is your DVM working well ? 12 volt battery just taken from shop and measures over 14 v ??
Cigarette lighter not best place to measure from - lots of inherent resistance here. Would use some other appliance.
With battery checked clean out the shop and still hard starting I'd consider fuel pump check valve. I had same business, and now I don't.
Fuel pump check valve maintains a certain amount of pressure within fuel lines. If valve fails, pressures are reduced and a few more turns of the flywheel on starting are required to allow fuel pump to increase line pressure enough for turning over. Will sound and feel 'electrical'.
OK, after living with the car like this for a few weeks and monitoring the voltage from from the cigarette lighter it would bounce around from 11.5 to 12.5, occasionally dropping to 10.8.
The starter also occasionally would crank slowly (as if the battery was low), but it always worked. The car never died electrically, it just ran at lower voltage.
I had originally assumed it was the alternator and that the battery was not being fully charged, hence the slow start. BUT - one thing that wasn't making sense was that even if I put the car on a charger for a few days to make sure the battery was fully charged, it would still act the same way - like the battery was weak.
I felt like a dog chasing its tail because some symptoms seemed alternator, some seemed battery.
So...this weekend I was under the car replacing the AC compressor and looking at the starter and saw the screw had backed out on the small yellow-black wire that goes to the solenoid. The wire was barely making contact to the solenoid. I put in a new screw, snugged it down and now everything works. Car starts solidly, but more importantly the charging system at the cigarette lighter is giving me 13.5 to 13.7V like it always used to.
I have not yet had time to look into the FSM to see exactly what this wire is, as I just figured it out yesterday afternoon. I presume its a ground for the solenoid??
Anyway - problem solved and the car is back to driving like it should.
The yellow wire is the 50 circuit from the "start" at the key. For automatic cars there's an interposing safety relay. It has nothing to do with charging, and so long as there is adequate power passing through the connection to draw in the starter solenoid, it has nothing to do with cranking speed.
But....
Sitting next to the yellow wire is the post that connects the starter to the wire from the battery, and the wire from the alternator. During your work on the yellow 50 wire, you disturbed the bigger wires there to restore the connections. Recommendation: If you haven't done so already, disassemble, clean and restore that big wire connection at the starter. Begin by lifting the battery ground strap. Then remove the outer nut and washer on the solenoid post where the wires come together. Use a wire brush to clean the post itself and the nut underneath until they are bright and shiny. Do the same to each of the connecting lugs on the wires, and the washer and outer nut. The copper is now bright and clean. Reassemble and carefully re-torque the nut; It's a copper nut on a copper stud so overdoing it risks stretching and breaking the stud. Renew the cable ties that keep the wires from moving around, so that vibration/movement doesn't loosen the nut. Once the connection is snug and secure, treat the whole connection and the 50 connection next to it to a coating of Vaseline or some battery terminal protector spray, so water and road salts don't undo your work.
For those playing along at home, this task is part of the car's annual electrical maintenance protocol, along with cleaning the battery terminals, the alternator terminals, the jump post connections and the positive connections to the positive bus bar in the CE panel where the three feeders from the jump post terminal connect. For all but the connections at the starter, use Vaseline to protect the work. Only the starter terminal is exposed to road crud, where the waxy battery terminal protectant spray may last longer.
Dr. Bob, At the risk of arguing with someone that knows a heck of lot more about these than me; I assure you this was the only wire I touched. However, to clarify as stated in my first post - it was not the yellow wire; it was the yellow/black wire shown in the photo.
The black & yellow wire goes to the ignition system for boosting power to the ignition coil while cranking. It will mean you will start easier with it connected properly - but it has no function when running (starter disengaged). It was not a cause of the strange voltage readings which still make no sense to me?
Did you change the DVM? - something else was going on - it wasn't this.
This explained your slow starting - but really not much else...
Well, its a strange phenomenon. Other than installing a new AC compressor and drier, I didn't make any other repairs. I've been driving her the past 2 days and everything is completely normal. Same voltmeter. And the voltmeter at the cluster (which had previously been hovering between 10 and 12 and was consistent with what was measured at the cigarette lighter) is now back into the 13V range like it always used to be.
I'm not doubting your information or diagnosis - and I suppose I must've touched SOMETHING, but have no clue what it could have been.
When the weather cools off I'll loosen that wire again and see what the effects are. If the voltage dips I'll update this thread with a video to prove I'm not crazy. If not, it will remain one of the 928 mysteries.
Hate to say it...but it looks like it's about time for some motor mounts. Oil pan looks very clean, so you may want to install them without removing crossmember. Did my 84 that way, it is somewhat of a struggle...but it can be done (obviously). And boy...did it make a difference in the way the car felt....much, much smoother on acceleration.
Glad you got your electrical issue straighten out.
Working on that wire almost undoubtedly had you bump into the other cables feeding the starter solenoid, since they are so nicely bundled together just a few inches from the lugs. So yourself a favor now, even while it's hot out, and at least slide under there with a 13mm box-end wrench and carefully snug up the nut on the lug where the positive cables gang up. You can probably do it OK without disconnecting the battery if you don't touch your wrench to anything else, but since you have the 13mm out anyway use it to lift the ground strap. Or use the wingnut if your car is still using that. I can almost guarantee you that you cured the symptom by that tiny bit of jiggling of the cable bundle to the starter.
Brian - please don't tell me I need new mounts. Those are OEM Porsche mounts I installed only a year ago to the tune of about $300 each and a pain-in-the-butt weekend of dropping that front cross member, as I recall! Maybe its just the angle of the photo. I'll take a closer look this weekend.
Thanks guys. As always this place is a treasure trove of knowledge.
06-06-2017, 10:25 AM
SOLVED** Strange electrical problem - Alan or electrical gurus, please chime in
