What just happened? M25

[Kenneth K]

Went to start the diesel today, just to charge the batteries. Started up just fine, but I noticed right away that I had no RPM indication. That was unusual. Then when I went to check battery charging--voltage read 12 volts. No output from the alternator. I scratched my head, knowing both problems were alternator related.

I let the engine run at low speed for about 15 minutes just to warm it up. Then I ran it up quite a bit. Once I hit a high RPM the tachometer came back online. Went below and checked battery voltage and saw that the batteries were now charging normally.

So, in the end all is good. I'm just wondering what happened electrically / mechanically.

 

[Tom Metzger]

Loose connections. Loose belt. You choose.

 

[supersailor]

Sometimes the field doesn't form until the rpm builds slightly. Nothing wrong. Just slightly lazy. RPM's below 500 rpm are the most likely culprit. Once formed, the field will stay intact unless something is wrong with the alternator.

 

[redbeard1]

Hi,

We had the same thing happen on our e-34 last fall. We could not get the tach to work without a burst of throttle. I finally traced our problem to lots of spider webs in the socket that houses the "alternator charging bulb" in our console. This bulb is part of the circuit that excites the alternator (most alternators need a little current before they can produce electricity). Once I cleared the cobwebs, the tach began to work normally.

Depending on the source of extra resistance in your circuit (spiderwebs, salt corrosion, etc), you'll probably be fine for a while, but I would find the source of extra resistance and clean it.

p.s. most of these set-ups have a back-up parallel excite circuit in case your bulb burns out. If the back up doesn't work, then there is a problem with that circuit as well. Our back up circuit was also broken.

 

[Kenneth K]

Redbeard, can you elaborate on "alternator charging bulb." Don't know what that is. If you're referring to a warning light for the alternator, none is installed on my '85 M25.

Yes, I know loose connections and bad grounds are common for this type of problem, but all the connections on the engine side are good, having been re-done last winter. I do still have to tackle the rats-nest behind the engine control panel (what were they thinking to wire it like that????).

 

[Christian Williams]

rats-nest behind the engine control panel

You may only have to tighten some nuts or replace rusted in-line fuse.

 

[redbeard1]

Hi,

Yes I meant the warning light for the alternator.

I've attached an image our our engine/console wiring diagram that highlights the primary exciter circuit. The circuit is designed with a certain amount of resistance. If the actual resistance increases due to corrosion, loose fit, spider-webs, the current delivered to your alternator drops and your alternator will not excite normally.

I've also attached an image of what I hope is your wiring diagram. As you can see, it does not have the same set-up but I would focus on the purple wire. That wire seems to be the return part of the excite circuit. I would check that wire for any source of corrosion or loose fittings. At a quick glance, I'm not sure how the current gets to your ignition but I would follow the red wire #5 to the alt gauge and from there some wire must connect to your ignition. Maybe someone who has your engine can comment.

Finally, this website was a great source in learning how these things work and how to debug.

http://www.ratwell.com/technical/ChargingSystem.html

 

[Kenneth K]

You may only have to tighten some nuts or replace rusted in-line fuse.

Tighten some nuts for sure. I still have the old ammeter in place (disconnected), and I know I need to replace the switches. Over the long term, I'd much rather re-rig to have a hot bus bar, a switched bus bar, and a ground bus bar. That would make it much easier to find and fix problems, and make future changes. As it is now, I have as many as 4 terminals screwed on to the back of some instruments.

 

[Kenneth K]

I've attached an image our our engine/console wiring diagram that highlights the primary exciter circuit.

Great info. I really appreciate it. This is why I always start my projects on this webiste!

 

[kloebereng]

Went to start the diesel today, just to charge the batteries. Started up just fine, but I noticed right away that I had no RPM indication. That was unusual. Then when I went to check battery charging--voltage read 12 volts. No output from the alternator. I scratched my head, knowing both problems were alternator related.

I let the engine run at low speed for about 15 minutes just to warm it up. Then I ran it up quite a bit. Once I hit a high RPM the tachometer came back online. Went below and checked battery voltage and saw that the batteries were now charging normally.

So, in the end all is good. I'm just wondering what happened electrically / mechanically.

Ken, if the battery voltage is up, the alt might not excite, so no AC output signal to the tach.

Do you charge thru the harness to the panel (Universal wiring diagram)?

There's two huge issues I have corrected over the years with C30 owners who have Universals - both are fire hazards and have caused fires:
1) Charging thru the panel harness
2) no over current protection on the main power feed to the panel.

Ken K