Well I didn’t get to check the timing or vacuum lines this weekend but I did find what might be the cause of many of my electrical gremlins on my 78 Shark. I checked & rechecked and it looks like my charging system isn’t up to snuff. While the previous owner did replace the Alternator recently (showed me the work order) it appears that it isn’t putting out the way it should be. At idle I get about 9-10volts and under power the best I get is ~12volts. Assuming a standard 12volt charging system I should be getting around 13-14volts. So given the fact that I can increase alternator output at the higher RPMs I first check that the belt wasn't slipping - it wasn't. That looked fine so I suspect that the (new?) alternator or regulator is the root cause of my electrical woes.

Also I’m still fumbling through my CD manual but I did find a couple references to how a poor operating Alternator can affect engine performance (ala misses etc). Whatever the case I suspect I’ll be chasing my tail if I don’t address the charging system before I delve too deeply into the timing issues.

 

When you did you measurements did you use a seperate volt meter othen then the one on the dash? They are known to be sketchy sometimes.

 

I used my trusty DMM.

While I’m aware that the gauge meters don’t read correct – it is within about half volt of the true reading. BTW I actually measured several locations under the bonnet, in the dash and the battery itself.

 

good deal, I will keep my fingers crossed for you.

 

Bret, I had to set my idle to the high end of the spec to get decent charging at idle. That means ~850RPM.

 

You probably have a problem too because a charging system shouldn’t care what your idle speed is. I mean if idling you should still see at least 12-13Volts for a charged battery with a max of 14volts for a battery that needs charging. But you should only expect to see that after having to jump a dead battery or if you drained it significantly by running a lot of accessories (stereo, lights etc) with the engine off.

In my case to achieve that max output I'd of had to set my idle to something like 1500-2000 RPMs just to maintain 11-12volts. Sorry but that just ain’t right.

The more I think about it I had a similar problem with a Lucas alternator on my 78 MGB. In one case it was acting very similar to what I am experiencing now with my 78 Shark. Turned out to be the Alternator had in fact lost its regulation and would brown-out at a stop-light idling with all my accessories running. BTW regulation can (at lease with the Lucas units) go the other way and put out too much voltage. I had another Lucas alternator (replacement for the first) that would run high – peaking at about 17volts! Believe it or not I got tired of fiddling with Locus Alternators and switched to 90Amp Bosch unit. That was about 4 years ago and it’s been functioning flawlessly ever since.

I just hope this turns out to be that simple.

 

Sounds like what I was experiencing with my 86 before I replaced the alternator/regulator and the auxiliary cooling fan (which had a short in it). Good luck.

 

Bret: You should have the alternator tested/rebuilt. Yes, the output should be 13-14 volts dropping to 12 at idle (that's with the lights on at night; mine is 13.5 all the time during the day). The idle output is marginal on these alternators and Dave (Sharkshin) spent a considerable amount of time trying to deal with it.

Mine runs 13-14 dropping to 12-13 at idle (dash gauge -lights on at night, as mentioned). Occasionally, if the idle drops for whatever reason below 675, I will see 10-11 volts and the headlights will dim but otherwise nothing unusual happens. Turn the headlights off or just slightly raise the RPMs, and it jumps to 13-14.

 

Sorry, I guess I should have clarified. When I said the alternators output shouldn’t be affected by the idle speed, I should have expounded by ending with “assuming idle is set correctly to factory spec and/or above a certain fall off point”

Example: with my 78 MG’s idle set to the factory settings 850 RPMs (100+/-) the “bad” alternator wouldn’t charge. Likewise the other bad alternator would put out between 13-15volts at the factory idle speed and increase to well over 17volts above 2000 RPMs. I still think I was extremely lucky that car didn’t turn into a burning hulk on the side of the road. It was then I got religion about on-board fire extinguishers.

With the Bosch 90Amp alternator (Lucas’ was 65Amps) I installed on my MG – it will charge the battery at an idle with all the accessories running and my stereo cranked. So I hope you see why I suspect there is a problem if an Alternator does otherwise.

 

Dang! The Alternator is definitely bad. Currently, I’m looking through the PO’s records to get the contact information for the shop that installed the Alternator the last time. While I know that most warranties aren't transferable, but I kind'a figure it's worth a shot to see if the shop wants my business too, seeing how they are so familiar with the car. Who knows they might make a good will gesture.

 

Sorry it took me so long to get back to this.

Bret, You're right, if you don't get the voltage till 2K with a good belt then the alternator is toast. I find that generally, if an alternator has one bad diode, it will take 1200-1500RPM to get good voltage. What you describe sounds like two bad diodes.

What's unique about the 928 alternator is that with stock pulleys it only turns 2x crank speed. So the alternator must be designed to charge at a lower speed when idling than is typical in most cars. This actually isn't as easy as it sounds, from the perspective of the designer of the alternator. I found out to my dismay that the Delco CS130D that I tried to use is typically designed to run over 4x crank speed, and looking around at other random vehicles seems to support the notion that 3x or more is normal... Once I started paying attention to this factor, I found that the test jig being used by the alternator rebuilder was spinning the alternator far faster than it spins when installed in my car.

With my alternator there was a very steep rise in output until about 800RPM(crank) at which point it is able to keep up with the loads on the system. I set my idle to 850 for good measure.

 

Bret, if you are around town tomorrow ( tues 29th) and still having problems, and want to take a ride up to me lets take a look together, I'm going to be working on my car anyway... give me a call or pm.
Neil

 

I set my idle at 6200RPM, and OFTEN ... no charging issues .... but seriously, what about Voltage Regulator?

 

Btw Dave it's nice to see you posting again Brother

 

Bret,

I find this comment interesting... Several people recently have said more or less the same thing: regulated voltage is higher when the battery needs to be charged.

I'm sorry but I think this is wrong. A charged battery will need less current and will allow the system voltage (generated by the alternator) to be higher. A battery consuming a high charging current will cause the system voltage to fall. The alternator and regulator have no real idea of the battery health - all they know is how much current is demanded total by the cars systems. If this goes to charging the battery or powering fans or headlamps doesn't really matter.

Usually at decent rpms (2K+) the alternator can generate all the power needed to charge even a badly discharged battery and run other equipment so the voltage droop is quite minimal and the regulator can do a good job.

The voltage regulator is designed to maintain a more or less constant voltage around 13.5-14v on our cars. It isn't perfect and still varies depending on alternator output which is better at higher rpm - at low rpms (idle) the alternator (not the regulator) starts limiting its output due to the inability to generate sufficent total power - this equates to a voltage (and hence current) drop. If this drops below about ~12.4v no charging happens and due to non linear voltage to brightness headlamp bulbs dim appreciably as voltage drops by a few volts. As Bill says reducing your system loading - headlamps off, big audio off, blower fan off, rear screen heaters off etc can have a major effect on charging @ idle under adverse circumstances.

This is particularly true if your battery is already significantly discharged and needs a lot of charging current - and conversely doesn't have a lot of reserve charge left (so you really need to charge it).

Best bets - make sure alternator is healthy and also keep the battery healthy - especially if you are going to be somewhere icy & dark next time you need to start and idle in traffic....

Alan