low speed engine miss

[ez73]

I have a 73 240 with 280zx distributer &coil,also has 280zx type 65amp alternator. About 3 weeks ago started missing at slow speeds. I thought it was fuel related ,air gettin in before the pumpas this has happened a couple times in the past. Ichecked out the hose clamps on my mazda elec fuel pump. Ithen changed out all the short fuel hoses and both fuel filters. put on my spare rx3 fuel pump still had the miss. Then proceded to the engine pulled all the plugs checked the plug wires and coil wire all looked fine. I just about gave up and was headed to my z guru buddies place and noticed the starter dragged when starting up. I didnt think much about it even though I had been on vacation for two weeks and hadn't driven the car much since 5/27 car show. Imade a stop at the bank and the car wouldn't start. Got a jump and noticed my amp guage wasn't even moving. Put my charger on the battery (10 amp ,5hrs then 2 amp overnight) drove the car and everythin g was fine. Alt. putting out 14.4 volts with fully charged battery, battery tested fine with batt. check guage (under load) This has me stumped ,alternator supposed to put out enough with a discharged battery to also run the engine,but it doesn't. All the equipment on the car appears to be fine, but it appears that the engine needs a "battery field" from a charged battery to run at lower speeds without a miss. At high speeds the alt. puts out enough to run everything. anybody got any thoughts on this. richard " ---" funk

[sblake01]

Sounds to me like the internal regulator is malfunctioning in such a way that it only puts out proper voltage at higher rpm. So you're not getting enough voltage at lower rpm to properly run the car or keep the battery charged. Pull the alternator and have it tested.

[geezer]

I agree with Stephen, Get the alternator tested. I've seen open or shorted diodes cause this before also. You can test the full output of the alternator, by bypassing the internal regulator. To do this, ground the field terminal and with an ammeter take a quick reading. If you get a reading of 50 amps or better, the internal regulator is at fault. Don't full field it any longer than it takes to get a reading. I am assuming that the battery is indeed OK.

[geezer]

After reading my post I realized I didn't mention that engine RPM will have a direct relation with the alternator output. I would guess that 3500 or so RPM's would be needed to generate 50 Amps.

You could also do a simple night test to evaluate your alternator. With the engine at idle and the headlights on, watch for a change in the brightness of your headlights after turning the engine off. If there is no immediate change and no detectable difference after a minute has passed, all is well. If the headlights dim, chances are the alternator is OK and I would look at battery condition or power bleedoff. If the headlights become brighter the alternator is at fault. Make sure all other accessories are off.

[ez73]

thanks for the replys I suspect I might have an electrical drain somewhere that caused the battery to drain and possible low rpm output problems with the alternator. In checking for drain does the amp meter go in series with the positive or negative terminal on the battery?

[geezer]

You didn't provide any details about any add ons installed on your car. Do you have an alarm, does the clock work, memory equipped radio or other accessories? These all contribute to battery drain. They shouldn't draw enough to cause the problem you are experiencing though unless defective. It is important to follow the correct sequence of testing proccedures to get to the root of the problem. You said you have ruled out the battery. The next thing to check is the starting system. You might think...what does that have to do with anything? Fact is, there is a possibility of current drain in this or any circuit. We used to have cases where the ignition system was the problem. Points were often the cause. After checking the charging system you can then connect your meter and pull individual fuses to locate the culprit. I will post a few trouble shooting pages for you to follow. Hope it helps. Check this out. Could be a handy item to keep in the glove box.

http://www.jaycar.com.au/productView.asp?ID=QP2251&CATID=24&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=619

[geezer]

I'll try later. The Manage Attachement feature is not there right now. Maybe Mikes working on it. Shout is gone too.