New Theory on Electrical Problem
 

I was talking to my friend who works for Bosch last night.

I told him that when I start up my car, the voltage reads 13.6 volts, but as I drive, it can drop as low as 12.5. What accessories I'm using does not seem to affect the voltage, only the amount of time the engine is running.

His theory? I have a bad diode (or more) in the alternator that works fine as long as the engine is cool, but as the engine comes up to temperature, it starts to fail.

Any thoughts?

What are you using to check your voltage? Dash gauge or external gauge?

External gauge.

Would Check/Clean your Batt Terminals and then move to the Voltage Regulator on Alternator. Found a good post on it. Has link to Rennlist post with pictures.
http://forums.pelicanparts.com/showthread.php?s=&threadid=249560&highlight=voltag e+regulator

If it's not the Regulator, Alternator might be on it's way out. Usually goes on the night trip when your headlights are on and the batt drops so low the Engine can't fire.

When wire get hotter the resistance goes up. As resistance goes up more electricity is converted to heat. Electricity making heat, makes less voltage. Thus why computers and such, hate heat.

The battery is brand new, so that is not an issue.

Getting to the alternator on this car is a royal PITA, being a 951. If the problem is either the voltage regulator or the alternator, I'm changing both so I don't have to go back in there for a while...

i am going to say that your friend is right about the heat and the electrical parts.

Maybe not an issue, BUT, are all the cables, terminals, and all main engine ground points good as new also? Make sure. Always the first place to start.

At this point, the only other thing it could be is the ground strap.

Well, it would'nt hurt to take a look see. A lot cheaper and easier than an altenator replacement. I'm just throwing out the idea of looking at all the options, and going after the easiest and cheapest first. Then, if that isn't it, then go deeper, and to the more expensive and labor intensive. Ya never know. Sometimes its the dumbest and easiest things. Eliminate all those first.


In your sig. Your RIGHT. there is no such thing as "No such thing as a free Porsche".

I got one. I found out too.

I can't see where the ground strap leads to on this car, but I'm guessing that it runs to the alternator. If that's the case, then changing the ground strap isn't much easier than changing the alternator.

(I did read Clark's Garage's instructions on changing the alternator in a 951. It's a major ordeal.)

I'm a thinking this is for your '86. I'm a thinking that you may not have to replace the ground, just clean the terminals. I'm also a thinking that the ground strap on your car goes from battery thru the firewall, and down to the back of block to the middle of block, mounted on the bellhousing. I would still think, if this is the case, its worth checking those terminals, and connections. Clean, dielectric grease, and re-tighten. Corrosion on the battery end of it, can and will KILL ya, and it can travel down that cable a ways. Poor conductivity at the bellhousing won't help either.

i do believe it's a stud on the back of the engine.

87951SmileWavy

Either way, I can't where it goes from the top side.

I have replaced the terminal clamps on both the positive and negative sides...