I have a 87 S4 TII, and I am wondering if I have a charging issue, or if I need to upgrade the alt. I want to get the idle to a constant 800 rpm

Basically I idle at 1070 rpm, when it is idling at this, the voltage is 13.5V.
Sometimes after starting the car, the idle drops to 800, it still idles, sometimes it wants to go on one rotor and die. When this happens the voltage is 12.6-12.4. This is why I am thinking the alternator may not be charging correctly.

If I upgrade the alt, I then have a few questions.

• I know FD and Taurus alts are very popular and easy to install
  • I would like to do the Taurus alt, but does it fit with the stock TMIC, would I be able to shave the Taurus housing a bit to make it fit?
  • Would the FD one even charge at idle, or would I have the same issue.

Most of the grounds are clean, I will be going around the car again in the next week to determine if any are bad.

 

These sound like two separate issues. And while the stock alternator is barely adequate, upgrading it won't help your idle issue.

The best thing to do is to start by checking free stuff. Vacuum leaks, TPS, grounds, etc.

If you can fix the idle issue and get a consistent idle every time, THEN measuring the voltage will tell you whether or not you have a charging issue.

 

Fwiw, i think it’s worth doin the upgrade to a fd alternator. My car was s4 n/a and all the voltage drops even at night with lights on was ridiculous. Now I’m running a fd alternator, s5 t2 engine, Taurus efan no problem.

as mentioned above, it might not be your problem, but can you test for it? Have your car hooked up to another running car via jumper cables and see if higher voltage improves idle quality…? I’d have to think about that for a little.

 

Did a smoke test and had no vac leaks. Tested the battery it's good, checked and cleaned most grounds. One thing to note, I can't have the headlights on or it won't charge enough for a good idle.

Does anyone know if the Taurus alt fits the oem tmic, I see some say no, and others say barely

 

The ford contour 130 amp(95-97 inline 4 one) alternator does fit the stock intercooler but barely ,...I had to shim up the right side(upper manifold mounts) of the intercooler with a very thick washer to clear the the top edge of the alternator casting..grinding the corner of the alternator casting a bit would have worked without the thick washer.