Hi all:

I am having the following problem:

When idling at 750 rpm,
if I do any of the following:

1) push on brakes, or

2) put on vent (heater), or

3) put on the lights.

the rpm drops significantly (sometimes stalls the car) then it goes back to 750 with a little bit of bouncing.

I don't have AC nor power steering.

I don't seem to have any vacuum leaks, and my idle is rock solid otherwise.

I know it has nothing to do with BAC valve,
since the BAC valve is not involved with 1) 2) 3)
(at least to my knowledge).

Note that I can hear a light squealing from the alt, which tells me suspect alt bearing.

I suspect my alternator is on its way out
although its voltage output is ok and seems to be working fine otherwise.

Can bad bearing on alt cause the rpm to drop significantly when electrical accessories are turned on ?

Can somebody confirm my hypothesis or point me to some other direction ??

Any help is well appreciated,
thanks.

 

Have you set the TPS? It did the trick on my vert. But, since your idle is solid, this may not have any bearing on your problem.

 

TPS is not the problem. You need to either: Add more grounds to the battery and electrical system, or get a FD alternator. The stock electrical system blows. Run a 4g wire from the positive terminal of the battery to the nut on the top of the alternator (under the boot), then another 4g wire from the negative terminal to the chassis. This should fix your problems.

 

yeah your stock alt is in the early stages of dieing.

Re-enforce your grounds that may put if off for a while but eventially you will need to replace the battery and alt.

 

Get a used FD alternator. They can be had for like $100. Hey Mark, you know what the amp rating on a FD alternator is? I am considering going electric water pump, so I will need to upgrade my alternator.

 

i think there 110A or 100A

 

i was thinking of getting an FD alternator myself i was just curious if they are just bolt in or does anything need to be modded?

 

Direct bolt in, from all I have read.

 

True, you guys are more than likely correct about the alternator dying and the bad grounds. My car would stall at red lights with the AC on, or when I turned on the AC the revs would drop dramatically. Also the idle was rough and bouncy. I set the TPS properly and it has been fine since. Perhaps a coincidence.

Cheers!

 

Two different things, the AC on and the brakes or headlamps on.

And the FD alt is 100A

 

I had the same problem a couple weeks ago. Got in my car to drive 30 miles. Step on the brakes and the charging needle dropped from 15v down to 9-10v and went back up. Needle would bounce with the turn signals. When I got where I was going and shut the car off, it would not start again. New battery and it seems fine, but I suspect the alt./reg. is failing. Do the ground upgrades and keep an eye on the alt. or get it tested in the car. From what I understand, if the alt/reg fails, it can sometimes take out the ecu.

 

While it may be another thing, the bac does react to the headlights or the fan being turned on. I've observed it by monitoring the bac when doing so. It compensates for the load put on the altenator when the lights are turned on.

 

Thank you all for your replies.
I am getting a new alt today, so hopefully this will solve the problem.

To HAILERS:
I have looked into FSM (page 4A-62 of FSM 88) for the BAC valve, and I haven't seen anything there about BAC being involved with lights being turned on. Note that I have an 86 NA std, so maybe the 86 setup is diff. from 88. I was suspecting the BAC for a while for the pb described in original post but ruled it out after looking up the FSM. You're saying the BAC is active when the lights are put on, that's quite interesting. Do you have more info on this ?

In all cases,
I'll let you know if the alt replacement solved my problem. If not, I will have to go deeper (I hope not) into the BAC valve.

Again,
thank you all

Hugues -

 

Here is an update from thread originator:

Replaced my alt. Same exact problem.

I am focusing on the 'applying the brakes' problem since it is the most problematic (note that it is probably all related)

I start the car, let it idle. Everything is fine. Idle rock solid at 750.

I step on the brakes. The rpm drops to almost stalling the car and goes back up to around 1000 and comes back to 750 (with a little bit of dampened bouncing)

I've had this pb for quite some time so I don't remember the correct behavior in terms of RPM when you apply the brakes. I don't think the above behavior is normal though

I don't think the BAC is involved with pressing on the brakes but I could be wrong.

I have checked my grounds and they seem fine.

I am gonna check the TPS next to make sure and check for vacuum leak at the brake booster, but after that, I dunno what to look at.

thanks for your help,
Hugues -