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Question wierd idle and tach

about 2 months ago I took the car out and it started to run real rough and the tach went to 6000 rpm when the engine was barely above idle. I brought it home and when going over it at home I found that my battery ground was loose, tightened and no problems since then.
Until today , as I was warming the car it did the same rough running thing and the tach went to 5500-6000. I shut it off and checked the battery and it was all tight. I waited about 15 min. and re-started the car and it ran perfect, took it for about a 25 mile spin shut it off twice, no problems with the way it started and ran.
Any ideas why its doing this? I'm thinking maybe one of the ground straps, how many straps are there and where are they ? any other ideas? Its an 87 930
Old 03-20-2010, 06:27 PM
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the alternator is going bad.
Old 03-20-2010, 06:48 PM
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Most likely alternator as mentioned. You could take a minute and crawl under the car and inspect the ground strap at the transmission.
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Old 03-20-2010, 07:20 PM
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So, "going south" alternator can be a cause for jumping RPMs? Mines is pulsating 700 to 1500.
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Old 03-20-2010, 08:08 PM
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mine has just started doing that too. First it didnt move at all then as I drove longer it would try to move and go back flat. After more driving It started going up and responded to throttle by moving up but flickered up and down. It did this once before but it just went away and functioned perfectly for a few weeks before starting to do this again.
Anyone know how to check alt function before buying a new one....Take it to auto parts store and have them check? Can they check alternators on these type of cars?
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Old 03-21-2010, 08:24 AM
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Quote:
Originally Posted by IDGAF27 View Post
So, "going south" alternator can be a cause for jumping RPMs? Mines is pulsating 700 to 1500.
I think what we're talking about here is the tach moving eradically independent of the engine's true rpms. If your engine is truely fluctuating in rpm's, then it probably has nothing to do with the alternator. More likely vacuum/air leaks or fuel delivery fluctuations. Go through the complete tuneup sequence (check timing, check WUR control pressures) to first rule them out.

When the alternator goes, it starts producing an inconsistent voltage...and for voltage sensitive stuff like tachs and maybe digital WUR's, they will react really weird. That seems to be what most people see when alternators start to go. Never experienced it myself....yet.
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Old 03-21-2010, 10:36 AM
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my rpms "feel" constant. not fluctuating, just the tach is. However, I started her in the garage today and let her idle. Not tach movement at all again BUT boost gauge was fluctuating around .2 bar???? Never done that before. RPMs sounded normal . Not fast or anything that would induce boost.
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Old 03-21-2010, 06:47 PM
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IMHO, if your going to pull the alternator to check it out, you might as well replace it with it being such a PITA to do. Lord knows as soon as you reinstall it, it's going to go bad..,
Old 03-21-2010, 07:02 PM
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holy crap! $695 for an alternator!!!!!!!!!!!!!!!!
Is there a way to check my current one to be sure its failing before buying this thing. Wow!
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1984 ROW 930 turbo with rarly8 exhaust, waste gate pipe and HEADERS
K27HFS TURBO, B&B IC, 1 bar spring,LEASK WUR with rpm solenoid, fab speed intake filter, front strut brace rennline. MOMO wheel, Innovative afr 16-1 gauge, custom 930 ELGIN cam
Old 03-21-2010, 07:43 PM
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Quote:
Originally Posted by zaidigonzo View Post
holy crap! $695 for an alternator!!!!!!!!!!!!!!!!
Is there a way to check my current one to be sure its failing before buying this thing. Wow!
Sticker shock alert!!!! You know, these things have to come over on a slow boat from The Mother Land.

Still, an ingenious person should be able to figure out a way to take a run-of-the-mill $100 Delco and somehow retrofit it. When mine dies (hopefully I die first) you can bet that I'll attempt to fab one up in my Fred Flinstone garage.
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Old 03-21-2010, 07:48 PM
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You know you could have your current one overhauled for far less. IIRC last time I had one rebuilt was like 3 years ago and I went thru a local p car shop and paid $250 or so. I know I overpaid. Next time I will skip the middle man and go straight to the source.
Old 03-21-2010, 08:56 PM
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Quote:
Originally Posted by zaidigonzo View Post
my rpms "feel" constant. not fluctuating, just the tach is.
Check the voltage across the battery with the engine running, check for (and clean up) puking battery acid if you've been boiling it with 18 volts...

A bad voltage regulator can cause screwy readings on all the gauges - because they're basically voltmeters. Most people notice the tach first, for whatever reason. Maybe it is just more sensitive.

Some voltage regulators are internal to the alternator, some external.

Quote:
BUT boost gauge was fluctuating around .2 bar???? Never done that before. RPMs sounded normal . Not fast or anything that would induce boost.
Unlikely to get boost on a turbo without load, regardless of how high you rev it in your garage.
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Old 03-22-2010, 12:05 AM
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put a voltmeter on the battery terminals when idling and take a reading. actually it should be producing 13.4-14.2 volts at about 1500rpm to charge the battery.
I had one go bad and I removed it and had it rebuilt at an alternator rebuild facility for alot less than cost of a new unit. P
Plus you know it will fit inside the fan housing when returned to you..... they have slightly different alternators over the years that changed dimensionally, so you have to be careful on which unit you order.
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Old 03-22-2010, 05:02 AM
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