Voltage Regulator...intermittant failure? Might need help in Mid-South PA/North MD

VaSteve · 03-23-2010, 02:31 PM

I decided to drive my 911 for work to Gettysburg for a week of training. Car ran great all the way, but an old issue re-emerged as I was parking it. The tach started bouncing like mad around 3000 rpm. I had seen this issue before in December and attributed it to a loose green wire. All that has been rectified and I have put several hundred miles on the car in the last week or so (including this trip) and everything was great. In fact, the car was at the mechinic last week who was setting the CIS back to proper mix.

The issue is intermittant. I drove to Hanover a few miles away and it started immediately. After shelling out the money for a multimeter (my 3rd!, other 2 are at home) I couldn't reproduce the problem on the ride back. Based on everything else being corrected, I'm suspecting the voltage regulator.

What kills voltage regulators? What would cause this to be intermittant? Alternator is 2 years old (VR is attached). I'm at a loss at how to approach this. I live 2 hours away in VA and will head back Friday. I'm heistent to drive without know if this is in fact the issue. I'm thinking of getting the part overnighted here, but don't have the tools to fix in the hotel parking lot.

Any thoughts, suggestions? Could it be something else? Anyone nearby if I source the parts?

lr172 · 06:45 PM

not sure if your car is CIS Or EFI. If it is EFI, the first thing I would expect would be the connection between the tach and the Motronic box, second would be the tach itself and third would be the motronic. If you have CIS, check the connection at the coil as well as the tach. My second guess here would be the tach itself.

Next time this happens, turn on your headlights. a voltage swing wild enough to make your tach go haywire should be having an impact on many other electronic components. You lights will get brighter as your voltage goes up and dimmer as they go down. While this is happening, turn on your lights and look for changing intensity.

This doesn't sound like a VR problem to me. The tach has a circuit board in it and it deciphers signals. These are usually designed to handle a fairly wide voltage range. I would expect high or low voltage to cause a mis-calibrated reading and not oscillating needles.

Larry

James Brown · James Brown's Garage

Agree Ir172, more like a intermittent open in the tach. circuit.

VaSteve · 03-23-2010, 07:01 PM

I did a bit of reading on here and all signs seem to point to VR failure, but I'm open to other ideas. When it was crapping out on the way to the auto parts store, I was getting what seemed like a rise in the actual idle. Wouldn't an open in the tach circuit, cause a drop? BTDT

lr172 · 09:31 PM

Quote:

Originally Posted by VaSteve

I did a bit of reading on here and all signs seem to point to VR failure, but I'm open to other ideas. When it was crapping out on the way to the auto parts store, I was getting what seemed like a rise in the actual idle. Wouldn't an open in the tach circuit, cause a drop? BTDT

No. The only effect of an open in the tach circuit would be problems with the tach. On the other hand, a failing coil or connections to the coil would cause both problems with the running of your engine AND a flakey tach signal. I assume your battery is charged, as you indicated this occured only briefly. Have you verified the state of your battery? If it is 12 volts, the VR is not the cause of your poor running engine. Ignition systems run just fine on 12 volts.

VaSteve · 03-24-2010, 03:04 AM

Quote:

Originally Posted by lr172

No. The only effect of an open in the tach circuit would be problems with the tach. On the other hand, a failing coil or connections to the coil would cause both problems with the running of your engine AND a flakey tach signal. I assume your battery is charged, as you indicated this occured only briefly. Have you verified the state of your battery? If it is 12 volts, the VR is not the cause of your poor running engine. Ignition systems run just fine on 12 volts.

Yes, I tested the voltage on the way back from the auto parts store. 12+ (don't recall the exact figure) volts with just the key and 13.65 with the engine running. The car ran fine on the way back. No symptoms (bouncing tach, changes in idle) or anything. You mention 12 volts (I assume you're speaking of a drop). What figure is too high? 14.2+?

Good thoughts on the coil. All the research I have done (here on Pelican) never mentions the change in engine speed (only observed at idle) only the jumping/bouncing tach.

Like I said, it's intermittant. No variable is the same...it was freezing cold in December when first observed and I had driven a god ways. Drove around all day Saturday in 70 degree weather with no issues. Observed problem again after 2 hours of dead nuts reliable driving on this past Sunday only when parking it...this was high 60's weather. Observed yesterday after about 5 min of driving...it was cold (50's) out.

At least I have the multimeter to test it again should it pop up. I'd like to just throw parts at this

, but which parts?

lr172 · 03-24-2010, 06:04 AM

You mentioned that you thought the VR was bad. When the VR goes bad, the engine run's off battery voltage, which is ~12 volts. I was making a point that engines running without alternator support show no difference in performance. This can change as a battery discharges significantly and voltage falls.

I would start by cleaning up all connections at the coil and from the coil to the tach.

Larry

VaSteve · 03-24-2010, 06:14 AM

I will look it over at lunch but I'm not hopeful. I've cleaned all this pretty good recently and thought I had it fixed.

What about intrmittant VR failures that show greater than 12 v per other folks experience. I don't understand the disconnect there...?

VaSteve · 06-05-2010, 12:18 PM

Update on this....I was been driving around with the multimeter in the car and have not had issues until today. It's about 1000 degrees outside and I drove an hour to the track to watch some friends drive. As soon as I pull up, I'm getting the bouncing and the slight fluctuation in idle (not consistent with what the tach is saying). I tested the voltage: 13.6 and no change at all with actual rise in engine speed (me stepping on the gas).

Shut it off, had lunch and no repeat performance on the ride home.

Larry, you still reading? Coil? (I'm going to go over the connections real good when it cools off a bit.

VaSteve · 06-05-2010, 01:40 PM

Connections all looked good, but I had another rotor meltdown. Even though I did the "MSD modification" it blew out the plastic/bakelite. I think this coil is suspect and is putting out too much juice and ruining my rotors amongst setting off the tach.

Canada Kev · 06-05-2010, 10:19 PM

When my VR went, the symptoms were interior instrument lighting not being a constant brightness. I checked the voltage at the battery when not on, idle speed and at about 2000 rpm. All readings were within normal expected range: about 12v when off 14v when running.

I was still getting the variable lighting, though. I took a cigarette lighter plug end and clipped the bared ends to my meter probes, plugged it in and went for a drive. A couple blocks later, I was seeing readings of about 16v. Time to park the car and order a new regulator.

VaSteve · 08-22-2010, 03:11 PM

Still having this issue creep up periodically. I drove a couple of miles this evening and when I go home, I was having the same issue. The tach would bounce and the idle would raise slightly. When I pulled into the garage, I forgot to roll up the windows and so immediately turned the key to "on" with the motor off. The needle jumped around again and actually swept all the way to 7000. I let the car sit a bit and went out after dinner and was having the same issue while driving.

I have been driving around with a "battery tester" type Voltage meter in the cigarette lighter. I had 13.6v consistently.

I have a new voltage regulator on the workbench and should probably put it on, but what if that's not the issue? What else should I be looking for/at?

E Sully · E Sully's Garage

You say you did the MSD modification to your rotor and it still melted. I see no reason from your testing to suspect the voltage regulator. The MSD uses an adaptor for the tachometer, doesn't it? It sounds like you should be looking into your MSD and coil for a problem.
I have no experience with MSD ignition, so I can't help you with that.

VaSteve · 08-22-2010, 04:09 PM

Yes, I did the MSD mod and when the last one let go, I put a solid rotor in there. I popped off the cap and all looked OK (this time). Maybe it is the coil....could the coil running down its charge make the tach jump around when the engine was off? I can't image the VR doing this since the alternator is not turning...I was cranking the windows off the battery power.

James Brown · James Brown's Garage

So you have a MSD 6al type of ignition? do you have a a tach adapter on it? engine reves over 3k all the time? Classic MSD problems. SC's do not require the tach adapter. The tach. can read the MSD signal correctly. If you are having problems like rev limiting, try reversing the polairty from the green wire (dissy) to the MSD unit.
After all the time you drove around with the meter, I don't think it's the VR.

VaSteve · 08-23-2010, 04:20 AM

James. That is correct. MSD 6AL with no tach adapter. The tach indicates around 3K even if the engine is at idle. There is a *slight* bump in actual revs. Tach will bounce all over the place 2-4K (not scientific

) when driving. When it's not acting up, the car seems to drive fine. The fact that I got bizzaro readings when the engine was off (but the key was on) has me puzzled. Perhaps could the coil be discharging and giving off this nonsense?

When you say "classic MSD problems" what do you mean?

wwest · wwest's Garage

The CDI drives the tach, the OEM/factory CDI is HIGHLY subject to intermittent failures if the "battery" voltage is unusually high, the battery is REMOTE from the alternator & REGULATOR.

An improved design would dictate a separate voltage sensing wire that doesn't carry the battery charging or load current, from the battery to the regulator.

Regulators have a temperature sensor that results in higher charging level of the battery the lower the the outside, ambient, temperature becomes.

It would not be altogether unusual to have a properly charged battery but with voltage spikes, "long" duration spikes, as high as 18 volts all the way back there in the engine compartment.

VaSteve · 08-23-2010, 08:58 AM

I'm not sure I follow....

I understand what you've said but how do I translate that to my current situation?

James Brown · James Brown's Garage

Classic meaning the problem of weird tach bouncing is common. In some cases, you might have to get a tach. adapter (MSD, Berry Hreshon). The big heavy red wire is hooked up to a hot wire all the time (like the starter or the battery). And the small gage red wire goes to the ignition (switched). So even with the key off, the unit is receiving power and might be "leaking" some voltage that the tach is seeing. I don't think that is a problem but contact the MSD troubleshooting forum and ask the experts

wwest · wwest's Garage

Quote:

Originally Posted by VaSteve

James. That is correct. MSD 6AL with no tach adapter. The tach indicates around 3K even if the engine is at idle. There is a *slight* bump in actual revs. Tach will bounce all over the place 2-4K (not scientific

) when driving. When it's not acting up, the car seems to drive fine. The fact that I got bizzaro readings when the engine was off (but the key was on) has me puzzled. Perhaps could the coil be discharging and giving off this nonsense?

When you say "classic MSD problems" what do you mean?

"..bizzaro readings.."

The SCR within the OEM/factory CDI box is not rated to endure the 400 plus volts the DC-DC inverter will generate if the "local" supply voltage is above, say, 15 volts. Note: The battery might measure a perfect 12.35 volts and the voltage in the engine compartment measure >15 volts, or at least have long duration higher voltage spiking due to the time constant of the alternator rotor inductance.

Oftentimes turning the headlights on, highbeam, will result in things settling down due to resulting lower supply voltage.