Voltage Drops at High RPM ...... Solved

[seanwmurph]

Hello, my 1987 T3 is giving me a real headscratcher and I haven't yet found information in other posts discussing this same issue. The symptoms are that when accelerating (typically shifting from 1st to 2nd, but seems to be consistent at RPM's greater than ~3000), the dash lights turn off, radio restarts, and the coolant temp indicator light flashes. Typically, the engine continues run nicely but sometimes will studder.

I've recreated the issue while parked (cool engine), and with a multimeter on the battery I see the voltage drop from about 13.5 to 7V at the high RPM. It idles great, there's a nearly brand new ECU installed, new distributor cap/rotor and spark plugs.

I went through and cleaned all the ground connections at the battery, starter motor, transmission, and the ground connections behind the fuse box. I'm not sure of my next troubleshooting steps so any advice is very much appreciated!

[Mikesarge]

How tight is your alt belt?

Add a jumper cable between the engine and the body and rev again.
_________________
1986 Syncro westfauxlia EJ-frankenmotor
1980 911 SC 3.1

[Steve M.]

When was the last time you cleaned the mounting bolt at the bottom of the Alternator? It should also ground through the belt adjusting brace as well.
Adding a ground wire as been said above is a great idea. Take from the Ground below the Ignition Coil to one of the alternator housing bolts.

How clean is the wire from Alt. to the Starter at the starter?

Do you carry a spare voltage regulator for the Alternator? Check the brushes on the Voltage regulator that is on the Alt. now.
_________________
This free advice is worth exactly what you paid for it.

There are seven days in a week. Someday is not one of them.

[Wildthings]

If you battery voltage dropped to 7v it wouldn't have enough charge to start the engine and the FI might not have enough voltage to function.

I find actually having 7v at the battery unlikely. If such was caused by a dead short something would blow up or burn up for sure.

[DanHoug]

where are you measuring voltage when you see a drop to 7v?
_________________
-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[Ronzo_volvo_guy]

Sean;

Dan beat me to THE question we'd like to see answered: WHERE are you measuring this V? A virtually instant drop, to a Buss V of 7V is impossible as there is a honkin' lead acid Batt, capable of delivering mega power, which is looking to keep the Buss at a steady V....so if V is indeed dropping a huge amount (wherever you are measuring it), that suggest a poor connection between that measuring point and the Batt ...so that tells you a lot already !...and the fact that the issue seems to be RPM dependent suggest a vibration factor affects it...also consistent with a loose or poor connection!...you might want to check your engine to frame hop-over chassis cables both Neg and Pos first!)...now it's just up to you to find and correct this poor connection location...remember: Shiny metal clean, and tight, and long-term protected with ACZP!

Good Hunting!

Ref: https://www.sw-em.com/anti_corrosive_paste.htm

[seanwmurph]

Thank you all for the advice! I'll be back at it today trying these suggestions.

To clarify, I was measuring voltage directly at the battery when it would drop and there's only two causes I can think of for this:
1. Measurement error, cheap multimeter and probes being wrong..
2. There's a sudden and heavy current draw from the battery (like a dead short, or perhaps activating the starter solenoid) causing a drop over the batteries internal resistance.

So, if I don't have measurement error than I think I'm looking for a possible intermittent short of 12V+ line to the frame. I will probably go in the following order and retesting along the way:
1. tighten the alternator belt - fingers crossed it's that simple!
2. Run a ground from the alternator mount bolt to below the ignition coil - thoughts on a gauge? My first inclination is just 10 or 12AWG.
3. Check and clean the wire from the alternator to the starter.

I don't have a spare voltage regulator on hand, but I can at least inspect it for obvious damage? And it looks like some local parts stores have them I can use for troubleshooting at least.

If those don't work, I'll try:
4. run a ground jumper from the engine to the battery post directly - if this fixes it, I will investigate the chassis grounds closer.
5. in addition to the ground jumper, run a positive jumper from the battery to the starter solenoid - if this fixes it, I'll replace the positive battery cable.

Thank you Ronzo_volvo_guy for that great reference on corrosion at contacts. I've been scrubbing with a wire brush and putting some off-brand dielectric grease on the connections..

[crazyvwvanman]

I'd be more thinking about replacing the entire alternator at this point.

You say it has a "nearly brand new ECU"
When and why did it get that?

How long has the problem been going on?

Mark

[Ronzo_volvo_guy]

Sean;

If you were measuring right at the Batt posts, and saw an instantaneous dip from 12.X to 7V, that suggests either an internal Bat issue (or very low State of Charge!) or as you say a massive short causing a big internal drop...please let us know what you find!

Wire gauge doesn't have to be a guessing game...just take the maximum expected normal current, for instance a 60A Alt, and look up the appropriate gauge for that in the tables (Search: wire gauge vs current)...it looks like they specify an 8ga for a 10 ft run, so a 10ga should be fine for a shorter run out in the open (able to shed heat), and not passing maximum I continuously.

Thanks also for the kind words to the ACZP suggestion over dielectric grease...I've been using and highly recommending and sending it out with my kits now for decades without exaggeration...and ain't had a bad connection on a so treated connection or bad feedback about it yet!...and BTW..."scrubbing" the contacts until they shine is and will always be a good first step!

Cheers

[dgbeatty]

I cannot imagine what would put a 500 amp load on the battery causing it to drop to 7vdc without causing a fire. A starter will not unless locked up. Have you load tested the battery? Modern battery testers do not load test.
Were you testing the voltage directly at the battery posts? More likely you were testing at the battery terminals.
From a far distance and without being able to directly observe I am swagging that you have two problems.
1 an alternator regulator with a brush problem such as hanging or worn out.
2 wiring problem in the battery area or possibly an internal connection in the battery affected by vibration.

Please keep us up to date on your progress.
_________________
Schau in das Buch

[Jake de Villiers]

Loose cable terminal?
_________________
'84 Vanagon GL 1.9 WBX
'86 Westy Weekender Poptop/2.5 Subaru/5 Speed Posi/Audi Front Brakes/16 x 7.5 Mercedes Wheels - answers to 'Dixie'
@jakedevilliersmusic1
http://sites.google.com/site/subyjake/mydixiedarlin%27
www.crescentbeachguitar.com
www.thebassspa.com

[seanwmurph]

Welp, I should have looked a little harder before posting to save myself some embarrassment... It's a miracle this issue was occuring intermittently without starting a fire. Turns out the positive cable from the alternator to the starter solenoid post (looked like an aftermarket parallel cable installed some 10+yrs ago...) was hanging lose and occasionally arcing to the frame 😬

Thank you all for the help! It was very much appreciated!

[djkeev]

Glad you found it!

That is a known weak point.
Be aware, on my last Vanagon on the replacement cable I purchased they used a cheap copper crimp on connector which engine vibration broke off at the alternator. This left a loose 12v wire and killed the engine.

This time I have a hydraulic crimper, I purchased top quality wire and top quality terminals from McMaster-Carr. I'm not repeating that broken connector experience again!
I made my own, much better than what I purchased last time.

Dave
_________________
Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[Steve M.]

One of the electrical mod's is replacing the stock cable with a bigger one to carry the load. Hence the reason somebody added an extra cable ti help carry the load.
_________________
This free advice is worth exactly what you paid for it.

There are seven days in a week. Someday is not one of them.

[Wildthings]

[djkeev]

[DuncanS]

Also glad you found it Along with the fuel lines the alt to bat wire would be oe of the first things replaced.

This from an earlier topic about the same issue.