Stumble when headlights on

[sixer]

Good morning all,

I've got a bit of a low voltage/ground problem. 1976 bus, 2.0L with stock FI.

Problem: When the headlights are on, the bus stumbles and hiccups. This happens from 2-5 minutes of driving, then goes away, and will not return until the next cold start. Colder ambient temperatures mean it does it for a longer period. Turning the headlights off during this period smooths the engine out and it runs just fine. Turn the lights back on, and the stumbling returns.

Relevant info: Just replaced the battery last night to ensure I had all the power I needed, recently cleaned and reinstalled trans ground strap, added a direct battery-to-fan shroud ground cable. Cleaned FI grounds, ground for double relay, any other grounds I could find in the engine bay.

Any further ideas? Thanks in advance for your help.

Edited for clarity that lights on/off make a difference.
_________________
1976 Transporter, Stock 2.0L
http://instagram.com/sixer.bus/

[busdaddy]

A volt meter of some sort you can see while driving would be the next logical step. First on the big red wire at the fuse box and if that checks out OK test the black wire.
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[sixer]

Thanks Bus Daddy. I set up my voltmeter and got this:

Testing setup: Voltmeter bridged from main + cable into fuse box (fuse #8 ) to ground.

Results:
Static voltage before start: 12.44V
Idle/running voltage (stays pretty consistent with increased RPM): 13.4
Headlights on: 12.7-13V, stumbling occurs.
High beams on: Engine dies

Pretty much any drop below 13V seems to make the engine stumble. Should I take my voltage measurements somewhere else? I looked for the big black wire but did not see such a beast.
_________________
1976 Transporter, Stock 2.0L
http://instagram.com/sixer.bus/

[Wildthings]

Repeat the test by reading the voltage on the #15 wire (fuse #11 or #12). I think you will find the ignition switch is on its last legs.

[sixer]

Same setup, this time on Fuse #12 (repeated with 11, same results).

Results:
Static voltage key off: 0V
Static voltage key on, not running: 12.5V
Idle/running voltage: 13.04-13.2V
Headlights on: 12.3V (@ beginning of test) to 12.7V (@ end of test)
High beams on: 12.4-12.5V

Of course, during this round of testing the problem resolved itself. By the end of this test I could use the headlights, high beams and all without stumbling. I will repeat this afternoon/tonight when I know everything is cold. Or as cold as it gets here (71° ambient temp right now).
_________________
1976 Transporter, Stock 2.0L
http://instagram.com/sixer.bus/

[Amskeptic]

[sixer]

Thanks for the input Colin. Just because I can I ran the test again, this time taking my readings directly at the battery.

Results:
Static voltage: 12.57V
Idle/running voltage: 13.28V rising through test
Headlights on: 13.00V @ beginning of test, rising through test
High beams on: 13.00V @ beginning of test, rising through test

As of right now, the problem is not showing any symptoms. It idles and revs fine with the high beams on, and everything else I can turn on as well. I will run the last two tests again in the morning when the bus is cold again. The alternator is a remanufactured unit from Advance Auto, not sure how old it is though. I am going to go ahead and replace the cable from it as there is a splice, and the end treatment leaves a little to be desired. I looked at the splice a few weeks ago, but it looked fine so I left it in.

I am still collecting advice and building my list of things to test, so please weigh in!
_________________
1976 Transporter, Stock 2.0L
http://instagram.com/sixer.bus/

[Amskeptic]

[sixer]

[aeromech]

The alternator sure would be suspect but I have to ask....

Even with the alternator putting out zero amps, the stored power from the battery should privide plenty of juice to keep the ignition system running for a long period of time. I think we've all heard stories of people driving their buses long distances just on battery power. Now if the alternator was somehow creating a dead short, then the battery power could drain quickly but I'm afraid you'd be seeing and smelling some smoke at that point. I'd also think that your system would operate fine down to about the 9-10 volt point and you report this is happening with battery voltage above 12 volts with the high beams on. I'm sorry I can't offer up an explanation of why you're seeing the stumble. You could try running a hot wire from your battery + straight to the coil to see if that solves it. If it does then I think you'd be looking at an ignition switch as was offered up earlier as a possible cause. Maybe you should reseat the connector at the resistor pack just for grins? You might also watch your fuel pressure during a stumble (if you can get it to repeat again).
_________________
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[sixer]

Thanks for the ideas aeromech. Some quick questions/comments:

I agree, the voltage seems fine, but I've hear the FI system is fickle about voltages too. Hence why I'm still seeking out further troubleshooting ideas like the ones you mentioned.

Regarding running a wire from the battery to + side of coil: any issues about doing this with pertronix? My thought would be: start engine, see if stumble happens, if so connect wire and see if it stops. This would essentially guarantee full voltage to the coil instead of having to pass through the ignition switch correct?

Oh, and what is a resistor pack? Still learning here...
_________________
1976 Transporter, Stock 2.0L
http://instagram.com/sixer.bus/

[aeromech]

The resistor pack is the thing in the upper left attached to the firewall

The fuel injectors operate at around 5-6 volts so you need the resistor pack to drop the system voltage. You can see the connector in the picture. To your question about the coil wire. Yes, I think you should rig up a good source of battery power and when you get a stumble, connect that wire directly to the coil +. I don't think it will harm the Pertronix as long as you connect the wire to the RED pertronix wire on the coil.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[sixer]

So after some testing, here's what I've learned: if I connect a cable from the positive straight to the battery to the + side of the coil (per aeromech's suggestion), the problem seems to go away completely. So I replaced the ignition switch, but the stumble remains. Doesn't last long after being warmed up, but it's there when cold. Colin, I still haven't tried replacing the alternator cable, although that is on my list for tomorrow. Any other ideas?

When I drove it today (after replacing the ignition switch), here's what happened: started fine, idled fine. Made it a block, then hiccup'd once, so I turned down a side street to do some further testing. If I turn the headlights on when cruising, the bus bucks badly. If I turn the lights on when idling, it wants to die. The symptoms lessen as the driving time increases, and eventually go away completely. Tomorrow I will be replacing the alternator cable and re-cleaning all grounds and connections.
_________________
1976 Transporter, Stock 2.0L
http://instagram.com/sixer.bus/

[LouFilia]

I had a similiar problem on a different Bosch injection unit. I think your looking on the wrong side of the power distribution network. I started by cleaning all my FI connections and the cleaning of the FI grounds solved the problem for a while (got the most results cleaning the grounds directly from the computer), but it came slowly back. It turned out my resistor pack was old and not suppling the enough voltage to what we call the double relay. For reasons that only a electrical engineering teacher could probably only explain to us correctly, as I drove the car, the voltage output would slowly go up. Thus the problem went away as the car 'warmed up'. It may be different on a L-series injection, but that's my first thought.
_________________
'78 Westy (P22), 2.0FI (GE), 4speed (CP), thought I was buying a VW, ended up with a Bentley.

[aeromech]

Let's go back to what you just said. Per my instructions, you connected a hot wire directly from the battery to the coil + side. You said the problem went away.

I know you've replaced the ignition switch but I'm curious if you go back and hook that wire back up if the bus will ever stumble?
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[SGKent]

[aeromech]

Is your battery ground strap to the bus engine or the bus chassis?

This makes me think maybe you could try running a special ground wire from the battery - to the engine case and see what happens.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[SGKent]

[Wildthings]

Rig up a jumper to go from hot side of the #8 or #9 fuse to the hot side of the #11 or 12 fuse. If this fixes your problem it is either the ignition switch or the wiring to and from the switch. Note that you will not be able to shut the engine off with the jumper thus installed.

You need to work your way through the #30 and #15 circuits and find out what the voltage is at each point along the way. If the voltage drops substantially at any point, you likely have found your problem.

[telford dorr]

Agreed. Note on the schematic that the power from the starter goes to the fuse block, across a busbar, out of another fuse block terminal, to the ignition switch, out of the ignition switch back to the fuse block, across another busbar, out of the fuse block, then to the rear of the bus to the coil.

That's four connections at the fuse block, any of which could be intermittent. Search, clean, tighten, reinstall. See if the problem goes away permanently.
_________________
'71 panel, now with FI
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