TheSamba.com :: View topic - Carbureted Vanagon- tach needle bounce, distributor issues?

Thu Jul 10, 2014 1:51 pm

I started having issues with my van stalling, with wild drops in the tach needle so I thought , "time for a new distributor". Any excuse to get rid of the crappy svda I had in there, so I promptly ordered a petronix iii, and while I was at it a flamethrower iii coil. When installed it it had way crisper timing judging from the light, but I was still getting a tach needle drop accompanied by a low idle. Now it revs down and mostly stays down around 200 rpms but the needle is still bouncing.

When I rev the engine up out of idle everything smooths out. I thought maybe it was a bad ground so I cleaned up all the connectors in the engine compartment. The wire to the water temp sender fell off while I was doing this so good thing I went at it. I also replaced the ignition switch, no change.

Here's the weird thing, when I pull off the spark wires on the distributor cap, the wires to the left side of the engine are bright and spark at a regular interval. The wires on the right side are dim and spark erratically. I replaced plugs and wires, no change. I called pertronix and they said the low resistance on the coil may be effecting the dwell angle but then what about the idle drops with the tach needle bouncing? That was the original symptom and it changed but didn't go away. I'm going to connect the old coil, per their suggestion and see if it's different.

I'd love any suggestions. Especially ground location for the ignition switch or anything else I might be missing.

Thu Jul 10, 2014 2:39 pm

If you use an analog meter the read the voltage on the 12+ side of the coil, is the voltage jumping around?

Thu Jul 10, 2014 5:45 pm

When I checked the voltage it was off the + side and ground to the chassis. No jumping Voltage, 12.4 v.

Thu Jul 10, 2014 7:09 pm

Actually it was a digital meter.

Thu Jul 10, 2014 9:16 pm

Digital meters don't always tell you what is going on, which is why I recommended an analog one.

Fri Jul 11, 2014 7:23 pm

do they even make analogue meters anymore? I had a friend dig around in his tool cabinet for one, no luck. He threw it away when he bought his digital. any recommendations for one?

Fri Jul 11, 2014 8:39 pm

Radio shack used to sell small analog meters. Analog is still important for a lot of hobby electronics

Fri Jul 11, 2014 9:36 pm

Radio Shack and many others. Analogs still have their place, for use on older autos they are my first choice. I actually just picked up a dual analog-digital meter off of amazon to give it a test. Still in the package at this point though.

http://www.amazon.com/gp/product/B000JJMIX4/ref=pe_62810_112114450_em_1p_9_ti

Sat Jul 12, 2014 7:58 am

Did you try installing the diode in the line from the coil to the tachometer?

Sat Jul 12, 2014 10:22 am

Randy, please elaborate. I don't have a diode in there now.

Sat Jul 12, 2014 3:09 pm

from egauges.com....

Purchase a diode #1N4005 from a local electronics store such as Radio Shack or Circuit City. Cut both ends of the diode so each is approximately 3/4” long. Crimp 1/4” female spade connector on the end of the diode with the silver band. Crimp butt-splice connector on the other end of the diode. Crimp the opposite end of butt-splice connector to the wire connected to the ignition signal source. Connect 1/4” female spade connector used in #3 above to terminal #4 on the back of the tachometer. Connect a ground (-) wire to terminal #3. Connect a switched 12-volt power wire to terminal #2. Set switches for appropriate amount of cylinders."

Sat Jul 12, 2014 3:47 pm

I think I'm making sense of this. The coil is doing something funning to the signal for the dwell angle and tach? You lost me at "set switches..." Should I put the diode in line on the + side going to the coil from the tach? I'm guessing the engine compartment would be the easiest for this. I'll get a diode in the meantime.

Sat Jul 12, 2014 4:42 pm

I checked the voltage at the coil (- to chassis) and got a pretty steady reading around 14v (it was 12.4 on the digital meter). The voltage was varying no more than 1v on the analogue.

I also put in the old coil, and it had the same jumpy idle with dimmer irregular spark on the distributor cap for cylinders 1 and 2, but bright and regular on 3 and 4. This same jumping tach was happening with my old distributor but it would just die instead of idling at 200 rpms while the needle jumped. I tried increasing the idle to see if it would change anything and it was just more erratic idling.

Could it be something going on with my tachometer thats causing this? I got the diode but I'm still not entirely sure where to put it. I'll try anything at this point. Anyone know a good psychic?

Sat Jul 12, 2014 6:50 pm

Would it be worth running the coil (-) to the chassis to check the ground?

Sat Jul 12, 2014 7:04 pm

To verify what & why the tach is reading what it is, do hook up a tach / dwell meter first.

If the hall sensor is putting out irregular firing signals it would cause the dash tach to bounce around.
Do check the wires entering the distributor to make sure there isn't any cracks in the insulation or chaffing wherever they enter the distributor.

This is I would start this investigation prior to adding diodes.

Sat Jul 12, 2014 9:48 pm

Surfy Murphy wrote: I think I'm making sense of this. The coil is doing something funning to the signal for the dwell angle and tach? You lost me at "set switches..." Should I put the diode in line on the + side going to the coil from the tach? I'm guessing the engine compartment would be the easiest for this. I'll get a diode in the meantime.

Some tachs (notably VDO) are very sensitive to "stray voltage" associated with points replacment modules, funky coils, or even currents running near the line heaed to the tach. I have put them right on the coil and right at the gauge before to get rid of the distrubance. Both places have worked well.

Sun Jul 13, 2014 10:29 am

Ok, the dwell angle reads roughly between 8 and 11 with the rpms around 5-600,also extremely erratic. I also separated the wires going to the coil from each other and and away from the spark wire. No change.

Sun Jul 13, 2014 10:31 am

How do the wires look going into the distributor?
The electrical feed wires, not the plug wires---

Sun Jul 13, 2014 10:36 am

They look good! Brand spanking new. There's a rubber boot in thE distributor for strain relief. Keep in mind this symptom was there before I got the new distributor and coil but before the needlewould just jump around and stall. Now it hangs on at low rpms and revs up and down.

Sun Jul 13, 2014 10:38 am

Then try the diode installation.
See what happens.