[jguy83]

88 VW Cabriolet. Strange things started happening today. It doesn't matter the gear, but as soon as I get in between accelerating and decelerating, the tachometer goes to 0 and the car starts bucking (best way I can describe it).

As soon as this happens, if I completely let off the gas or start accelerating again, the tach goes back up to where it should be and the car drives great. The motor runs great at idle, while accelerating, or while decelerating.

This car has had recent cap, plugs, and wires.

Any ideas what's causing this?

 

[matty kirk]

I would check the ignition coil. The procedure is in the Bentley.

It sounds like your car is mis-firing pretty badly, and since the tach is dying as well, it makes me suspect the coil.

Maybe Hall Sender, but I don't know that that would affect the tach. Whatever it is, it definitely sounds like ignition system.

 

[briano1234]

I would check the ignition coil. The procedure is in the Bentley.

It sounds like your car is mis-firing pretty badly, and since the tach is dying as well, it makes me suspect the coil.

Maybe Hall Sender, but I don't know that that would affect the tach. Whatever it is, it definitely sounds like ignition system.

How many key's on your key ring?
Have you replaced your battery to frame and frame to engine/tranny ground?
Have your checked your Coil, or hall sender?

 

[flipty]

Seconding what everyone's saying. The coil isn't a very expensive part ($50-100) and is pretty simple to replace.

(edited to add) also the coil may have a ground cable that goes to the engine block (on the valve cover's top-rightmost bolt, looking at it with the hood up)... might want to just remove that from both ends and clean the ever-loving daylights out of it, or replace it.

Also Briano asks about the keys because everyone seems to have a million keys on their ring these days, and it's hell on these old ignition switches.

 

[jguy83]

I would check the ignition coil. The procedure is in the Bentley.

It sounds like your car is mis-firing pretty badly, and since the tach is dying as well, it makes me suspect the coil.

Maybe Hall Sender, but I don't know that that would affect the tach. Whatever it is, it definitely sounds like ignition system.

I checked the coil about 2 months ago for a different issue and it was working great. I'll give it another check.

I'm wondering if it may be the Hall Sender. Will look into that too. Thanks!

 

[jguy83]

How many key's on your key ring?
Have you replaced your battery to frame and frame to engine/tranny ground?
Have your checked your Coil, or hall sender?

Just one key

 

[jguy83]

So this morning on the way to work I wasn't having this issue anymore... Drives like a champ again. I'll see what happens over the next couple days.

 

[tolusina]

......Maybe Hall Sender, but I don't know that that would affect the tach. Whatever it is, it definitely sounds like ignition system.

A missing Hall signal will certainly affect the tach, I'm 100% on that.
Further, that the tach is affected narrows the issue down to the ignition primary, that's the low voltage side of things, not the high voltage sparky stuff.
---
My first guess based on reported symptoms is a broken wire to the Hall generator inside the distributor.
Normally, as part throttle vacuum is applied to the vacuum advance diaphragm , part of the Hall generator plate rotates relative to the distributor housing and plug, wires internal in the distributor flex as this happens.
What my guess is, is that that normal wire flexing opens the break in the wire(s).
Randomly, some such breaks are repairable, other cases require replacement of the Hall generator assembly.
Here's a link to GAP's Hall generator listing, note the "square plug" comment, there are at least two plug versions, the plug may need to be changed as well to match.
http://www.germanautoparts.com/Volkswagen/Cabriolet/EngineElectrical/45/1

A quick, easy and free test of my guess is to remove the vacuum hose from the vacuum advance unit on the distributor, plug the open hose with a golf tee, pencil, sheet metal screw or what ever suitable you have on hand, go for a drive.
Power and throttle response should be noticeably reduced with the vacuum advance disabled, the centrifugal advance (if working properly) should keep the car reasonably drivable.
With the vacuum advance disabled, there will no longer be any rotation of the Hall generator relative to the distributor housing and plug, therefore no wire flexing to open the break, if my guess is correct, your cutting out and tach dropping symptoms will cease while being replaced with new symptoms of reduced power and throttle response.
---
A slightly more involved test, still easy and almost free, involves a length of vacuum hose and a vacuum source, the vacuum source being you.
Connect a couple foot long length of vacuum hose the the distributor's vacuum advance port, idle the car, suck on the open end of the hose. Sucking the hose will rotate the Hall generator relative to it's plug, if the car's issue is a broken wire, the engine will quit (randomly) as you apply your vacuum.




.

 

[mattchirhart]

having this same problem but only on acceleration, and only if i push it. if i am gentle i can control it but it is getting worse by the day.

 

[mattchirhart]

also, my tach just drops with the bucking, never dropping more than 500 rpms

 

[berkeleyjeff]

and the ruling was?

So this morning on the way to work I wasn't having this issue anymore... Drives like a champ again. I'll see what happens over the next couple days.

So, what was the verdict on your lost tachometer reading?

Coil, Hall sender, something else?

Chasing a similar problem, car bucks at 5K until you lift and shift. Haven't had a tach reading in along time. Heard mine could be a lack of signal to the ECU running through the tach.

Thanks.

Jeff

 

[briano1234]

So, what was the verdict on your lost tachometer reading?

Coil, Hall sender, something else?

Chasing a similar problem, car bucks at 5K until you lift and shift. Haven't had a tach reading in along time. Heard mine could be a lack of signal to the ECU running through the tach.

Thanks.

Jeff

When was the last time you checked your grounds, Battery to frame, frame to engine, and Valve cover rear to coil bracket.