VW T3 diesel starter clicks but won’t crank — starter tested good on bench

[Vodim]

I’m troubleshooting a starting issue on my diesel VW T3 / Vanagon and would really appreciate advice.

Background / history
I had almost the exact same problem last year.

Symptoms back then:
- Turning the key would sometimes do nothing, sometimes produce strange buzzing/whining noises.
- No proper cranking.

I removed the starter motor and bench-tested it using jumper cables from another car battery. On the bench:
- the solenoid engaged strongly,
- the pinion gear shot out properly,
- the starter spun fast and seemed perfectly healthy.

Since it looked fine, I cleaned/sanded the contact surfaces:
- starter mounting surface against the gearbox
- some electrical contact points / terminals

After reinstalling, the van started perfectly again — but only for 1–2 starts. Then the exact same problem returned.

Current symptoms
This year the van sat unused for a while. When trying to start:

- ignition warning light comes on
- glow/preheat circuit seems to activate
- when turning to START, the starter does not crank the engine
- symptoms vary:
- sometimes nothing
- sometimes buzzing/whining
- sometimes a distinct click from the starter solenoid
- sometimes repeated clicking

Recently it became more consistent:
- now I usually get a clear click from the solenoid every time
- but still no engine cranking

When trying to start, dashboard / instrument illumination goes completely dark.

Battery / electrical tests
Using a multimeter:

Battery voltage at rest:
- initially ~12.0 V
- later ~12.5 V

Battery voltage during start attempt (earlier test):
- around 10.6–10.8 V

Voltage drop on ground side:
- battery negative to engine block during start attempt: only ~0.04–0.05 V

Large positive terminal at starter:
- ~12.6 V present

Terminal 50 (small solenoid trigger wire) during START:
- only about 7.6–7.9 V (seems low?)

Additional tests
I also bypassed the ignition circuit by feeding battery positive directly to terminal 50 on the starter solenoid.

Results varied:
- sometimes only a free-spinning / whining sound
- sometimes only a strong click from the solenoid
- still no proper cranking of the engine

No reliable full cranking even with direct feed.

I also tried adding an extra ground cable from battery negative to engine/gearbox, but saw no obvious improvement (though I’m not 100% sure the jumper cable had perfect contact).

Other notes
- The small spade connector on terminal 50 is not ideal — the connector is slightly oversized for the spade terminal.
- Because the problem temporarily disappeared after removing/reinstalling the starter last year, I strongly suspect an intermittent high-resistance connection somewhere.
- However, I’m unsure whether the fault is:
- bad starter / solenoid under load
- bad internal solenoid contacts
- poor main positive connection
- poor grounding through starter mounting / gearbox
- ignition switch / wiring to terminal 50

Given all this, where would you focus next?

[TwinTurbo]

given the testing you have done there seems only three thing realy left.

1) Battery is goosed ( need to use a "Load" Clamp metter not a volt meter )
2) starter is ducked
3) engine is stiff to turn due to internal resistance. ( pull plugs and check with spanner on crank pully )

 

[colinthefox]

I had a similar problem a while ago.
It was a bad connection on the interconnector braided cable that goes from the solenoid to the field coil through the casing.
It wasn't obvious until I operated the starter in the pitch dark, when I got a bright flash and firework display from the offending connection.
Not saying that this is your problem, but worth checking out I think.