91 W250 electrical issue

[greenallgreg]

Having an electrical problem with my truck. When driving the voltage gauge drops to around 10v and dances while driving and when turn signal is on. Headlights also dim due to the low voltage as well as the speedometer being off around 5mph. Had a test done on the charging system which resulted in the battery having 12.76v and the alternator having 12.53v with no load and 12.42v loaded. Drove the truck to work one night and didn't start when leaving. Got it home, turned it off and tried to restart with no success, not even a click. Put a good charge on it and didn't have a problem starting since, just the low voltage.

 

['93 W250]

This probably isn't the reason but I had charging issues and it turned out to be a jacked up ASD relay. It was intermittent and then finally died all together. Maybe something similar... it would be odd though if your alternator is getting power. Mine wasn't and that's how I narrowed it down.

 

[B.G. Smith]

The voltage reading you show are too low to charge the battery. It should be 13.8 to 14.4 volts unloaded. May be the voltage regulator. I think it is inside the ECM on your truck. bg

 

[dodgenstein]

91 has external voltage regulator

 

[Fastback67]

I switched to a Transpo C3813 VR on my 1990 W250.... I also added a good solid ground wire between the alternator and the VR, just for good measure. Haven't had an issue since. Would highly recommend a relay setup like the A3012 as well, just buy a similarly rated, but less expensive relay, they're a dime a dozen. Cleaning all those associated 20 some year old electrical contacts and plugs is a good idea too.

https://alternatorparts.com/external-voltage-regulator-high-output-alternator-kit.html