BH,

The Pre-Glow Relay's Instrument Cluster (Yellow) light will/should come on as
soon as you turn the Key to the "Glow" position.


The Glow Plug relay you've pictured DOES NOT HAVE A
FUSE (80 Amp).
It Has ELECTRONIC sensing protection.
In case of a Dead Short or other dangerous Electric
Anomaly,the Relay automatically Dis-Connects itself.
,Until the Fault is Rectified.
All OM602.962 and some OM603.XXX engines came with
these Intelligent Relays.

And Yes the Relay's actions are telling you one or more
GP(s) are Bad.

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'84 300SD sold
124.128