Compression Ignition Addict
Join Date: Aug 2008
Location: Madison, WI
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A starter relay WILL NOT WORK as a GP relay, for two reasons:
1) It is NOT rated for continuous closed duty. The GPR needs to energize the glow plugs for up to two minutes at a time. A starter relay is rated to be closed for only 30 or so seconds at a time
2) Its coil circuit is grounded through its mounting base. The GPR coil circuit is switched on and off by the PCM through the ground side of the circuit. If you mount the relay, it will always be energized while the key is in the RUN position.
* Get a relay made for the purpose. You can get a Western Plow relay with an isolated ground and (reportedly) rated for continuous duty for $15-ish, or a Stancor / White-Rodgers 586-902 for $50-ish. A lot of controversy about which is better or more worth the money, but either one will be more appropriate than any starter relay adaptation
* Get a volt meter. They're $10-ish at Harbor Freight, occasionally going on sale for $5. Except for the small fuses and bulbs, that's less than the cost of any one electrical component on our trucks. Pays for itself the first time it's used to determine that something _doesn't_ need replacing.