Glow plug system, 123

I have a 1979 300SD with 256,000 miles on it. I haven't used it much as I have the wagon as a daily driver. It starts fine cold after i replaced the
one bad glow plug (it only needed #3 and it's the easiest to replace), and starts fine when warm. If i start it cold and only go a very short distance, the glow plug relay assumes it's warmed up and it won't re-start unless I either wait until it's dead cold or i jump the glow plugs. Has anyone fooled around with adding a resistor or something to the temp sensor to let a tired engine start easier?
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I would change the glow plug relay... the new one will let it run another 3 minutes before it shuts off... But as with any relay, it should have turned the glow plug system on again as long the ignition was turned off and then back on.
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Do you suppose the one AC brand plug may be different than the ones of unknown age causing the problem? I remember with VW diesels there was a change to a "fast glow" system, plaugs and relay were swapped.
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Measure the resistance of the plug. Since all the power is used to make heat, there should be a linear relationship between the resistance and the amount of heat it produces.
To test your relay, hook up your meter and see if you have voltage and for how long after you turn the key on.
You could also have a carbon problem. sometime there is a carbon build up in the glow plug chamber. They sell a tool to clear this out.
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Ditto on Roger's suggestion.
All your glow plugs should be about 0.6 ohm... For your car, glow plugs are dirt cheap... why bother anything other than Bosch? Ebay seller sells the whole kit cheaper than glow plugs at dealer.
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For your car, glow plugs are

my thoughts exactly, but there was a set in the glove box when i got the car
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It's not that cut and dried. While th cheap Chinese Monarch plugs do not last as long as BOSCH or BERU they do burn hotter which sometimes helps. I use them.
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I put in the other 4 plugs ysterday, tested it once, and sold the car today. Now back to the 81 Wagon and other projects Thanks.
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