I have a 1990 350 SDL and in the morning it wont start up.....before (bought 1 month ago) would start on the first crank (after glow plug light went off), now I have to crank and crank and crank and 10 or so minutes after she will start...is it my plugs, evrything electrical is working fine (except rear pass. left window and my lumbar controls)..please help so I know what to tell the mech to look for..... thanks people..DIESEL 4EVER

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Are you sure the glow plugs are working? Replacing them never hurts it also my be a relay not lightin them.

 

take out the handy multimeter and check that the plugs are getting voltage. secondly your plugs are run in parallel this means that even if on goes out the others will still function. use the multimeter on the ohm setter and check each plug from the positive terminal nut to the negative terminal on the battery. you will need to remove the leads that go to each plug first. you are looking for a very low resistance something on the order of 1.5 ohms. if you have a very high resistance or an infinite resistance that plug is bad.