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Old 02-03-2019, 05:42 PM
Jay Gibbs's Avatar
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87 300TD glow plug time relay question...

I have a no start problem with my 87 300td. Turning the ignition on, the glow plug light comes on and I hear the glow time relay “clunk” on. Light goes off after about 10 seconds, and after about 20 seconds later I hear the relay clunk back off. Car just cranks and fire at all. It’s about 50 degrees today so I attempted to continue cranking to build heat in the pre chambers...lo and behold after about 60 seconds of cranking, it very slowly starts to fire, and with enough continued cranking it eventually starts.

The glow plug system is clearly not working...yet there are no blown glow fuses and the time relay controls the glow light properly and “clunks” on and off where I expect it should.

Is there an alternate failure where the glow time relay still seems to function properly yet doesn’t warm the glow plugs? I had this problem a week ago with the same symptoms but it was too cold outside to warm the prechambers by just extended cranking. Miraculously, the next day it started immediately at an ambient temp of just 14 degrees and start and the preglow worked perfectly.

Is there a glow time relay failure mode that matches these symptoms?



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Old 02-03-2019, 06:34 PM
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You may have a crack in the strip fuse of the glow plug relay, or the points on the relay may be so badly worn that it no longer reliably makes connection to power the plugs. A voltmeter is your friend here, measure from one of the GP studs to the block when it should be heating, if you have voltage, you may have dead glow plugs or fuel supply problems.
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Old 02-03-2019, 07:00 PM
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Originally Posted by Diseasel300 View Post
A voltmeter is your friend here, .
A voltmeter is NOT your friend!!! More often than not, you will get burned in your diagnostic using a voltmeter. That function of my multimeter hasn't been used in my shop in years... You want to substitute loads, a test light is a basic load - depending on the circuit load I'll be using headlight bulbs.

Use a test light to ensure your system is capable of carrying current. Start with your glow plugs. With the glow plug connector disconnected on your relay, end of test light to bat +, touch each glow plug stud for a 5-10 seconds to make sure the test light will light up. If it is NOT lighting, or beginning to dim after a few seconds, you have a confirmed dead plug in that cylinder. Lighting up bright, is a "good" plug.

Now switch test light to bat - and do a similar test on the relay side. Relay "ON" , touch each side of the strip fuse, test light should be bright. Then go and check each pin for the glow plugs, still with the relay "on". Light should be bright for each one.

This will get you by for basic glow system test.
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Old 02-03-2019, 07:02 PM
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If you have voltage at the glow plugs when cranking, check the resistance of each glow plug. You can check at the glow plugs or at the glow plug connector at the relay. The connector socket pins are numbered if you look closely. It should be 0.7-1.0 ohms.

You might have air in the fuel system. Crack the injector lines then see if fuel dribbles when you crank.

Someone recently discovered a clogged oxidation catalyst causing difficult starts.

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Old 02-03-2019, 08:20 PM
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I use the Multimerter and almost never use a test light. I like to know how many volts there actually is something a test light does not tell you. And of course the multi meter has the Ohm Meter setting and the AC milliamp setting which are useful for troubleshooting other stuff. So for myself I have learned to use the Miltimeter to good effect.

The Test Lights I have used in the past don't survive well in a tool box. Multimeters hold up better inside of the tool box then a Test Light does. However, now that the have LEDs that could be a different story.
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Old 02-03-2019, 09:20 PM
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Extended cranking time, when the glow system is working, and from your description it is, is from air in the fuel more often than not.

10seconds light on, no blinking, and a full 30 plus seconds of relay clunk... if the cabin light is dimming the glows are at least pulling amps. Starting in 14F weather pretty much confirms they are working at least SOME of the time.
The 87 glow relay DOES have a fuse. Unless someone has changed the relay for a 91 or so, it will have a strip fuse that can sporadically work.

Notice if parking on a downhill slope forward or backwards makes a difference.
Forward hard starting focus on glow system.
Rearward hard starting, focus on fuel system. The 87 is notorious for delivery valve seals leaking air...
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Old 02-12-2019, 01:42 PM
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Bit of an update...I left the car alone for the day and attempted to start the next morning...started perfectly fine and has ever since. I’m figuring it must be a slowly failing relay or the fuse on the relay. Definitely something in the glow system and not a fuel related problem. I ordered a new relay that just arrived today and I’ll keep it handy in the glovebox for the next time it acts up, swap it in and see if that solves it.

Thanks for the help.
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Old 02-12-2019, 04:49 PM
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FWIW, if the light glows and goes off normally then the plugs are fine and the relay is working correctly. The relay compares the actual combined current draw of groups of may be the no 1 plug to the combined draw of the other five on your car...and the light would not come on if the expected ratio wasn't measured. I think an air leak or fuel line clog may be at work here. Or maybe you have carbon built up in your prechambers.
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Old 02-12-2019, 06:06 PM
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Jay - what you describe is a classic air leak into the fuel system, not a problem with the glow plugs.
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Old 02-12-2019, 06:36 PM
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Totally an air in the system problem

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