I've has some squirrel related wiring issues (chewed wires) which I think I have repaired (soldered back together). I'm not sure what the wires are for (they are near the battery case on 300td) My car is now not starting...It cranks fine, there is fuel in the filter, the glow plug indicator light is working, but absolutely now combustion when I crank it. I'm thinking that there might be a glow plug problem (its also been pretty cold here). I checked the resistance of all the plugs by probing the "output connector" at the glowplug relay and they all checked out ok. How can I check if my glow plugs are actually recieiving current? Can I just remove the "output connector" on the glow plug relay and see if juice is coming out of the relay. Any other ideas on why my car is suddenly being so stubborn? Thanks

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'85 300td Frybrid WVO Kit

You can 'see if juice is coming out of the relay' by using an ammeter or a clip on current probe to monitor the current delivered when the plugs are activated. The plugs should initially draw about 15-20 amps and will taper down to about 7-8 amps after 20-30 seconds of operation.

I noticed you are using a WVO kit - you know WVO and cold weather don't mix very well?

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98 Dodge-Cummins pickup (123k)
13 GLK250 (135k)
06 E320CDI (323K)
16 C300 (62K)
82 300GD Gelaendewagen (54K)

Since it's unlikely that you'll have an ammeter capable of 20 amps, you can just touch the + probe of your voltmeter to the tip of one of the glow plugs...........the - probe to ground...........and you should see approx. 12V during the 30 seconds immediately following the turning of the key to the #2 position.

Plugs do not receive current. They draw current (its an important distinction).

The easy and safe way is just to test for voltage across the plug.

Actually, it's the pole on the battery that's drawing the current. The glow plugs just happen to be in the way.

...........and, it's not an important distinction..........

I tested the voltage at the glow plug tip (wire still connected to glow plug) and it was only 9 v. I also tested the voltage coming out of the glow plug relay (tested all 5 male probes) and it was 12v. Both of these tests were conducted when the glow plugs were being activated. Does the fact the voltage seems to drop at the glow plug mean anything.

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'85 300td Frybrid WVO Kit

If the test was performed properly, it indicates that there is high resistance in the wiring from the glow plug relay to the glow plug itself. This would be practically impossible on factory wiring unless it's been damaged in some fashion........

Just for the hell of it...........test the other four glow plugs in the same manner..........see if they all read 9V.


There is one other possibility:

If the battery is very weak, the system voltage will drop severely when the glow plugs are activated. So, check the battery voltage during the 30 seconds when the glow plugs are activated...........see if it also reads 9V. If so, then you can forget about testing the other glow plugs.........you need a new battery.

Well I got an update. I checked the battery voltage under load from the glow plugs and it was 10.9 (down from 11.9 not under load). I also tested the voltage coming to the Glow plug relay (while under load from plugs) and it is 9.9, the same as the voltage at the glow plugs when they are under load. It seems like I'm losing a volt or so between the battery and relay. Does this indicate anything. Does a 1 v drop in voltage under load indicate that my battery is no good, or maybe just undercharged? Thanks for the help

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'85 300td Frybrid WVO Kit

Battery is weak. Should have over 12V no-load (12.5 is preferred). Losing one volt under load is typical.

Do you think it just needs a thorough charge, or replacement? What about the loss of 1 v from the battery to the glow plug relay? is that normal?

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'85 300td Frybrid WVO Kit

How old is the battery? Heat and cold are what kills batteries.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Well, I pretty good........but, I cannot diagnose a battery over the internet.

It needs to be fully charged.........first. Then, any of the McParts can put it under load to determine if it's still in the game.

1V is more than I'd like to see from the battery to the relay..........there could be some wiring issues at the junctions.

Did you do these two tests from the same ground point? If not do it again and use the same ground...go right to the negative of the battery would be my suggestion.

You may have a bad engine gorund too..check the strap which grounds the engine to the chassis to make sure it is intact and in good shape without corrosion under it.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

All those measurements were taken with a coomon ground (the negative battery post). It could be due to the wiring issue I eluded to in my initial post. I'm going to fully charge the battery, install a diesel giant block heater then give it a try. I'll get back to you all after that. Thanks for all the help.

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'85 300td Frybrid WVO Kit