Seems my glow plugs only want to fire up when the car is warm. When its
cold, the light will not come on and it is hard to start. After I get to my destination, I shut er down and turn the key and the glow light comes on.
What gives? I tested the relay with an ohmeter and all 5 plugs register less than 1 ohm. And I get 12.5 volts on all three screws on the relay.
Any thoughts? I'd hate to sway out the relay for $150 if I am not sure if it need it. I did not however test the ignition plug on the relay. How does one test it?

what kind of car?

__________________
-Trevor

OBK #12
1980 300SD 333,XXX miles - Totaled
1986 Mazda RX-7 212,XXX miles - impounded and auctioned off
2005 Jeep Liberty CRD Limited 33,000- SEGR, Provent, Fumoto

sorry.....83 300d Turbo lots of miles!

he is saying once he's at destination he turns the car off then turns the ignition on and the glowplugs are working because they've "warmed up" during the trip.

I would start at the relay. It's basically the only electrical component between the ignition switch and the glowplugs. If it's not working until it "gets warm" then the relay is most likely the problem.

I'm going to also suggest you reanalyze your car and find out if your glowplugs DEFINITELY won't light before they're warm or if it's just a battery issue or something. I just don't see "warmth" being an issue in this electrical component...it's made to operate at extremely low temperatures and this is why I just can't understand the need to get warm....there may be something else that the trip does other than just warming up that may enable your glowplugs to function properly.

Good Luck!

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1974 240d "Peace, Love and Empathy" - Kurt Cobain
1982 300sd "It all depends upon the Benz!" - Me
1983 300sd
1985 500sel
1992 300se

I just went on another road trip and prior to starting, I turned key and no glow. I pulled out my multimeter again and there was voltage at all points and all plugs were reading less then 1 ohms at the relay. So the plugs appear to be ok. After I got to destination, 30 min drive, I stopped and restarted and glow light came on. what gives? Do I just chance it and swap out the relay? Every time I check the voltage/ohms I notice the adapter for the ignition on the relay is very loose and easy to come off. Can it be as simple as a bad connection at the relay where the ignition wires connect? Is there any more troubleshooting steps I can take to prove that the relay is bad before buying a replacement?

Location? How cold is it when you start the car without the glow plug light?

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

Certainly. A poor connection at the smaller connector could cause the same condition as turning the switch off and on.

Can you hear the relay click off? If so, how long does it remain on?

Turn the key on and see if you have 12v at the plug...touch the multi-meter lead to the end of the glow plug and the other lead to ground. You only have 30 seconds to do it, so be quick.
Also, you can glow and put your hand on the block near the plug and see if its getting warm.
The ultimate check is to remove the plugs and give them 12 volts and see if they glows.
When I changed my plugs, I got them from this website and also got a new relay that glows for 3 minutes after starting to smooth cold starts. It wasn;t too expensive, 5 plugs plus the relay was less than $150.

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now

I just had my wife turn the key while I had my ear to the relay. No glow light on the dash and no clicking whatsoever. We tried it a couple times. I will do some further testing. I am assuming it is either a bad ignition wire connection at the relay or the relay is simply bad. Can't believe it is $160 for the relay though. Thanks for all the helpful advice.

I had the same problem recently on an 87300d. I have had poor starting for a while and like you sometimes the light would not come on so I decided to go with a afterglow system from one of the later models. Even after installing the new relay my light would not come on when the car was cold ( and it aint that cold in Alabama), but would come on when the engine was warm. In fact after the engine had been running after about 15 to 20 seconds then the light would come on. Well I had to replace a hood release cable so I had the lower dash on the drivers side out. Decided just for kicks to unplug the cables to the ignition switch and plug them back. Well after that the light comes on when cold. Today I replaced by glow plugs and I had two that were bad. Weird stuff.

HA, Eureka, I think I've got it. As I found earlier, when pulling off the ignition connector on the relay, it came off without effort. It was a loose fit. So, just for kicks, I put a small piece of tinfoil around each post and put the connector back on to make the fit more snug. Prior to doing this, I tested by turning the key and no glow. As soon as I added the foil, the relay is kicking on and glowing. Started after 3 cranks! This was a good test. Will see how it fares throughout the week. May have just gotten lucky this morning. Anyone have any better, more secure way of tightening the connection w/o using tinfoil?

Solder maybe?

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1985 MB 300D/T

Fell off the WVO bandwagon when people started trying to charge $2.50 a gallon for dirty vegetable oil.

If the cover comes off that plug, you may be able to pinch each one of the connectors. I think most of the plugs can be opened up.

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss