82 300cd glow plug issue
 

I've recently acquired a 1982 300cd with 107k miles. The car is in great condition. The glow plug light seemed to turn off rather quickly, but the car always started. One morning when I tried to start it, the glow plug light didn't come on at all, and it started hard. I've checked all the fuses, and the metal strip under the rectangular plastic box on the driver's side fender. Everything is fine. Do glow plug relays suddenly stop working? I am assuming that the aforementioned plastic box is the relay, and if so, can I pirate the relay out of my 82 300d and plunk it in? My 4 door doesn't see much action these days, because the coupe is such a blast to drive, I'd like to troubleshoot this before spending any money. Thanks in advance.

yes,
yes,
and
Yes.

Wonderful. Thank you much.

It seems everyone suspects the Glow Plug Relay first.
It most often is an issue with the Glow Plugs themselves.
The Diesel Giant Site has a good pictorial on how to check the Glow Plugs with an Ohm Meter. (This is not an endorsement to buy parts from that site.)

If you do not have one Harbor Freight sells inexpansive Digital Volt Ohm Meters (Multimeters).

Reguardless of what the Glow Plug Light is doing if you leave the Key in the Preglow poisition you can preglow for about 30 seconds; after that the Relay timer shuts it off.
You can also cycle the Key back and forth and start another Preglow Cycle.

If you remove any Glow Plugs ream out or otherwise clean out the Carbon from the Glow Plug Holes.
Only Bosch and Beru Glow Plugs hold up well.

The tips on the Autolite Glow Plugs can swell (this happened to me).
The Monark Glow Plugs do not last long (ya, I tried them also).

I swapped out the relay, and still no light. There is power at the relay, and both sides of the strip fuse. Will a faulty glowplug keep the light from coming on? I will test them as per the DIY on diesel giant tomorrow. I don't seem to get any kind of glow from the plugs at all. Right now, the car starts like my four door when it's 5 degrees out, no matter how long I leave the key in the on position before I try and start it.

"No light" is how the fault indication system indicates a failed glow plug. Or plugs.

Its a long series connection. Any faulty part in the entire series will stop the whole series from working.

El Cheepo DVM's as many have posted here work very well and at under $10 are a deal you cant afford not to have. I actually just won another won, $4.16 delivered.

Been several yrs since I bought one and my current buy just confirms the price.

Some magazines will have a full page add with coupons, a 20% off coupon and something free. screw driver set, tape measure, gloves etc... or some times a digital volt meter. I just got another one last week, not Iam up to about 20 of them. :D

Rod and Custom, Hot Rod and the AAA Via mag have the adds.

X2 on checking the GP`s, if the second Relay didn`t change anything. pull them out and put power to them, the ends should get bright red, if not, toss them.

Charlie

On an '82 300CD, the glow plugs are wired in parallel, not in series. A failure of any combination of glow plugs has no affect on the functioning plugs.

I'm fairly certain he's discussing the glow plug indicator circuit... not the plugs themselves.:P

The GP relay on the 123s uses a current sensor to determine if all the GPs are drawing current. it announces the failure by not allowing the dash light to come on.

It is possible to be one plug down and the light still come on. However when you get two plugs down the light should not come on.

To verify the light still works pull the 4-pin connector from the relay and jump battery to pin 3. This should turn on the dash light.

Or he could spend that time ohming his glow plugs. From the description he probably has two or three bad ones, with #1 very likely being one of them. They aren't hard to check, they aren't expensive, and they aren't hard to change.

alright, so I got five new bosch plugs from a parts house around here, slapped them in, and all is well. Starts like it's 1982 again. I have yet to test the plugs I pulled out, but if I find any that are good, they're going into my 300d. Thanks for all your help guys

Thanks Tango... I thought they were all in series.

Still boils down to a very simple ohm check of each glow plug or pull them and connect them to the battery.