glow plug relay
 

Hi, great forum, lots of good info.

I just purchased a '75 240d, and it was slow starting it in the cold so I pulled the glow plugs to test. This car was upgraded with the bosch fast glow plug kit. Glow plugs tested fine. I put everything back together, now the glow plug indicator light doesn't come on, and car won't start.
Unfortunately, when I hooked everything back up, I attatched the ground(to chassis) wire to glow plug #1, and apparently this was a bad idea, as it isn't needed with the new style relay/plugs.

Did I potentially short the relay?

I removed the glow plugs and tested them again, jumped to the battery, and they glowed hot. Also, I am getting a .5/.6 ohm reading on all of them. The jump wires all tested ok as well.
I have 12V into the relay, but can't get a reading out of it. I tested it shortly after turning the key. It read .8V, then the relay clicked and it dropped to .3V. Now it reads zero when I test it.
I also don't see any fuses in the system anywhere with the new kit, I guess the old ones had a strip fuse in them.
Is it possible to service these new relays? I hate to crack it open for nothing.
Thanks for any advice. Dave

You did not mention if you had Strip Fuse on the Fire Wall.
There is more than one setup on Cars made with the Loop/Filiment type Glow Plugs. One type has a Relay that needs a Fuse on the Fire Wall; another has no Fuse but has a Reed Switch inside of the Relay.

Also if your upgrade was one that came with an Relay different from the Stock Relay that makes the issue even slightly different.
How, about some pics of the Relay and see if you have a Fuse on the Fire Wall.
Below is a pic of a complete Bosch Kit.

you popped a fuse somewhere.
start at the harness, and work your way back on the power wire.
you'll find it.

Hi, thanks for the replies.

The Relay/Wiring harness are the exact ones pictured in your post Diesel911, with no visible fuse on the relay. Does this relay have a Reed Switch that you mentioned? There is also no fuse on the firewall, or wire coming off of Glow plug 4 that on the older setups seemed to lead to a strip fuse.

I followed the wiring harness from the relay all the way until the four wires dissappeared under the dash, no fuse in-line as well. Now I couldn't see the last 4-6 inches of these wires under the dash, without taking some of the dash apart, but I would guess there isn't one under there, as it it would be really difficult to change.

There are a couple of threads on here indicating that the upgraded glow plugs wont work with certain early relays. Yours may be an example of this. Can't recall the exact reason why they won't work but one guy troubleshot for a long time before reaching that conclusion.

I do not know if the Kit Relay has the Reed Switch or not.

Sometime last year there was a memeber looking for the Fuse but could not find one.
I looked up some of the Wiring Diagrams and found one of ths systems has no Fuse but the Relay had a Reed Switch in it. So I assumed that the Reed Switch subed for the Fuse.

Because the Relay came from a Kit there is no Wiring Diagram in the Service Manual. Also I do not think all of the kits are made by Bosch.

Is there anyway to peak inside of the Relay?
I guess with too fast and too much of an over load it might be that the Read Switch (if the kit regulator has one) took a hit.

Most of the electronic modules on these older cars come apart with a few screws, or a little careful prying.

I'm not sure how the fuse-less relay works, but it shouldn't be too difficult to at least get in to take a look.

When it was sticking engaged, I reworked the glowplug relay on my 300sd, just a matter of filing the points that were sticking.

thanks for all the posts. I am going to try and look at it all again this weekend. Seems to me I remember reading here that one could disconnect the relay completely and jump a wire directly from the battery to glow plug #1 and try and start the car. If it started, wouldn,t that mean the relay(or relay wiring harness) were the culprit?

Dave

Look at this picture. If your relay looks like this (and you confirm a bad relay) I have a couple new ones with warranty left.

The relay in this pic is actually for a 5 cylinder. The power output pins would actually be four but the pin patterns are the same.