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Jack English

USA
1065 Posts

Posted - 08/31/2017 :  13:36:33  Show Profile  Reply with Quote
Selected this forum as these relays are common to most cars. Window lifts stopped working; fault traced to relay grommets on the bracket that hold the relay. These grommets are rubber with a central brass sleeve for the supporting screw. Old rubber dried and crumbling allowed the relay case to contact the brass sleeve. Thus shorted to ground the relay no longer works. Such grommets NLA for years according to Tom Hanson. OK, I can make a substitute, BUT, what is the purpose of the relay inside being grounded to the case?? Why do the mounts need to be isolated from ground? I assume the engineers had their reasons, otherwise a bakelite case would have been used.

Jack English
300SEL 6.3 #4768

Ron B

Australia
11545 Posts

Posted - 08/31/2017 :  22:24:42  Show Profile  Visit Ron B's Homepage  Reply with Quote
Sounds like the guts of the relay fell down inside the case then shorted because the relay case is normally dead.The relay has it's own ground terminal (85 or 86 ) and the rubber mounts are just to protect against vibration. I make new ones from a piece of thick rubber .
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Art Love

Australia
6139 Posts

Posted - 08/31/2017 :  23:27:25  Show Profile  Reply with Quote
Jack,
It makes no sense to me that the relay case contacting the bracket would make the relay fail. The base of the relay is made of Bakelite or equivalent so the terminals and the contents of the relay have no contact with the steel case. The only source of those grommets is to buy a complete relay which includes the grommets. As far as I know, Bosch never sold the grommets as a separate part; they were just part of the relay. Tom was selling these relays not that long ago - I bought some from him, because Bosch re-manufactured them. If your relay is failed, buying a new one includes the grommets routinely.
Art
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Jack English

USA
1065 Posts

Posted - 09/01/2017 :  13:27:50  Show Profile  Reply with Quote
Art & Ron: I would agree with you about the relay being isolated from the container but the second relay from Michael did the same thing. I am using other grommets and plumbing faucet washers to isolate the can from the bracket. I think I will go over to the left side of the engine compartment and test other relay shells for continuity with ground. Tom no longer has the relays. Neimoeller wants US$175 for one.

Jack English
300SEL 6.3 #4768
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Ron B

Australia
11545 Posts

Posted - 09/01/2017 :  20:26:43  Show Profile  Visit Ron B's Homepage  Reply with Quote
They normally run forever because of the quality of them,I have a few here ,used ,if you get stuck . But most finnys,W108's etc etc all used the same part, only the time delay switch can be a bit difficult to track down.
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Jack English

USA
1065 Posts

Posted - 09/02/2017 :  14:11:54  Show Profile  Reply with Quote
Ron, thank you for the offer but I have two (four post) relays. As long as they are not grounded they work. The one that's in now is just left dangling, not fastened to the sheet metal bracket. Window lifts work normally. As noted above, when the weather cools I will fasten it down with plumbing faucet washers. Everyone thinks the original bushing is NOT intended as an electrical ground isolater but it looks to me that is what is intended. Any engineers chime in?

Jack English
300SEL 6.3 #4768
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Aaron H

USA
427 Posts

Posted - 09/05/2017 :  20:25:44  Show Profile  Reply with Quote
Jack, Ron and Art made mention of the fact that the bottom of the relay is a phenolic material....which completely isolates the inner workings of the relay from the metal housing that the inner working sit in. In light of the aforementioned, it would be redundant to isolate the relay housing from chassis ground since it's already isolated via the phenolic base. Suffice to say, and as Ron mentioned, if there is a dead short to chassis ground,then there is a problem in the wire harness, window switches, wiring in doors, or in the relay itself.

The rubber grommets are nothing more than a means to thwart vibrations. The brass sleeves inside of them are only there to provide support for the relay and to act as a stop for the screws that hold them on.

The actual housing of the relays is grounded to chassis ground via the brass sleeves in the grommets. That is to say that the brass sleeves inside of the grommets make constant contact with the bracket(s) the relays screw to. This is why when the relay fails, or anything else that causes a dead short to ground, a fuse will burn. It doesn't matter if the rubber grommets are deteriorated or not....as long as the relay is screwed to its mounting bracket or the car, it will always be a chassis ground. Again, you have a problem either in the relay or in the wiring.

Also, if you're working on a later W108/W109 chassis, there are 2-3 relays for the electric windows. One of them will be up in the firewall that is a round cylindrical shape. Don't forget about that one in your testing.

The previous brings me to another notable point...whether you're working with the old 3 wire system or the newer 2 wire system, or if you're working with a round or rectangular relay, the same will happen if there is trouble down the line.

There are several ways to find dead shorts. Some are quick and risky, and others take longer but are safer. To remain safe, you'll essentially need to check each and every wire that is NOT a ground wire by checking for continuity to chassis ground. If a hot wire has continuity to chassis or relay ground, then something is awry....you will have located the trouble. Luckily, the short to ground is usually a wire in one of the doors, and in more unusual cases, where the wires pass through the "A" pillar to the door.
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