LH relay won’t engage
 

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what was the last thing you fixed or washed?

What Stan said.

LH Relay Won’t Engage
 

Hi,
Please forgive my ignorance of acronyms. What does “LH” stand for?

The 'LH' stands for German: "Luftmasse-Hitzdraht" - the hotwire anemometer technology used to determine the mass of air into the engine

To expand on that, you will often see LH used to refer to the the LH Jetronic ECU, which sits next to the EZF or EZK ( Ignition) ECU in the passenger footwell.
https://cimg3.ibsrv.net/gimg/rennlis...cffaf6770e.jpg

What is the battery voltage at the front PS terminal? Not that you're suffering from a bad grounding strap in the trunk....

Weak ignition is never good. I'm not surprised, that the LH-ECU refuses to work. The master in the system is the EZK, it provides a signal to the LH-ECU and tells it to wake up and start working. I guess that the necessary signal is as weak as your spark. This means "no go" für fuel. Solve the EZK issue first.

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Post 2 thats a serious question please answer it

The LH relay on my '89 is actuated by the LH box itself, by grounding terminal 85 on the relay from LH terminal 30 (black with violet tracer) via V14.

Power supplied for the relay contacts and the fuel pump relay (shared feeder...) comes from the battery positive terminal, and one of the smaller RED wires that connects to the battery terminal there. Power through that feeder comes into the CE panel at U12.


Since your symptom includes both the fuel pump relay (XX in my 1989 car) and the LH-Jetronic relay (XXV in my 1989 car), I'd start my troubleshooting with those small feeders and the connections at the battery. The ring terminals are tinned brass, so get a gentle scrub with a brass toothbrush until they are no longer dull. Avoid scrubbing the tin plating off. Do all the terminals while you are there with the battery disconnected, loosening, disassembling, cleaning, reassembling and snugging each one. Once everything is working correctly, coat all the connections with some Vaseline to keep the battery acid vapors away.

While you are there in the battery well with the spare wheel out, take a few minutes to clean the ground point in the forward area of the spare wheel area, hidden by that black plastic cover and one nut holding it. Inspect the battery ground strap. Then replace it if it hasn't been replaced already. The copper braid inside the insulating sleeve rots from the acid fumes from the battery, so a new tinned-braid replacement with an insulating sleeve should be in there. You can make one or get one from our favorite 928 parts vendors here.

Getting full voltage to the LH and fuel pump systems will help immensely. Be aware that the power feed to the LH and through the fuel pump relay is not fuse protected, and is hot so long as the battery is connected. Use Appropriate Care when dinking with this wiring and the relays.