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LH relay won’t engage

 
Old 04-11-2019, 04:06 PM
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Shark2626
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Default LH relay won’t engage

hello!

i’m stuck and looking for direction. I have an s4 that is acting like it has a bad cps, it cranks strong but the fuel injectors are not firing and the spark is super weak, not visible. the fuel pump needs to be bridged to work, but it’s pumping lots of gas. I have changed the cps, twice, both are new bosch 944 units, but with no luck with either unit. the tach will bounce, but not constantly, just at first.

the car will run, just barely, with a shot of starter fluid. i had the lh tested and it’s good, but the lh relay does not close when I crank the engine. the relays are new and the grounds are clean.

wondering... can a bad ignition switch cause symptoms like these?

Last edited by Shark2626; 04-11-2019 at 04:38 PM.
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Old 04-11-2019, 07:54 PM
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Mrmerlin
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what was the last thing you fixed or washed?
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Old 04-11-2019, 08:11 PM
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Randy V
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What Stan said.
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Old 04-12-2019, 07:09 AM
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Ed Petry
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Default LH Relay Won’t Engage

Hi,
Please forgive my ignorance of acronyms. What does “LH” stand for?
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Old 04-12-2019, 08:23 AM
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FredR
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Originally Posted by Ed Petry View Post
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
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Old 04-12-2019, 09:55 AM
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KenRudd
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Originally Posted by FredR View Post
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.

Last edited by KenRudd; 04-12-2019 at 12:22 PM.
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Old 04-12-2019, 11:58 AM
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Schocki
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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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Old 04-12-2019, 12:24 PM
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Shark2626
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Originally Posted by Schocki View Post
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.
sorry if I am measuring the wrong terminal, but I assume that you are referring to the jump post at the front of the car, passenger side? if so it is getting full battery voltage, 13.15 volts at the moment.

i am going to go back to the CE panel and the two computer connectors to do some quick checks again with a multimeter, but I’m curious how the ignition switch works with regard to the three key relays???
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Old 04-12-2019, 12:41 PM
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Post 2 thats a serious question please answer it
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Old 04-12-2019, 09:51 PM
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dr bob
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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.
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