1990 MB 560 SEL W126, surging/stalling/Wildly fluctuating tachometer - MBWorld.org Forums

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1990 MB 560 SEL W126, surging/stalling/Wildly fluctuating tachometer

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Old 07-04-2007, 09:45 PM   #1
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1990 MB 560 SEL W126, surging/stalling/Wildly fluctuating tachometer

In Canada, I own a 1990 MB 560 SEL W126. I have had years of faithful service from this car but whilst on a recent trip of about 50 km, at approximately 100 130 km/hr, the car started surging (as if fuel starvation was occurring). Shortly after this started, I encountered a traffic jam with speeds of 20 60 km/hr. The surging continued, with complete engine cut out. I had to restart repeatedly while crusing. This pattern continued while stopped at traffic lights as well. I nursed the car home. I took the car to the mechanic the next day. Of course it started and performed as if new, on the way there, while there and for a following lengthy trip. Since no symptoms presented, I had to describe in detail the symptoms of the problem. The mechanic suggested he thought it might be the fuel relay. He located a used one and changed it for the existing one (to ensure it worked). We put the old one back in and I was on my way. 2 or 3 subsequent trips were uneventful (except for my cracked radiator top hose connection. No overheating. I replaced the radiator).

Symptoms are

1. Wildly fluctuating tachometer. Anywhere from 0 3000 rpm. This lasts
for up to 20 seconds.
2. Although tachometer fluctuating, engine working normally.
3. Fluctuating tachometer eventually registers 0 rpm and stays there.
Meanwhile the engine hunts with actual fluctuating rpm. Although my foot is not on the accelerator and the car should be idling. After about 10 seconds at 0 rpm (tach), the engine stalls
4. The engine will immediately restart, by key turn, but will stall
either instantly or by repeating the sequence above.

On my last trip, when this occurred, I switched to the borrowed fuel relay.
This did not change the symptoms and I had to restart the car about 30 times to get home.

Please help.
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Old 07-23-2007, 05:28 PM   #2
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Souds like the OVP (over voltage protection relay). It is located on the firewall between the ABS computer and the brake booster.
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1990, 560, 560sel, engine, fluctuating, mb560, overheating, ovp, r129, relay, sel, stalls, surging, symptoms, tachometer, w126

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