Bouncing Oil Gauge
 

At idle the needle bounces up and down. Engine idles great. Is it time for a new oil pressure sensor?

I'm not sure on your 1989 420, but on the W140s this happens, and it is the gauge making a false contact on the instrument cluster. The gauge just probably needs to cleaned with fine sand paper to clean the oxidation on the pin terminals. The gauges plug into sockets on the instrument cluster.The same may need to be done to the socket on the instrument cluster.

Best of luck,

Martin

OK Thanks

if you replace sending unit buy new one from dealer only.I made that mistake,so mine doesn't work well.

Do you think I need one?

more than likely,they are made to be replaced.But first pull out you instrument and check your wires.I added a extra ground wire to my instruments,just in case.Or you might use electrical contact cleaner first on your sending unit to make sure its just not oily.Mine is still working with 343000 miles

W0W Expensive.

find out what brand dealer sells then look on ebay I buy 99 percent of my parts there

OK Will do. thanks

My '91 300SE did that: it was just the slip-on wire connection at the sending unit. Sanded the pin and squeezed the connector a bit, all good.

DG

My '86 420 has done this since I got it in December 2003. A rather rhythmic bounce during idle. it pegs at max at anything above idle.

Mine quivers a bit when warmed up, on my SD it would bounce all over the place until I changed the sender unit. Key is to use a VDO one, no aftermarket!