Oil pressure gage acting strangely

[Full Monte]

Hi Guys,

The oil pressure gage goes up to "2" when I turn on the switch but before I start the engine. Doesn't seem right. When I turn on the engine, it goes to "3". I borrowed a temp sensor from a friend and installed it, knowing it worked on his car. It didn't improve things. So the gage operates between "2" and "3". I took out the cluster and looked at it, but didn't see anything obvous to fix. I took photos, too. Do you have any suggestions?

Monte

 

[The Warden]

I would say "replace the sending unit", but it sounds like you already did that.

I would try unplugging the wire from the sending unit and, with the wire hanging, see where the gauge reads. Then, ground the wire and see how the gauge reading changes. If the gauge stil only goes between 2 and 3, the gauge in the cluster is probably bad. I don't know if the gauge can be replaced by itself or if you'll need to replace that entire 1/3rd of the cluster...never tried it.

If the gauge acts normally (should be at 0 with the wire off and 3 with the wire grounded, I believe), I would go ahead and replace the sending unit anyways unles it's ridiculously expensive.

Sorry I couldn't offer more...hope this helps some. Good luck!

 

[Full Monte]

Thanks Warden,

I'll try that and post what I find.

 

[Full Monte]

I would say "replace the sending unit", but it sounds like you already did that.

I would try unplugging the wire from the sending unit and, with the wire hanging, see where the gauge reads. Then, ground the wire and see how the gauge reading changes. If the gauge stil only goes between 2 and 3, the gauge in the cluster is probably bad. I don't know if the gauge can be replaced by itself or if you'll need to replace that entire 1/3rd of the cluster...never tried it.

If the gauge acts normally (should be at 0 with the wire off and 3 with the wire grounded, I believe), I would go ahead and replace the sending unit anyways unles it's ridiculously expensive.

Sorry I couldn't offer more...hope this helps some. Good luck!

Warden,

Got around to doing it today. With oil pressure sending unit disconnected, switch in "run" position, engine off, I get a zero reading. I couldn't ground the wire for the second part of the test because it is a recessed connector.
I did run a wire with an alligator clip at each end to ground and to the sending unit terminal with it connected, and got a pegged reading of 3.
Does this point to the sender again?

 

[The Warden]

Looks like it to me. While anything is certainly possible, that is a strong indication that the sender's bad. That still doesn't explain why the other sender didn't work either, but I would go ahead and replace it and go from there.

I have a friend who works at Beshoff; if you want, he can probably get the sender for you at a discount...

 

[Full Monte]

Warden,
Thanks...I may take you up on that. There's one idea that keeps plagueing me...the old sender unit and the new one affected the needle in exactly the same way. My friend said the sender unit he gave me had been working fine.
All my fooling with the wire connection now has the meter reading 1.5 instead of 2 in "run" position, not started. I'm wondering...could this be a symptom of a bad connection/connector/wire? If it's a "high resistance" wire due to connection/age/heat, would it do this?

 

[towcat]

monte-
you verified that the oil pressure is good with a mech gauge already? If you want to use my test kit, gimme a yell.

 

[The Warden]

All my fooling with the wire connection now has the meter reading 1.5 instead of 2 in "run" position, not started. I'm wondering...could this be a symptom of a bad connection/connector/wire? If it's a "high resistance" wire due to connection/age/heat, would it do this?

That's certainly a possibility, especially since messing with the connector made a difference in the reading.

Would it be possible to take your sending unit and put it on your friend's car and see how it works? I didn't even think about that until just now but that would be a sure-fire way to confirm/eliminate the sender.

Again, I haven't seen an oil pressure sending unit connection on an M-B before (both my 123 and my 116 have mechanical gauges with an oil pressure line running to the instrument cluster), but if it'd be possible to take a paper clip or something similar and clean the inside of the connection, that may make a difference. Also, it wouldn't hurt to do an ohm test between the connector at the sender and the connector at the gauge.

Hope this helps some

 

[Full Monte]

Good idea, Warden....I'm gonna have to get behind the instrument cluster soon anyway.

Towcat, I don't know what the oil pressure is...just that it's sufficient to run it...I drive it daily. I would like to put in a real gage, but that's a "nice to do" project down the road. First item is to get this turkey working.

I'm finding with these old MBZ, I always have a list of things to fix, including things that I thought I fixed, but later turned out not to be. I hate it when one problem turns out to be two or three...