Oil Pressure Gauge pegging

[sr54x4]

I have a new to me 1988 Formula 242 with a 7.4 Mercruiser (rebuilt with 40ish hours on it). The issue I have is the oil pressure gauge reading on my dash. I have the VDO gauges. Key off, the gauge is at 0. Key on, but engine off it pegs. Engine on, at idle it sits at full pressure. At speed 3-4,000 RPMS it backs down to 40-60 depending. Its almost like it works opposite how it should. I know there is an issue because its pegging with just the key on. I am having trouble determining if I have a gauge, wiring, or sending unit problem. Here is the trouble shooting I have done so far.

Pulled the sender wire from the sending unit, and gauge pegs with just key on, engine off.
I have proper 12v and ground at the gauge.
Key on, about 50 ohms at gauge end of sending wire.
At idle, getting about 70 ohms at the end of the wire that connects to the gauge. At about 3,000 RPMS goes up to a little over 100.

What do you think is the issue here?

 

[Bt Doctur]

Pulled the sender wire from the sending unit, and gauge pegs with just key on, engine off.

Theres the problem, with the sender disconnected the gauge should be at 0

 

[IamMe]

Remove the wire from back of the gauge. If the gauge no longer pegs the wire is grounded out somewhere. If the gauge still pegs its a bad gauge.

 

[sr54x4]

Pulled the sender wire from the sending unit, and gauge pegs with just key on, engine off.

Theres the problem, with the sender disconnected the gauge should be at 0

So you think its a bad gauge?

 

[sr54x4]

Remove the wire from back of the gauge. If the gauge no longer pegs the wire is grounded out somewhere. If the gauge still pegs its a bad gauge.

I did this, and when I do the gauge still pegs. so bad gauge then.

 

[tpenfield]

The Formula's of that era used the VDO Blue Line gauges with the rectangular bezel (I believe), which are no longer made. You might find a used one or left-over stock on eBay.

 

[sr54x4]

The Formula's of that era used the VDO Blue Line gauges with the rectangular bezel (I believe), which are no longer made. You might find a used one or left-over stock on eBay.

Can I replace it with a regular gauge? Or will only the VDO work?

 

[sr54x4]

After looking at some new gauges, its also possible my sending unit is not correct. I have been reading that VDO gauges need VDO senders, or they work in opposite. The sending unit I have doesnt really look like the VDO 2 post ones. It just looks like the single post mercruiser one. I will have to pull it and get a part # off it. I read that using that sending unit will make the gauge read opposite and eventually cook the gauge, so I may have 2 problems here. It also has the engine alarm wire and sending wire all on the same single post on the sender, which doesnt seem right.

 

[tpenfield]

The VDO gauges often use the European standard for resistance ranges within the sender units, rather than the U.S standard. So, your gauge reading may be off if mixing a non-VDO brand gauge and the VDO sender. Kind of depends on what you want to mix and match of old versus new. some folks end up replacing their entire set of gauges in order to stay consistent.

I think the resistance ranges are lower in the VDO senders, so they will draw more current through the gauge . . . not sure if it will cook them or not.

Engine alarm wire and the gauge wire should be on 2 separate senders . . . or separate posts on a combination sender (if there is such a thing).

 

[sr54x4]

The VDO gauges often use the European standard for resistance ranges within the sender units, rather than the U.S standard. So, your gauge reading may be off if mixing a non-VDO brand gauge and the VDO sender. Kind of depends on what you want to mix and match of old versus new. some folks end up replacing their entire set of gauges in order to stay consistent.

I think the resistance ranges are lower in the VDO senders, so they will draw more current through the gauge . . . not sure if it will cook them or not.

Engine alarm wire and the gauge wire should be on 2 separate senders . . . or separate posts on a combination sender (if there is such a thing).

Thats what I was thinking. I am going to have to pull the sender and see what it is. It looks like it is a non-VDO. Either way, I think the gauge is bad because it shouldnt peg with the wire off and key on like it is. So I just have to see what sender I have and then either pair a gauge to it, or just start fresh with a new gauge and new sender it seems.

 

[sr54x4]

Thats what I was thinking. I am going to have to pull the sender and see what it is. It looks like it is a non-VDO. Either way, I think the gauge is bad because it shouldnt peg with the wire off and key on like it is. So I just have to see what sender I have and then either pair a gauge to it, or just start fresh with a new gauge and new sender it seems.

On second thought, I believe I have the correct sender, if it reads 70 ohms at idle and works its way up, thats the way the europeans are supposed to work, from 10-180 ohms.

 

[sr54x4]

Put in a new gauge, ran a new wire and still pegging. At this point im thinking it has to be the sender.

 

[sr54x4]

I ordered a new oil pressure sending unit, it was from Ebay, and it was listed for VDO gauges, 80 PSI, 10-180 OHM range which is what my gauge calls for. It didn't specify if it was for auto or marine. Is there a difference? I know things like starters, alternators, fuel pumps, etc. need to be marine specific. But is it safe to use any sending unit that matches the specs for the gauge?

 

[Bondo]

It didn't specify if it was for auto or marine. Is there a difference?

Nope,.....

 

[sr54x4]

Ok, new VDO sending unit is in and gauge works! Thanks for the help so far.

Next issue... I connected the light blue wire to the gauge post on the sending unit. Everything works. Went to connect the tan wire with the light blue tracer line, which should be for the engine alarm, and when I touched it to the post it sparked. Turns out I have 12v going through this wire at all times, key on and off, even batteries switched to off. Is this normal? I actually don't have an engine alarm installed right now, the previous owner took it out for some reason. I was just hooking up that tan wire so it was dangling in the engine bay. A full alarm system is on the "to-do" list. Right now, just need to know if the engine alarm wire is supposed to have 12V? I am having trouble tracing where that wire goes to.