Sender-gauge mismatch on 3.6 swaps

[Jack Olsen]

This is a minor issue that's been around for a long time -- so maybe someone has solved it. When you put a 964/993 3.6 into an earlier car, you get doubled oil pressure readings when the later oil pressure sender sends a signal to the older dash gauges. Not a huge problem -- you know when you have pressure and you know when it goes low.

But I recently ordered a replacement sender after mine failed, and I accidentally got one with M10 (x1) threading where the sender goes into the temp/pressure sensor console. It's supposed to be an M18 (x1.5) thread for the console, so I'm going to have to get the right one.

The senders seem to come in 0-5 bar and 0-10 bar configurations, although that doesn't seem to me like the thing that is causing the mismatch. The sensors probably send different signals out (measure in Ohms) to the gauge.

But can anyone tell me why I can't run an early sensor (like 901 741 551 02) with an adapter to convert their threading to M18?



Previously, Ingo on this board talked about using a resistor to modify the signal from the old sender to the new gauge, but I don't think anyone has ever bothered to work that out. Jim Patrick sells some sensors that use an M10 to M18 adapter.

Does anyone know what the threading was for the earlier sensors? Is it M10? I just can't see why swapping in the earlier sender with an adapter wouldn't work. But that also seems like too simple a fix.

Did the early single-pole senders have a different accommodation for the no-pressure light? I can't remember.

[mreid]

You can, but I just ordered the earlier sensor with the correct threads and screwed it right in.

[Jack Olsen]

Are all the VDO sensors 10-184 ohms of resistance? Is the difference simply whether that range is applied over 0-5 bar or 0-10 bar?

[Jack Olsen]

Well, tentatively, I'm going to say that it IS as simple as switching in the earlier sender and using a brass adapter to make it fit with the 3.6 console. The older senders are M10 and the newer ones are M18. The older senders us the 10-184 ohms to cover a range of 0-150 psi, and the newer senders use the same 10-184 ohms over 0-75 psi. It seems to be that simple.

The complicated part was doing the swap without a partial engine drop on a '95 engine and my own remarkable ability to somehow crack the aluminum console during my initial re-installation. So I had to wait a few days for a new console piece, but now it's buttoned down and I'm going to do a little leak-checking next week.

I suspect one could get a VDO sensor that is 0-150 psi with M18 threading -- they make a lot of them. But I didn't find it.

[Tom '74 911]

Originally Posted by Jack Olsen

The senders seem to come in 0-5 bar and 0-10 bar configurations, although that doesn't seem to me like the thing that is causing the mismatch.

I could be wrong, but I think it may be that simple?

I can't remember exactly which sender I have in my car ('74 w/3.6L), but I do remember that the first one showed double the oil pressure as you mention. I swapped senders and the gauge now reads as it should. the 0-5 and 0-10 bar distinction rings a bell with me. I will try to look back through my receipts to see if I can find part numbers... but it sounds like you are on the right track anyway.

Tom

[Trackrash]

Jack, did you get a dual sender like you showed in your original posting? If so what is the part number?

I ordered what was shown in the catalog as a dual sender, but received a single sender.

Thanks,

[Jack Olsen]

Yes, mine has both the pressure reading and the failure light circuits. I got mine from Patrick Motorsports, but it's possible that it's the same early-type VDO unit I initially got from Pelican (which didn't come with an adaptor).