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Discussion Starter #1 (Edited)
Had some time today, so I decided to check the correlation between the Alfa dash gauge and the aftermarket sending unit I got from AlfaHill about nine months ago. I believe the sending unit to be a SunPro CP7577 (std 240-33 ohm, 1/8" NPT) Although the OEM gauge is a straight thread, the SunPro unit's NPT screws-in, snugs-up, and seals fine. 240-33 ohm spec is generally a standard spec for aftermarket gauge units.

UPDATE Note: The "SUNPRO CP7577" brand is no longer available except via new-old-stock (NOS). Check ebay. There might be some left for sale there. Otherwise, I believe that the "Bosch FST7577" is exactly the same unit as available in the JEGS and Summit racing link below:

http://www.jegs.com/i/Bosch-Actron/885/FST7577/10002/-1?parentProductId=1528585
https://www.summitracing.com/parts/bgs-fst-7577

The sending unit has been working flawlessly, however, I noted that with the engine off the dash gauge read about 7 psi. And at speed, the gauge seemed to read slightly higher than I would expect.

I could not find the specification for resistance values for the Alfa dash gauge, so I fabricated a test tool from a 1k Cermet Variable Potentiometer (Radio Shack #271-0342, $3.19) soldered to test leads. The POT is very sensitive and finely adjustable. Using this and a multimeter, I was able to adjust the POT's resistance to set the gauge needle on the division markings, then use the multimeter to measure the resistance of the POT.

Further, I contacted SunPro and requested the resistance specs on the 7577 sending unit, which they sent me within minutes of my initial email. Below is the correlation between the aftermarket sending unit and the Alfa dash gauge. As you can see, they do not match exactly, but are fairly close. In general, with the 7577, the dash gauge will be slightly optimistic (reads higher than actual oil pressure) at normal pressure ranges. At the higher end of the gauge (where the oil pump can't achieve that pressure anyway), the dash gauge will read lower than actual oil pressure.

To bring the sending unit to more closely match the dash gauge specs, and therefor more accurate readings, I installed a 22 ohm 1/2w resistor (Radio Shack 271-1103, $1.19 pk of 5) inline between the sending unit and the wire going to the dash gauge. Now the dash gauge reads slightly lower pressure than before, but more accurate. Also, keep in mind that the low pressure warning light is not affected. It runs on it's own sender and circuit. Min hot idle oil pressure is 7 psi.

To be sure, the SunPro sending unit is well useable as is (without the resistor) as long as you're only relying on it for general engine monitoring, and not for more accurate purposes. While my car is an "S", I'll go out on a limb and say the L engine oil pressure gauge is the same. Note the fairly large accuracy tolerances at the lower end of the scale.

It would appear that any 240-33 ohm sending unit can be adapted to the Alfa 164 dash gauge. Here's a link to a list of similar sending units, although the SunPro unit is half the price and works well.

http://www.egauges.com/0-100-PSI-240-33-Ohms-U-S-Pressure-Senders-s/23232.htm LINK NO LONGER GOOD.

For those of you out there (and there's a lot of you) running around with oil pressure gauges that are not useable due to bad sending units . . . . no more excuses. Git'r done!

Here are the official values for oil pressure from the Alfa Factory Mechanical Manual, page 1-95:

Engine at running temperature (194F)
800-900 rpm . . . . 11.6 psi
5000 rpm . . . . . . 65.2 psi

Here are the spec values for the OEM sending unit from the Alfa Factory Mechanical Manual, page 1-86:
0 psi = 290-320 ohms
6 psi = 270-290 ohms
58 psi = 103-133 ohms
116 psi = 0-25 ohms


I crosschecked the values of the Sunpro sender w/22 ohm resistor against a mechanical gauge and they matched within a 2-3 psi. Here's the matrix with the values:
 

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I also have a sender from Alfahill, 2 actually. I installed one in my 164S. Unfortunately, it is suffering from the bad insulator that Goats reported on. The needle bounces up and down. At one point, I got so frustrated with senders that I plugged the hole with a bolt:)
 

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"At one point, I got so frustrated with senders that I plugged the hole with a bolt"

Gee, my "fixed" ones for my 164's have lasted fine now for at least a decade or more, lol.

Yes, I can understand being frustrated if something goes wrong. You just had a little bad luck. Put a plug (sheet metal screw) in the hole you drilled to plug that hole, but the sender should work ok. If not, then you had something else break within the sender, or the sealing washing inside blew out, instead of just leaking a little.
 

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That one is long gone to sender heaven. I've got 5 new ones in stock:) Shows how much confidence I have in them.
 

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But Mad North-Northwest
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I did Del's drill trick in my Milano sender a few years ago. Still working well.

The 5 year old sender in the Spider started acting up recently as well. Drilled that one last weekend and now it's good again.
 

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Nice work Roadtrip!

Thats an A+ job in my book --
 

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Yes, it is very interesting. Looks like that in the working range of about 35-40 psi, one might use a 150 ohm resistor. Do I read that correctly?
 

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Roadtrip, I applaud your meticulous and valuable work with the sender. Can a similar approach be taken to correct (or adjust) inside cabin temperature? For example, on both my 164s (one now retired), when ambient temperatures were below 50 and you have the heater on, "64" is too hot for me (seems like 75) and the notch below ("low") is too cold. Might it be possible to add a resistor somewher to "fool" the temperature gauge in the overhead console?
 

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The way I read it, with a 22ohm resistor added, at 40 psi, the gage reading is spot on.
 

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Ok, I was reading the diagram wrong. I'll maybe give it a try to see what happens, although I like seeing a higher pressure value, lol, even if it is not spot on.

I do tend to use the gauge as just a general "state of things" indicator rather than looking for specific values. I like it to read something, however, chuckle.
 

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I'm afraid I don't pay much attention to it:(
 

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My first Alfa, the Giulia Sprint GT, actually had not only an oil pressure gauge, but an oil temperature gauge as well, as stock. It was most interesting to see just how the temperature rose, depending on how the car was being driven, and what the weather was like outside.
 

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Does anyone know what the thread size is for the OEM Alfa oil pressure sensor? I'd like to use an adapter bushing rather than screwing the aftermarket 1/8" NPT sensor directly into the block. I found the www.egauges.com site mentioned above carries adapter fittings, but I'm not sure what size to order.
 

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Discussion Starter #14
I believe it's 1/8-27 straight thread, but why introduce another joint when the sender screws into and seals directly in the engine casting.
 

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But Mad North-Northwest
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1/8" diameter? That's way smaller than the OEM one I have in box. I thought it was M10?

I could be wrong, though. I can measure tonight when I get home...
 

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It is straight thread M10x1.00 on OEM Alfa oil sender

1/8" diameter? That's way smaller than the OEM one I have in box. I thought it was M10?

I could be wrong, though. I can measure tonight when I get home...
Yes Alfa OEM sender it is straight METRIC thread M10x1.00 fine threads on oil sender not 1.25 or 1.50.
 

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I believe it's 1/8-27 straight thread, but why introduce another joint when the sender screws into and seals directly in the engine casting.
I wasn't sure how well the NPT threads on the sensor would mate with the block. I just didn't want to mangle anything in case I switch back to the OEM sensor in the future. Sounds like it is not a problem though.


1/8" diameter? That's way smaller than the OEM one I have in box. I thought it was M10?

I could be wrong, though. I can measure tonight when I get home...
That is 1/8" NPT. Approx. 10mm outer diameter. I believe NPT is loosely based around the inner pipe diameter.
 

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But Mad North-Northwest
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NPT 1/8 is tapered thread and 27 TPI. Per the NAPA auto catalog the oil pressure sender is straight thread M10x1.0 as Steve said.

NPT might be close but it's not correct: 9.9 vs 10mm, 27 TPI vs. 25.4, tapered vs. straight. Not sure I'd try that as you're likely to bugger something up when you tighten it.
 

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teflontape

give a couple turns teflon tape, and really you only need to tighten it hand tight
 

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It seems to screw in easily and seal well. I've had mine in for about a year with no leakage. As Goats says, teflon tape and hand tight.
 
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