Heat Sensor Ideas

[sqbk1971]

If my research is correct this is the Head heat sensor for the FI that has been removed from my 71 sqr.

Any good ideas of for a useful repurpose of the sensor or the hole?

[beetlejc]

So THAT's what that hole is! Thought for sure it was for a sensor of some sort but knowing it was part of the FI (that has been removed from my car) makes sense now.

Could it continue use as a temp sensor of some sort?

[KTPhil]

Those are NLA, and suitable substitutes have not been found and tested yet. Folks are working on it.

That is a cylinder head temp sensor, and it is not a major driver of fuel mixture. For those still running FI, however, you can't just disconnect it like your clipped off sensor is.

You should substitute a fixed-value resistor that represents fully warm resistance. This will let it run, though it may run a little lean during warmup. A fixed resistor of about 2KΩ is about right. You might get a couple more at values like 1KΩ and 500Ω and then see which works best hot and cold.

You install them to the harness like the stock sensor, and ground the other end.

For your non-FI car I'd just leave it there. The wire is broken off flush and so you probably can't use that sensor for anything. That sensor location is a little slow-reacting to be used for a cylinder head temp gauge.

[raygreenwood]

[KTPhil]

Unfortunately Russ' website is lost, or I could show his table of values, which is more finely graded. All I have now is the lower graph from the Bentley that did a curve fit, causing the odd shape to the lower pink curve, which can be ignored-- the actual level cited is flat but a curve fit makes it "J-shaped at the right.

It shows it leveling out at 1KΩ to 2.5Ω. Maybe you are thinking of TS1 (intake air, upper graph?) with a value as low as 200Ω?

My point is that the car will not run at ALL without SOME resistance... infinite resistance of an open wire means dead rich running and likely a no-start. At least the 1KΩ to 2KΩ values will let it start and run.

The right solution is the type you are working on... a modern sensor that is somewhat tunable. These above are just band aids to get it running.

[raygreenwood]

No...I am not thinking or TS-1.

And...that graph you have for TS-2...is totally incorrect and off by a factor of about 9ut 200F on the temperature line...and missing punctuation from the resistance line on the left. These sensors do not go to 10 or 15 Kilo ohms (thats 15000 ohms). They start out at the coldest at about 2300 ohms.

You will also NEVER find a TS2....that does not drop below 200 ohms in weather that is 65F and above.....nor will you ever see head temps at 150F running temperature.

What that graphic is showing is true....that the sensor can reach down to 250 ohms or slightly lower by 125F.....but the problem is that your heads are heading for temps in the 300-325F range and the TS2 keeps dropping.

What is missing here in that graph is the complete scale and the time factor from start to finish.
Depending on how cold the ambient air is flowing over the heads.....the time factor for a sensor in 32F weather to go from 2000+ ohms to 125° to 150° F.....might be 7-10 minutes. You get a nice slow curve in that respect.

However in 90° weather....that time factor may be 2 minutes. In that short amount of time....the cylinders and case are barely warm....the oil is cold and you run it lean too fast.

These are the problems with the factory system. And these are the problems you get when trying to test and assign real world values to this sensor...when its not installed on the head...wrapped in a mass of metal....with a limited amount of airflow over the outside of the sensor body.

In short...the operative range of the TS2....is about 2300 ohms cold down to about 58 when its as hot as it can get.

For some cars....depending on how your MPS, fuel pressure and timing is adjusted.....2300 ohms in cold weather can be a nearly flooded cold start with a lot of cranking. 1900 to 2000 ohms may work a little better.
Likewise.....on most of the systems I have worked on....again depending on tuning level....when it drops below about 125 to 150 ohms.....while being no leaner than it is at 200-225 ohms......it cannot recover fast enough to add enrichment when needed like right after the sun goes down or after a short shutdown while you are in the store (hot start syndrome).
Ray

[trythis]

Regarding the replacements for this sensor, I have done some testing since I am still using the FI system and plan to keep it.
The Autometer is a 2259 (~$15) but requires an adapter
The Moped meter is from a piaggio or Vespa moped part number 82622R (~$6)
The Bosch is NOS [0 280 130 003-000] No Longer Available. (keeoing mine for testing, not using on my car)

The Home Made sensor was made by another Samba member DougS and you can read the discussion here: http://www.thesamba.com/vw/forum/viewtopic.php?t=6...highlight=


I am thinking that reproducing the exact results of the original is going to be difficult but running one of these $5 or $10 sensors with a resistor inline will get me close enough.
_________________
Laverne, our Squareback project:
http://www.thesamba.com/vw/forum/viewtopic.php?t=664462

[raygreenwood]

[DougB]

Hey all,

I'm the guy that Graham (trythis) was talking about. I did make a homemade sensor as well as located the moped sensor (which I haven't had many opportunities to try in an actual vehicle yet - the one time I did the engine had a hard time starting - which I attribute to me not nailing the parallel resistor that Telford Dorr suggested).

I stopped back at this thread because my '73 FI Fasty is not exhibiting a similar issue that affected my '75 FI Bus (and that went away when I replaced the sensor in that vehicle):

Occasionally I'm getting what I thought was a miss, but I now believe to be a loss of power because it seems to last for more than one cylinder of time sometimes. It seems to happen upon acceleration...the car idles like a dream, and cruises at speed pretty well, too. It happened both before and after I replaced my dist cap, rotor, and plugs (they all needed replacement). I do seem to have the proper fuel pressure (inline analog gauge installed) but can't monitor it while driving as it's attached to the engine. My fuel pump is aftermarket, but because the car drives decently well most of the time I don't believe it to be a major issue. Fuel filter is pretty recent but I haven't checked it for issues yet. I am running a Pertronix ignitor so I know one of the first suggestions will be to replace it with points and see what happens. I might replace this sensor as well.

Anyway, just wanted to see if people think it could be sensor related in this case. I am blessed to have a spare, original Bosch sensor I can use, if needed.

Thanks,
- Doug
_________________
'75 Campmobile (tin-top to SpaceRoof)
'73 Fastback
'52 BMW R67/2
'41 Zundapp KS600
'55 Puch SGS250
A very, very understanding wife

[KTPhil]

If it is an occasional problem, I'd sooner suspect your wiring harness or connectors, rather than the sensor itself.

[trythis]

[raygreenwood]

[DougB]

Hello all,

So I replaced the sensor with a new Bosch one I had and...well...I hate to say it but in 3 separate drives of the car the engine hasn't missed or lost power once (knock on wood). I'm thinking that Ray's point that the connection between the old sensor might not have been the best might apply here. The old sensor wasn't very tight, and anti-seize looks to have been used in the hole in the past. I tightened the new one down a bit more than the old one so maybe that will help. I'm not really sure that the old sensor was bad, maybe just looser and anti-seize-ier than ideal. I'll let you know how things go.

Man, I wish I could go back in time and buy a bunch more of these sensors before they went out of production :-/ I know there are some promising alternatives, but OEM is always best/easiest.

Thanks,
- Doug
_________________
'75 Campmobile (tin-top to SpaceRoof)
'73 Fastback
'52 BMW R67/2
'41 Zundapp KS600
'55 Puch SGS250
A very, very understanding wife

[raygreenwood]

[blues90]