[T3] Bad day at the (FI) office

[Daniel Nohejl]

For resistance readings of the temp sensors, I’ve been going off of the info in this chart

http://classicvw.org/gallery2/v/Fuel-Injection-Manual/Fuel_Injection_035.jpg.html <http://classicvw.org/gallery2/v/Fuel-Injection-Manual/Fuel_Injection_035.jpg.html>

A few years ago, Joe (harness builder and engineer) came up with an equation based on these numbers

=-50.92*ln(64)+469.8

where the number in parentheses is the resistance for the CHTS. I dropped the equation into Wolframalpha.com <http://wolframalpha.com/> and at 64 ohms, the temperature would be 258* which seems reasonable on the top of the head and close to an exhaust port on a warmed up motor at idle. I know it’s not exact, but that’s all I have. Here’s the graph Joe made as well:

http://i.imgur.com/25qfIVM.jpg <http://i.imgur.com/25qfIVM.jpg>

I get low resistance numbers on both of the CHTS I’ve used since installing the new motor.  One of them was used on the previous motor and gave the same low resistance after warm up. Interestingly, in the fall, I discovered that the sheath on the wire of the CHTS I had been using for almost three years was torn and leaving the wire exposed in two places, so I swapped that sensor out for a new one. Right away, I had lean idle hunting and and difficult acceleration (hesitation, some bucking). Since the only change I’d made was to the CHTS, I had to conclude that it was the source of my issues. I measured resistance of 58 ohms fully warm whereas on the old sensor it was around 100 or so. I added resistance inline, made a slight MPS tweak, and drove that motor until installing the new one this winter.           

All the CHTS I’ve ever bought were sold under Bosch part # 0280130012. I wonder if over the last few years there’s been a change in how they’re manufactured so that they can perhaps be used in a wider variety of applications with different cars?

Anyway, yes, all of the readings I’m getting are with the MPS turned CCW (rich) from the pre-set position. Thus, if I keep the resistor inline, I’d have to lean out a bit by turning CW and back toward the original position. If I remove the resistor, then I’d have to go further CCW to avoid running way too lean. 




> On Mar 25, 2016, at 11:16 AM, Jim Adney <jadney at VWType3.org> wrote:
> 
> On 25 Mar 2016 at 8:22, Daniel Nohejl wrote:
> 
>> In general, is 64 ohms too low for the CHTS when fully warm? 
> 
> Yes, the CHTS should be 2000 - 3000 Ohms at room temp and will come 
> down below 1000 when warm, but I don't think I'd expect it to ever 
> get below 100 Ohms. I'm not certain about that, but to come down 
> another order of magnitude in resistance, I'd expect it to have to go 
> up an order of magnitude in temp. That put's it WAY out of the 
> reasonable range. You're unplugging it when you measure it, right? 
> Otherwise, you're actually measuring the parallel combination of the 
> sensor and the brain circuitry. I don't know how you could get 64 
> Ohms. I didn't think Bosch made anything that had that range. Maybe 
> yours is partly shorted, or .... Isn't that a part that you've 
> swapped? Hard to imagine that you could have 2 with the same 
> behavior.
> 
> Somewhere, I have a graph that I made of the temp curves for each. It 
> turned out that the curves were straight lines if you plotted then on 
> log-antilog paper (IIRC.) I can't find that graph right now, but I'll 
> keep looking. If I can find it, I should be able to give you some 
> idea of what temp 64 Ohms might indicate.
> 
> Just for shits and grins, I grabbed a CHTS that measured ~2400 Ohms 
> at room temp and stuck it in ~185 F water. The resistance dropped to 
> just under 400 Ohms. So this resistance falls faster than I expected, 
> but I still think 65 Ohms is too low. What does it measure when cold?
> 
>> Is 64 ohms too low for the air temp sensor when fully warm? 
> 
> No, that sounds about right. For a given temp, that one measures 
> about 1/10 as much resistance. And, on your car, that sensor actually 
> senses the oil temp. It sits in a place where it gets splashed by the 
> cam gears.
> 
>> I´m not using a K-jet manual....I was talking about  the manual that
>> Russ Wolfe often posted pages of on The Samba and on his website. Tram
>> uses it as well and refers to it as a workshop manual they used at the
>> dealership. All the section headings I´ve seen begin with "K" so I
>> just called it the K manual. Like these for example (they?should link
>> to Photobucket?: 
> 
> Ahhh, Okay. THAT K manual. K stands for Kraft: German for Power. It's 
> the portion of the dealer service manual set that deals with carbs 
> and FI.
> 
>> I think that in my frustration I might not have been clear. The meter
>> isn´t giving conflicting info. It´s telling me I´m rich with the
>> resistor installed at the CHTS and this is in accord with my poor fuel
>> consumption. 99.9% of my driving the last month has been in this
>> state: resistor installed, bad MPG, rich AFR readings. 
> 
> All done with the pressure sensor tweaked, right? What happens if you 
> put the pressure sensor back to where it started and leave the 100 
> Ohm resistor in?
> 
> 
> -- 
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
> 
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