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Question OK to ground Temp Sensor II as a test?

1974 914/1.8, AFC L-Jet. Tested my Aux Air Valve; closes fine on bench in a few minutes when 12V applied. Wanted to see if TS-II was at fault by grounding it and seeing if I then had 12V at the connector to the AAV. resistance at operating temp is supposed to be about 200-400, so I want to be sure the computer will be OK with a value of 0.
Also, if I am reading the waterfall diagram correctly, there should there be 12V all the time at the AAV connector/pin 12 of relay when the fuel pump is running. So, does the ECM provide the ground for the AAV once it sees that the engine is up to temp?
Old 10-29-2001, 06:03 AM
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BTW: which head it TS-II on? On the top or the bottom of the engine? TIA.
Old 10-29-2001, 06:04 AM
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The TSII is on the right-side head near the #3 cylinder. It is accessible from the top of the engine bay.
Old 10-29-2001, 07:38 AM
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yes on a 1.8 grounding the cht will cause it to run rich vrs the djet will run lean
on d jets people put a resistor to run rich and ljet the resistor causes lean

and it is top of cylinder 3



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scott thacher
75 1.8 on the road again
75 under rust
(fine german rust)
Old 10-29-2001, 01:03 PM
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Grounding the CHT should cause the 1.8 L-jet FI to run lean, just as it would the D-jet EFI.

The AAV is, I think, grounded through its case in the 74 cars. The 75 may have another ground, but I think the 74 is like the 70-74 1.7s and 2.0s in that the AAV is grounded through its case.

--DD

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[This message has been edited by Dave_Darling (edited 10-30-2001).]
Old 10-30-2001, 09:30 AM
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I'm not sure what the early AAV looks like, but mine looks like a drum on a cone that sits on its side, with an L-bracket attached to the drum to mount the whole thing. The inlet air comes in one side of the drum, and goes out (to the intake) from the other side. The two wires (two is why I think one is ground) plug into the end of the cone. It definitely doesn't look like the pic of an AAV on the Pelican site, which is more like a little barrel with both hoses on top. If it helps, it's the same as the later Bus, because I got some L-Jet FI parts from a Bus and the AAV is the exact same Bosch number. I was able to get it to close with 12V by connecting to the two spade connectors sticking out the end of the cone.
From what I can see on the early D-Jet wiring diagrams, the AAV was grounded all the time, and the 12V+ was supplied as long as the fuel pump relay was engaged. The L-Jet looks like it also has 12V+ power when the pump is running, but the ECU only grounds it once the head temp gets to the right point.
Old 10-30-2001, 11:31 AM
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I don't have the diagrams here in front of me for the 75. However, Pelican shows the 74 diagram in the Electrical Diagrams section. It shows the AAV grounding directly, without going through a wire. That means it grounds through its case.

Check the resistance between the two wires in the AAV. If it is around 30 ohms, then one is power and the other ground. If one or the other of them shows zero resistance from the pin to the case, that is ground--otherwise it doesn't matter which is which.

--DD

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Old 10-30-2001, 01:16 PM
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dave, from one of the few old time mechanics
i have spoken to said grounding the 1.8 cht ( same part ) would cause it to run rich

and so does one of the books i have i will find and email it to ya

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scott thacher
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(fine german rust)

[This message has been edited by scott thacher (edited 10-30-2001).]
Old 10-30-2001, 01:25 PM
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Quote:
Originally posted by scott thacher:
dave, from one of the few old time mechanics
i have spoken to said grounding the 1.8 cht ( same part ) would cause it to run rich

and so does one of the books i have i will find and email it to ya

Well, considering that all of these sensors are NTC (negative temperature coefficient) resistors, meaning that the resistance decreases with increasing temperatures, AND we know that a cold car wants a richer mixture than a warm car, then a LOWER resistance will give a LEANER mixture... right? So....grounding the sensor should produce a leaner mixture.

Brad Anders

Old 10-30-2001, 02:45 PM
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Well, I managed to start the car without reconnecting the TS-II, and it was DEFINITELY rich. It starts at about 4,000 Ohms and drops with increasing engine temp, so I think everything there is OK and lower resistance = leaner running. I also got 12V+ and a ground across the plug, so that circuit checks out OK and I should he all set. Thanks for the help.
Old 10-30-2001, 06:28 PM
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