Re: 75 t-spark motronic 4.1

[jonathan coates <alfa@domain.elided>]

In message , Lars Arvidsson <arvid@domain.elided> writes
>[John wrote]
>75 T-spark idles Ok  but takes a couple of over shoots to settle down
>however it is then stable.
>when opening throttle there is a lag then surging up to desired revs
>dropping back to near 1000rpm
>anyone with any ideas?
>I haven't had time to look at it yet hope to check it out tomorrow
>
>[Lars replies]
>I suggest check the throttle position switch, it's located on the the
>throttle housing.
>That switch tells the ECU when you're in idle and when not.
>If the switch says "idle", but the throttle is opened, the ECU tries to
>maintain the rpm at 850
>by means of adjusting the idle control valve (that silver cylinder,
>located at the plenum chamber)
>But when the ICV is fully closed, and the rpm still is too high, the ECU
>cant cope with it, the result is you get far too little
>fuel, then the enginge is surging. The situation is like a air leak at
>idle, because the ECU doesn't know that it's the throttle
>that's letting the air through.
>I replaced mine a few months ago, it was intermittent failing, mainly
>when hot. Costed about 120 USD.
>If you're brave, try to open it, inside is an ordinary microswitch, wich
>costs just a dollar.
>The other failure mode is, of course, the opposit; the TPS doesn't tell
>the ECU that the throttle is in idle position.
>Then you get no idle at all, no extra air through the ICV. If the TPS is
>sticking, both types of failure often occour.
>
>It's easy to check, and to replace.
>Check: First, with no engine on, slowly open the trottle by hand and
>listen. There should be a "clic" just above idle position.
>If not, replace the switch.
>To measure: remove the air filter cover, air flow meter and rubber
>manifold. Disconnect the connector from the TPS.
>use a resistance meter, measure between the middle and one of the other
>terminals (cant remember wich one, try both). At idle position,
>there should be zero ohm, above idle infinite. Repeat the checking
>several clicks, they can be eye-servants...
>
Just my observations:-
The switch is easily replaced if you are careful. I did mine recently,
fortunately I had a suitable switch lying around out of a dead
controller so I didnt find out where to get a new switch.
However, mine never did click, even when it worked OK. The other switch
in there, the throttle nearly full open switch clicks though.
Hacking open the cover of the switch housing is no big deal as there is
nothing very delicate in there close to the edges of the case.
Taking the throttle and switch off involves disconnecting the coolant
pipes, so find something to plug the end which doesn't connect to the
header bottle. Remove the four socket head screws holding the throttle
to the plenum, watch the screws though as they are differing lengths.

- -- 
jonathan coates

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