Strange TPS behavior

Hello everyone,

I am facing a strange behaviour on a Volvo 1800ES MY72.

Preliminary checks show that every and each part of D-Jet injection is the right one (correct Bosch ID).

The car has some hesitations at low speed / low load. It starts fine and runs quite well. CO is around 2.3% at idle.

Pressure sensor was not holding vacuum (from -0,4 bar to almost 0 in 20 seconds), I changed it for a good (electrical and pneumatical) one.

In order to find the culprit of those hesitations, I tested the TPS, at first on the engine with ignition ON and fuel pump fuse removed. 

Here is a video of the test (please raise up the sound level in order to hear the clicks) : 

The problem is that the injectors are triggered either on acceleration and deceleration, although the internal switch that cuts the circuit for deceleration works well. I cleaned the TPS tracks gently with electrical contact cleaner, with no success (same behaviour).

Wires on the car harness correspond exactly to the numbers engraved on the TPS, so the TPS is correctly wired.

To confirm this problem I removed the TPS from the engine and tested it on the table.

Here is the video link :

This time, pulses are only generated when turning the axis CCW, means acceleration. When turning it CW (deceleration) no pulse is generated.
Close examination of the TPS contacts and other elements did not show any failure 

I removed the ECU to check any strange thing around it but everything is clear and clean (at least outside and around the connector).

The question is : what makes the TPS do different things when on the table and on the engine ? As it is directly wired to the ECU, I would incriminate the ECU.

Any suggestion or correction would be greatly appreciated !

 

Hi Pierre,

a TPS cannot principally work different inside and outside engine compartment. TPS should not give any pulses on pin 9 and 20 if gas pedal ist released.

I also have seen some continuity between 9 and 20 during sweeping CCW and CW. What happens in the ECU if 9 and 20 are connected ?

I progressed today :
- after deep cleaning of the sweep contact on 9 and 20 tracks, engine hesitations are still present
- unplugging the TPS eliminates any hesitations. Accelerations are smooth
- unplugging only 9 and 20 to keep idle contact gives same results as previous test : no more hesitation, smooth car accelerations 

I suspect the contact that sweep on tracks 9 and 20 to be worn, so that its contact surface permits to contact at the same time 9 and 20. When checking this hypothesis with a multimeter between 9 and 20 it rings when sweeping.

Here is a picture as zoomed in as possible of this contact :



As I own a NOS TPS that has never been installed, I will try to take a picture of a perfectly new contact, for comparison.
In this situation, changing the PCB will have a limited effect... what is your opinion about that point ?
 

Hi Pierre,

if you have shortcut between 9 and 20, yuo have to remove. It must not be the case.

Hallo, 
Ich empfehle, die Platine zu ersetzen und meine Modifikation mit einer zusätzlichen Diode hinzuzufügen. Siehe meine frühere Nachricht # 18333:

Hallo,Zeit kurzen war das ruckeln bei konstanter Geschwindigkeit bei meiner R17 Gordini wieder zu spüren. Beim Test der DKS, Zündung eingeschaltet, Motor lauft nicht, waren beim öffnen UND schließen der Drosselklappe die Injektoren zu hören. Dies war auch der Fall wenn der 0V (Masse, Leitung 12/14) nicht angeschlossen war !  Also ist meine Schlussfolgerung das bei Bewegung der Drosselklappe, das Schleifkontakt über die Leiterkartespuren 9 und 20 (Eingängen zur „Flip-Flop“ Schaltung im ECU) kurz eine Verbindung verursacht zwischen 9 und 20 und damit den „Flip-Flop“ Schaltung im ECU aktiviert und dementsprechend Benzin Einspritzen lässt.Mit ein zusätzlichen Diode in Leitung 20 lässt sich dieses Problem lösen.  Nach Modifikation mit einer 1N4148 Diode, waren keine Injektoren zu hören beim Schließen der Drosselklappe. Beim öffnen der Drosselklappe haben die Injektoren funktioniert. Ich könnte mich vorstellen das die Schleifkontakte durch ein wenig Verschleiß etwas breiter werden und damit kurz 9 mit 20 verbinden. Mit der Diode wird es wieder funktionieren wenn nur nach Maße geschaltet wird über Schleifkontakt und Beschleunigungsschalter im DKS.Habe ein Bild dabei wie die Diode angeschlossen wird.Wurde mich freuen auf eure Meinung und Erfahrungen. Gruß,Ton
  
 

Hallo Ton,

ich sehe das anders. Beides ist in den Fällen, die ich bisher hatte, nicht nötig. Mit gutem Reinigen und Polieren der Kontakte habe ich bisher noch jeden Drosselklappenschalter wieder hin bekommen.

Und die Schleppschalter werden mit einer neuen Platine auch nicht ersetzt.

Also; Bangemachen gilt nicht !

Thank you for your replies.
I also assume that wear has extended the spherical contact so far that contact between 9 and 20 can occur.
I have found a brand new TPS that has solved the problem. So I can now study the possibility to replace the board. I would like to measure the contact surface diameter to objectivate the contact wear.

Ton, I am interested in a picture of the diode installed on your TPS. Thx !
 

You find the schematic here: jetronic.org/de/forum/jetronic/82-volvo-...er-platine-dks#18333

Regards
Norbert

Danke Norbert.

Thank you for this interesting tip. I'll test it next time I have the jerking problem.
I also notice that from your experience this problem occurs more on Volvo than other cars with D-Jet. That's an interesting point, although I have no idea about the reason why.

Heard about this problem from Volvo owners. This is the summary by AI:
Wear of the throttle valve axle (shaft) in a Volvo P1800 is a common issue, primarily resulting in a vacuum leak that causes poor idle quality and difficulty in adjustment. The solution usually involves installing replacement bushings in the throttle body or carburetor housing.
Symptoms and Causes
Symptoms: Excessive play in the throttle shaft can prevent the throttle plate from closing completely, leading to an inability to set a proper low idle (high idle speed), intermittent idle issues, or the engine stalling.
Cause: Over time, the constant rotation of the metal throttle shaft wears down the soft aluminum or pot metal housing (or the original brass bushings) where the shaft passes through the body. This wear creates a gap, allowing unmetered air to be sucked into the intake manifold, creating a vacuum leak.

New axle info: www.v1800.org/index.php/en/bosch-dj-en/d...eparatursatz_en.html

Yes sometimes the bushings are a little bit worn but how does this affect TPS ?