Disconnect Vaccum Line Marelli Ignition

[Michael J.]

I was experiencing a ragged throttle response on a 90 XJS conv. with V-12 and Marelli ignition. Throttle potentiometer voltage was checked and found to be good. Idle was smooth, although high (1200 rpm). Open throttle and the engine got rough like a bad throttle pot.

I disconnected the red vacuum line for the Marelli ignition module and everything smoothed out. My guess is that the sensor for the ignition ECU that advances or retards timing is shot. I'm now trying to find another ignition ECU. Reconnect the vacuum line and the problem occurs again. /ubbthreads/images/graemlins/confused.gif

 

[John S Farrington]

Michael, I am not folllowing your description of the red vacuum line to the Marelli ignition module. Are you referring to a red vacuum line coming from the rear of the right (A bank) intake manifold to a firewall port at the right rear engine compartment? Let me know what line you have referrence to and I will research the problem. The main ECU for the engine management controls timing in Marelli ignitions.

 

[Michael J.]

John;

The red 1/8" diameter line from the back of the R/H intake manifold does connect to the Marelli Ignition ECU P/N DAC5871 in the R/H passenger footwell. It connects to a manifold pressure sensor on the ignition ECU and I think that it controls ignition spark advance/retard in some way. I have another Fuel Injection ECU and swapped it out, but did not correct the problem. I have located another Ignition ECU and am waiting for it to arrive.

The symptom is ragged throttle just like when the green coolant sensor on the R/H Thermostat housing goes bad. With the Marelli ignition there is a wire connection between the ignition ECU and the Fuel Injection ECU, but I do not understand the theory of operation yet.

If I disconnect the red vacuum line and plug the manifold vacuum port, the problem goes away, except for at higher RPM.

 

[John S Farrington]

Michael, I now understand the vacuum rred line and the ECU. I have not had this type of problem before but you have done a good job of isolating it. It sounds like either the Marelli ECU (ignition) or the vacuum sensor in line is the culprit. The FI ECU is receiving an erroneous signal. Try replacing those componenets in turn.

 

[Michael J.]

Well...

The replacement Marelli module that I was able to get is from a 91. There does not seem to be a difference in the explanations in the factory book describing 90 and 91 model year ignition system and fuel management systems theory of operation. There are changes for 92.

The replacement module is identical in appearance, but the p/n on the 91 box is DAC 7485 rather than DAC 5871. The 91 also has p/n MED 122J on the label, whereas the 90 has p/n MED 122C.

Stamped into the 90 box on the end opposite of the connector is JA/12/P1/TC/01. The corresponding number on the 91 module is JA/12/P1/TC/13.

Installing the module and reconnecting the vacuum line did not solve the problem, but the roughness was less on accelleration with the 91 module than with the 90 module that was removed.

Here is the strange part. I disconnected the module vacuum line to the 91 module and capped the port on the manifold and the thottle response is as smooth as when I did so with the 90 module before it was removed. It is obvious that there is not enough ignition advance on hard accelleration, or, perhaps too much?

Two other possibilities:

How would I go about checking the microswitch that rides the cam on the throttle turntable? What function exactly does that switch serve?

How do I check the vacuum switch that is in parallel with it? That is the light blue vacuum switch at the right rear.

Any other ideas?

I have also not gotten to the bottom of why the fuel boost pump continues to slowly turn when the car is off. I am still disconnecting the battery everytime I leave the car unattended for more than an hour or so. I am wondering if that is part of this problem.

 

[John S Farrington]

Michael, Remembering back to earlier problems, your XJS has a manual shift transmission. I am sure that the factory set up the fine tuning differently (including the ECU) from an automatic. I cannot tell you what the differences are but you are going about the problems correctly. You are trying to fine tune your XJS for optimum driveability and smoothness by experimentation (hopefully small adjustments). That is the best approach because the textbook or shop manual solution which uses the automatic transmission is not pertinent. Unfortunately Jaguar North America and Jaguar Coventry are now controlled by Ford and their policy is to treat all 12 cylinder Jags like they were built by Ferrari!! Dont expect any help from that organization. Just be thankful that you have a magnificent 12 cylinder power plant at a bargain price. Continue on your approach and be satisfied with a solution that has the performance and driveability that you want. Best regards,

 

[Michael J.]

The micro switch is referred as the Throttle Position Switch. It, in parallel to the vacuum switch, signals the Fuel Injection ECU to go to "Open System" with a 12% enrichment when the roller on the arm of the switch rides up on the cam of the throttle turn table. I want to rule out getting lean at higher RPM.

What I am trying to find out is what is the gap between the roller of the switch and the cam of the turntable when the throttles are closed. The switch is held in place with two small bolts that ride in slots on the bracket the switch mounts to, at least on the 1990 model. There is considerable room for adjustment by moving the switch in the slots and retightening the bolts.

That gap or clearance tells the unit ECU at what point (RPM ?)to change to open loop. Normally the fuel system runs "closed Loop". The switch has very high electrical resistance until the roller rides up on the ramp at which point the switch closes with almost no electrical resistance.

The only procedure listed in the manual is for a combination throttle potentiometer/position switch. The illustration is not even close to what my switch and throttle pot look like.

Setting the correct position for when the switch closes should have no bearing on the manual transmission kit or visa versa.

I would appreciate if someone can check the gap with the car sitting static between the edge of the cam and the switch roller and tell me what their's is set at. I would also like to know at what RPM the switch closes.