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Problems

Vehicle suffers from poor idle speed control following work in engine compartment – especially tune up

VOLVO (CAT and NON CAT) 440/460/480/ MODELS fitted with FENIX (35 pin) (RENIX on Renault) engine management ECUs 1989 onwards.

Symptom

Vehicle suffers from poor idle speed control following work in engine compartment – especially tune up. Possible unrepairable damage to ECU idle control circuits due to crossing over idle control valve and throttle position switch connectors. These connectors can be undone whilst setting base idle speed or mixture.

This problem is most likely to occur on vehicles (both Volvo and Renault) with 1.7 litre engines.

On SOME cars these connectors are identical unkeyed 3 way connectors. The effect of crossing over is to apply earth directly to idle control IC within ECU – It is destroyed immediately ignition is turned on. This part is not commercially available so ECU cannot be repaired apart from robbing another ECU.

Diagnostic checklist
1) Confirm vehicle has 35 pin FENIX/RENIX system (BENDIX or SIEMENS stamped clearly on cast alloy ECU cover.
2) Confirm vehicle has idle control valve and throttle position switch with identical 3 way connectors in wiring to each component.3) Confirm wire colors match each side of connectors.
4) If wire colors don’t match, open ECU and look for obvious internal damage.
5) Even if wire colors match, open ECU and look for obvious internal damage (connectors could have been changed back).