505 Tachymetric Relay Replacements

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Having just been through a fight with the tachymetric relay in our 1983 505 with the XN6 motor and Bosch K-Jetronic fuel injection, I figured that I'd post some of my experiences in case they're of any help to someone else in the same boat.  This post is long, so if all you need are replacement part numbers skip to the end of this post.

For anyone unfamiliar with the tachymetric relay, it controls power to the fuel pump and its failure will leave your car utterly stranded.  This happened to me last week and resulted in a call-AAA-for-a-tow-home moment.

There are three key circumstances under which the relay operates:

1. When the ignition key is first turned to the 'on' position, it provides a one-second burst of power to the fuel pump.  This is basically a priming cycle to make sure that fuel has reached the injector nozzles.
2. During cranking and after starting the engine, it provides a constant source of power to the fuel pump in order to maintain fuel delivery to the cylinders.
3. If the engine speed drops below about 400rpm (i.e., the engine stalls or is switched off), it cuts power to the fuel pump so that fuel isn't continuing to be sent to the injectors.  This is accomplished by checking the negative pulse from the ignition coil.

A little background: some time ago, our car was having intermittent start / no start / run / no run issues.  These were traced down to the tachymetric relay; after pulling it and opening it up, bad solder joins and damaged components were found on the PCB.  Those were repaired and replaced (details can be found here), but the car started randomly showing similar symptoms again.  Unfortunately, until it decided to just flat-out not start one afternoon in a parking lot, I'd never been able to reproduce the problem at a time when I could catch it and try to figure out what the issue was this time.

As it turned out, my repairs had held - but the coil in the relay had finally given out and was no longer closing its contacts when 12V was applied to it.  This was confirmed first by removing the relay from its case, connecting it to the harness, then watching what the relay coil did when the key was turned from off to on.  Normally, the coil armature should be seen to move with a click as it made contact followed by hearing hear the fuel pump kick in; none of these things were happening.  Manually moving the armature to the contact caused the pump to turn on, however, so this gave a starting point for investigation.

With the relay out and connected to a bench supply, 12V was put directly to the coil.  No movement at all.  It was done.  Replacing the coil was a definite possibility (provided one of the correct dimensions could be found, which would likely involve cannibalising one from another relay), but given the history of problems with that relay I decided to just replace it and keep this one for eventual rebuild.

This is where the fun began.  The tachymetric relay as used by Peugeot was a Bosch unit, part number 9250624280.  This is out of production, and has been for some time.  It's rare that they crop up and when they do they're expensive - one source wanted $725 for the relay!  Apparently being installed in Porsches of the same era justifies this price to that particular seller, but it was way out of the question for me.  Fortunately, there's no shortage of other cars from the time that also had K-Jetronic injection.

Some research led to figuring out that the 1983 Volvo 242 with the 2.1-litre turbo engine uses a compatible relay.  The one that I got (from rockauto.com) came in a green case rather than black and had the Volvo logo moulded into it, but it's clearly a Bosch-made part.  After having it in place for a couple of days and approximately 100 miles I can't tell the difference between it and the Bosch-branded unit that was in there originally.  Pin assignments are exactly the same between the two, the connector is completely plug-and-play, and aside from some minor componentry and layout differences on the PCBs it appears to be virtually identical in terms of construction.  The Peugeot relay doesn't cross-reference directly with the Volvo unit (that I've found), but the Volvo unit does seem to be a direct replacement.

The Volvo relay is an Airtex / Wells-Gardner 1R1862 and was $56.79 at the time of writing.  Others that may also work (but which I haven't tried, so cannot confirm): Üro Parts 3523639 ($14.89); Beck / Arnley 2030066 ($27.79).