by Chris Miles
The RPM relay is essential for fuel pump operation. It powers the fuel pump when it detects an RPM signal, meaning there’s an ignition signal, and the engine is turning over. It’s a black six pin relay usually found mounted to the underside of the relay tray behind the passenger seat. Or, just look for the large relay similar to this photo.
If the relay fails, you can be left stranded, and you likely won’t get a warning of a failing RPM relay ahead of time.
To avoid RPM relay issues, I recommend buying Dave McKeen’s solid state RPM relay (https://dm-eng.weebly.com – use the “Contact Email” link at the top of his page for pricing). Unlike the original RPM relay, McKeen’s solid state relay runs cool and will essentially last forever. Since the RPM relay is essential for running the car, why rely on a 30+ year old piece of technology. Dave will rebuild your original relay with solid state components, or you can send him a compatible relay to rebuild. Search eBay and other websites for Volvo relays. You can also go to junkyards and search for 1980s Volvos (commonly Volvo 240s) for compatible relay cores. If you send McKeen one of the Volvo relays, you can keep your original DMC relay as a spare. Alternatively, please see my for sale link here (RPM Relay Core),if you would like to buy a relay core from me.
This photo gives you an idea of what to look for on the INSIDE of the Volvo relays. Make sure the pins “stacked” when they attach to the circuit board. Other relays cannot be re-used for McKeen’s conversion.
I’ve been using these relays in both of my DeLoreans since about 2010 and they work great.
If you don’t want to spend the money for McKeen’s relay, it’s not a bad idea to make a three-wire lead you can use to jumper the RPM relay socket in case of emergency. The three wires are all connected to each other at the top (so that they make a single circuit,) and when plugged in as the photo shows, the fuel pump circuit will be powered – allowing you to get home and buy a new RPM relay.