Sunday, May 18, 2008

The RPM Relay – A modern update

(Edit 02/21/16:  Updated links for Dave's website of related products.)

RPM Relay, a synopsis
The RPM relay, resides in your fuse/relay area. Physically it is a small boxy plastic case made either by Bosch (black), or Volvo (green). In our cars a thick wiring harness plugs into to it, just above the fuse box, under the door lock module. The RPM Relay’s function is essentially simple, as long as it senses a tachometer signal (engine use), it supplies power to the fuel pump. Additionally it also provides power to the warm up regulator heater, and it turns on the main fuel relay (Lambda relay). When the RPM Relay goes (or starts to go) bad, your cars stops dead due to no lack of fuel being pressurized into the system. Now, guess what part on a DeLorean is (still) a notoriously high failure part? Answer see 1st and 2nd words in sub-title above.

If your RPM Relay fails on the road, there are some options to get get you home.
One thing you can do, is to remove the RPM Relay and jumper (a thick paper clip will do in a pinch) terminals #30 and #87; this will supply constant power to the pump. This is only temporary as it will affect ECU/Lambda controlled fuel mixture, and overall is probably not safe to run for an extended period. John Hervey's SpecialTAuto site has an excellent reference of this emergency jumper solution here.

The other option up to now has been, to carry a spare RPM relay. Just in case.
Available at:
SpecialTAuto: here
Your local DMC affiliate: here
Remember though, these have the original late 1970's/early 1980's design.

Well, now that we are in the new millennium, we have a new option thanks to Dave M. (an electrical engineer and an active DeLorean community participant), aka “Bitsyncmaster” on DMCTalk and the DML. Read on...

Dave’s re-engineered RPM relay
In February of 2007, Dave sketched up plans for a modern update to the notorious RPM relay. His schematic replaced commonly used 1980’s discrete components with a modern solid state components and a micro-controller/processor. To really appreciate the difference between the original design, and Dave’s, look at the picture below (my old OEM relay on left, and Dave’s version - click picture for larger view).

The old relay reminds me of the results of an old Radio Shack electronic kit, and the new one just looks so modern with its really tiny surface mount components. Beyond duplicating the basic function, he also added an optionally active “Hot Start” feature. This will actually prime the fuel system, for 1 second at 1 hour intervals when the car is parked. Some consider this a controversial feature because it is workaround, to a different problem that needs to be fixed. Again, it’s optionally enabled by the user, and this could mean the difference between a drive-able car or not, at least until the complete fix to the problem is addressed.

Features of the Re-Engineered RPM Relay
Back to the main feature of the this new RPM Relay design. There are some very big difference in this new and modern relay:
a) Less current draw: This new design draws 16ma on ignition, and 20 ma in operation. The original relay draws 260ma. That’s a 92% reduction of current on the new design!
b) Eliminates a constantly energized coil: In the original design this relay maintains a constantly energized coil when it is on, this function is now replaced with solid state components (the differences are visible in the picture).
c) Less heat: Since the original relay sucks up more current through the discrete parts (see coil above as an example), they generate more heat. So much so, that you cannot touch the internal components on the original relays after a long drive. The new design, on a similar drive, runs barely warm. I know because I performed that test with the two designs.

To anyone who has dealt with electronics and reliability knows, the three benefits above, translates directly into a now significantly more reliable part. Cool.

As of this writing Dave is currently asking $75 to upgrade your old relay, along with a refundable $50 "core" charge. The upgrade consists of a completely new and professionally made circuit board, with modern components soldered in, and using your existing case and connector.

RPM Relay in use
I have been running this relay as a beta tester for Dave, since January 2008 (5 months as of this writing). In that time, I have driven my car every weekend on either 10-30 mile cruises, or often to longer distances to local car shows (including during high 90+ degree Florida days). In that time we also took the car on a 600 mile round trip that included 3 hours of continuous driving. The relay sits quietly in its place, performing its function, and frankly I have forgotten that it is there - which is exactly what this part should contribute to our cars.

Hat’s off to Dave and his improved design of the RPM relay, this is an excellent one person contribution to the maintenance and continued longevity of our cars.

These kind of upgrades are typically not as “fun” as other things that you could add to your car: You plug it in, and apparently you don’t really “get”, or see, any noticeable improvements. While that is true, you should consider this a preventive maintenance, a reliability improvement, an upgrade. And while those are often not “sexy” upgrades, in the end, it is money well spent on your hobby/passion/baby (as it were).

If you’d like to consider or purchase one of the new solid state relays from Dave, you can contact him at: or PM “Bitsyncmaster” on

Edit 01/03/09:
One Year Update
My solid state RPM relay is the oldest production unit in operation. It has been running without incident now throughout an entire year, including some near 100 degree ambient Florida temperatures, and several 3+ continuous hour trips. Many other owners have now been running Dave's relay with similar excellent performance.

Edit 02/21/16:
Eight (!) Year Update
Yup. Still all good. Great product, a must have if you own a DeLorean.

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Unknown said...

Does it have a priming function when first powered up?

Ozzie H. said...

Yup, it does.