RPM relay oddness

[Kenny_Z]

I've got my original RPM relay and a SpecialT relay. I purchased the SpecialT with a relay kit shortly after I bought Red and I never used the RPM relay for some reason. I think I remember why now. I put in the SpecialT relay and I can not hot start the car. And by hot start I mean I drove less than a mile with the AC on to the gas station, filled up 5 gallons, and then tried to leave. As soon as I put my original relay back in she fired up without hesitation. The first time it happened I was able to jumper the fuel pump and then put in the relay back in and she fired up. I did that because I had just replaced the fuel pump and thought maybe I had a different issue. The second time it happened I realized I was using that "new" relay and swapped them and she started. After the fill-up I drove to a fast food place about a mile away, got my food, and then left. She started right up with the old RPM relay. I might be imagining things but with the SpecialT relay the starter sounds like it is turning faster. Red will cold start with both relays but the SpecialT takes longer and she sputters for a bit like she's low on fuel.

This relay is around 2 years old (I was delorean dumb back then) so the odds of me sending it back are nil. It almost feels like the new relay isn't triggering the fuel pump but thought I should have enough rest pressure to at least sputtered. The accumulator is about a year and a half old.

Make any sense to anyone?

[DMC81]

Can the RPM relay cause your car to bog down and act like it's starving for fuel?

[Jonathan]

Any chance the RPM relay having problems has a pin bent slightly and isn't making good contact with the opposite end in the wiring connector?

It doesn't sound like anything else is coincidentally changing other than the RPM relay itself, so to me it seems the problem is either internal to the relay or something to do with how it is connected. No idea what they are supposed to look like inside, but if you put both of the connecting ends side by side for a comparison, do they differ in any way?

EDIT: didn't realize how old the original post was. Did you get this sorted out Kenny?

[DMCflux]

When I first got my D I bought an rpm relay from special t. It was defective new and never worked.

[DMCMW Dave]

The RPM Relay feeds two circuits. It runs the fuel pump so it's pretty obvious if that circuit is bad - the car is dead.

The other circuit is the Lambda control, and if that side fails the car will run but not very well (the indicator here is that you will not hear the frequency valve buzzing).

The RPM relay can be completely replaced by a three-legged jumper (brown to yellow/red AND white/purple), if that cures the problem its the relay for sure. Don't leave a jumper like this installed after you turn the car off, you'll notice that the fuel pump and lambda box will run with the jumper in place whether or not the key is on.

[DMC81]

The RPM Relay feeds two circuits. It runs the fuel pump so it's pretty obvious if that circuit is bad - the car is dead.

The other circuit is the Lambda control, and if that side fails the car will run but not very well (the indicator here is that you will not hear the frequency valve buzzing).

The RPM relay can be completely replaced by a three-legged jumper (brown to yellow/red AND white/purple), if that cures the problem its the relay for sure. Don't leave a jumper like this installed after you turn the car off, you'll notice that the fuel pump and lambda box will run with the jumper in place whether or not the key is on.


Thanks Dave, I was just reading about the jumper you mention. I'll give it a try and see what happens.

[DMCMW Dave]

Thanks Dave, I was just reading about the jumper you mention. I'll give it a try and see what happens.

You can experiment a little here too- once you have it running with the jumper, pull the jumper off the yellow-red only and see if it duplicates your running issue.

[DMC81]

You can experiment a little here too- once you have it running with the jumper, pull the jumper off the yellow-red only and see if it duplicates your running issue.

It typically runs ok at first but once I'm a few minutes down the road it wants to die out if I push on the gas with any pressure at all.

[Bitsyncmaster]

The RPM Relay feeds two circuits. It runs the fuel pump so it's pretty obvious if that circuit is bad - the car is dead.

The other circuit is the Lambda control, and if that side fails the car will run but not very well (the indicator here is that you will not hear the frequency valve buzzing).

The RPM relay can be completely replaced by a three-legged jumper (brown to yellow/red AND white/purple), if that cures the problem its the relay for sure. Don't leave a jumper like this installed after you turn the car off, you'll notice that the fuel pump and lambda box will run with the jumper in place whether or not the key is on.

+1
Most original RPM relays have two contact pins on the relay inside. You can get one circuit contacting and not the other (my car had this problem). Some of the newer OEM RPM relays only use one contact pin for those two circuits.

[Chris4099]

I've also heard of the RPM relay failing in that it will open/close the contacts with each RPM signal. Cycling the pump power would obviously cause performance issues as well.

[Farrar]

So the RPM relay is basically a dual SPST relay?

[DMCMW Dave]

So the RPM relay is basically a dual SPST relay?

AKA DPST relay. (double pole-single throw)

Driven by a 555 timer triggered by the ignition ECU pulse to the coil. When the pulses go away the timer drops the relay.

[Farrar]

AKA DPST relay. (double pole-single throw)

Ha ha! Thanks, Dave! I can't believe I typed that while forgetting what the letters actually stand for. I must be getting old...


Driven by a 555 timer triggered by the ignition ECU pulse to the coil. When the pulses go away the timer drops the relay.

Neat! :) Thanks for the info!

[LordFly]

The other circuit is the Lambda control, and if that side fails the car will run but not very well (the indicator here is that you will not hear the frequency valve buzzing).

Dave, can you define "run but not very well"? I'm wondering if maybe this is part of my cars problem. I have fairly major lack of power until about 3000rpm. And it takes about 20-30 seconds of cranking to hot start. New fuel accumulator didn't help either problem.

[Bitsyncmaster]

A failure of the Lambda side power depends if the mixture was adjusted when it was running open loop. Now if the RPM relay would suddenly of intermittently fail your mixture would be way off. Not sure what the symptoms would be with the working lambda and mixture correct and suddenly you went open loop.

[Elvis]

Who can see the problem ?


26198 26199


Now guess what it takes to repair this !?!?!?

BTW - this relay came to me to get a reliable "Primer" modification installed,
he didn't even know that his car/fuel pump was about to fail sometimes soon.

Here a picture of my "primer" mod. Now I also have the primer pulse each time
I turn the key (I never had that) and the engine starts in less than a second
cranking :-)
26203

[Bitsyncmaster]

Who can see the problem ?


26198 26199


Now guess what it takes to repair this !?!?!?

BTW - this relay came to me to get a reliable "Primer" modification installed,
he didn't even know that his car/fuel pump was about to fail sometimes soon.

I've seen quite a few of those. Many times the Lambda pin just does not make contact because there is not much movement of the contacts once one of them contacts. I guess that is why they went to a one contact relay. There have been about 4 or 5 circuit board changes I've seen.

[andy blackmon]

Dave didn't say so but on his site he has solid state rpm relays for sale. I have been running this for a couple of years and like all good products it works great.Put one of these in than you can move on to the next problem and with a Delorean there will be one.
Andy B 03513

[Elvis]

Well - push the relay back in place and resolder it - done !

[Kenny_Z]

EDIT: didn't realize how old the original post was. Did you get this sorted out Kenny?

I just stopped using the SpecialT relay and still have my original in place. She's given me zero issues.

[John U]

Yes, as Andy said, Dave M hasn't taken the opportunity on this thread to advertise his upgraded RPM relay. Just ordered one....can't wait to try it out. My car runs fine, but when it sits for a couple weeks or more (its a sin, I know!) it needs a few primes to start. Hoping this new relay solves that issue with it's 1.5sec initial prime.