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RpM relays ( are the 2 the same?) - Fuel System/Carbs - The Lotus Forums Jump to content

RpM relays ( are the 2 the same?)

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I am trying to trouble shoot a few things on my ESprit.

I took the RPM delay and speed sensing bypass relay out of my car. They look the same on the inside and the outside. My ECON light only flashes while I am cranking the car at start up then never comes on again.

Any ideas?

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Tim, they look the same , BUT the inside is not the same. Don't switch them.

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Posted by: "John Arkwright" lotusesprit87

Hi All

I have received a lot of great help from everyone over the years so I wanted to share a recent problem I debugged in the hope that it will help someone. I have a 1987 Lotus Esprit HCPI with the Bosch fuel injection.

For months now I have had an intermittent issue with the Lotus not always starting. The problem would most likely show up on colder days and when the car hasn’t run in a while.

The issue was the fuel pumps wouldn’t always come on during cranking. So the first things I looked into were the fuel pump and RPM relays. I saw the RPM relay wasn’t providing the 12 volts at times to the fuel pump relay. So I replaced the RPM relay with no change. I then looked into the over-speed module and saw at times during cranking it was not providing the ground to the fuel pump relays. So I now had the RPM relay and the over-speed module not providing the power and ground to the fuel pump at times during cranking. But when power and ground were missing it wasn’t always at the same time. So at this point it looked like I had a duel failure. So for the next step I ensured the power and ground to the RPM and over-speed module were good during cranking. My thought was possibly a voltage drop was present at these modules during cranking. But this was not the case. The only common required inputs for these modules were the ignition pulse from the
negative side of the ignition coil and power and ground. Since power and ground were verified, I then dug into the ignition.

To do this I purchased an 8 channel digital scope to help diagnose the issue. It is a cheap Chinese made PC scope made by Hantek and will operate reliably on an older operating system. The scope is only $100 USD and I also bought a used Windows XP laptop for $100 USD. So the cost wasn’t too bad. The problem was very frustrating to diagnose. I would have the car fully instrumented for weeks at a time. But it seems every time I instrumented the car, the car would always start.

I tapped into the pick-up coil voltage from the distributor along with other points such as fuel pump voltage and the primary and secondary of the ignition coil.
Figure 1 in the attached file shows the voltage from the pick-up coil. It measure about 0.7 volts peak to peak. The waveform also shows the amplitude sometime being high enough at times to allow the ignition amp to fire the ignition. Also once the car started, the pick-up coil voltage increases to a few volts. Since the amplitude of the pick-up coil was irregular I took the distributor cap off and looked at the pick-up coil. This is rather difficult since the distributor is buried underneath the intake manifold. I could see the pick-up coil was loose. But the 2 mounting screws were tight.

This was due to the threads of the screws that hold the coil plate down don’t go all the way to the plate (Figure 2). I had to use a fiber optic camera to see this with the distributor in the car. So I installed brass lock washers to take up the non-threaded area.

After fastening down the coil, the pick-up coil now produces a steady 2 volt peak to peak level during cranking (Figure 3). And now, the car starts the instant the key is turned. This makes total sense. On warmer days the oil is thinner and the engine cranks faster. So when the magnet spins across coil faster, a higher voltage is produced and triggers the ignition amp to allow the car to start. This also explains why once the car was started, there wasn’t an issue running. Also some days the coil was probably in the correct position, allowing the car to start.

One note - when the ignition input was missing from the over-speed module, it would not provide the ground to the fuel pump relays. So in a way it acted like an RPM relay. I also dug into the design of the RPM relay. It is based on a LM555 timer chip popular in the 80s. I hope this information becomes useful to someone.


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Edited by MrDangerUS

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