How To: In-dash potentiometer install for manual idle control

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VAPRAI
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Joined: September 11th, 2012, 4:17 am
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How To: In-dash potentiometer install for manual idle control

Postby VAPRAI » January 8th, 2015, 6:10 am

Here is a fairly easy way to get manual control over the idle by force feeding desired resistance values directly to the ECU.
This how-to would work on any OBD0 Honda but the part I used is a good fit for our cars specifically.
In the past there have been tutorials to wire in a static resistance amount but a potentiometer will allow a variable amount of resistance.

The coolent temperature sensor information is one of the main variables used by the ECU to set the idle rate in addition to contributing to the amount of fuel distributed. All the temperature sensor is doing is sending a resistance value to the ECU, specifically the amount of Ohms correlated to temperature.



Here is the diagram out of the FSM

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The problem that occurs is the wiring and connector for the temp sensor thermistor goes bad, especially as our cars approach 30 years of age.

I personally had my connector wiring go completely haywire resulting in erratic resistance being sent to the ECU and the resulting unstable idle.
Instead of replacing the connector I decided to take manual control of the resistance given using a small push button potentiometer that fits nicely within one of the empty spaces of the 86-87 dashboard. I used a Bourns 3683S-1-502L which is a variable potentiometer between 0 and 5K ohms, correlating between 32 and 248 F engine temperature to the ECU. At least a 2K ohm unit should be used for a high idle / fuel mixture. Bourns also makes a round knob potentiometer that fits nicely in the 88-89 dash, part 3610S-1-502.

This is the potentiometer I used, ran about $20 at a local electrical supply shop
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First you have to get access to the 2 wires that feed resistance into the ECU.
These are just the wires that go into the temp sensor in the engine block right under the distributor and crank angle sensor.

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This is the connector to pull. Mine was completely thrashed with the original harness wiring disintegrating out of it.
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I decided to strip back the original wires until I got past the corroded areas. It took a few inches to strip until the copper had a normal bright tint to it. Just keep stripping the jacket until there is no more discolored or dull copper. I then used some automotive quick connect adapters to splice in some copper 18AWG wire to run into the dash. Alternatively a wire could be run directly into the ECU but I decided to salvage the existing harness wiring. The hole used for the speedometer cable works well to feed the wire into the dash area.

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This is the spot that it fits in on the dash.
I used some quick connects here as well to easily be able to connect/disconnect the potentiometer later.
LS models with cruise control will have this spot filled but there is a free opening on the left side that will work.
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Now to prep the potentiometer, this is an optional step to wire in a resistor to prevent a CEL from ever occurring.
The ECU is set to look for at least 20 ohms of resistance, anything lower will trigger a CEL.

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The resistor goes between lead #2 and #3 on the potentiometer.
Wiring in a 20 ohm resistor between these 2 leads will ensure a minimum of 20 ohms at all times.
The resistor can be soldered or secured in a number of ways. I used a compression pliers to just heat the wire up and crimp it to the lead.
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The wires to the ECU connect to leads #1 and #3
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I used a non-conductive hot glue to secure the wires but potting silicone or soldering would work just as well.
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Once everything is wired up the quick connect adapters can connect to either wire.
The resistance value wires to the ECU are not polarized.

The potentiometer fits in perfectly into the dash spot.
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Now with the push of a button the exact amount of ohms sent to the ECU can be set.
Cold starts require a higher amount, in the 2K ohm range to ensure enough fuel is distributed to start.
Once warmed up the potentiometer can be tuned to any desired value. Typical warm values are between 200-400 ohms when the temp sensor is wired up but I have had a very smooth idle under 100 ohms and enhanced fuel economy as well. The Bourns push button potentiometer uses a percentage value between 0 and 5K ohms (500 = 2.5K, etc).

So far this has worked out great. Slight changes in the temp sensor resistance value caused the ECU to sometimes erratically adjust the idle, especially after cold starts where the engine is warm but the ECU is delayed in processing the lower idle. Forcing consistent values of your own choice gives complete control once you get used to which numeric value gives the best performance for a given situation.


awright3331
G1 n00b
Posts: 5
Joined: January 7th, 2015, 2:01 am

Re: How To: In-dash potentiometer install for manual idle control

Postby awright3331 » January 14th, 2015, 2:17 am

Nice man, unfortunately the 88&89 dash is different so it wont work exactly the same, but still I may try this at some point


Topic Author
VAPRAI
Legit G1Tegger
Posts: 108
Joined: September 11th, 2012, 4:17 am
Location: Arizona

Re: How To: In-dash potentiometer install for manual idle control

Postby VAPRAI » January 14th, 2015, 6:28 am

awright3331 wrote:Nice man, unfortunately the 88&89 dash is different so it wont work exactly the same, but still I may try this at some point

3610S-1-502 is one of several round knob parts that fit in the 88-89 dash
Really any round knobpot could be fashioned to fit.


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