Speedometer converter disables cruise control

[mk1_nutter]

I have a 1991 Jdm nsx here in the UK. Recently I discovered that the speedometer kmh to mph is disabling my cruise control.
The unit installed takes a power feed from the yellow and black wire on the instrument cluster. It then intercepts the yellow/red cable.

When I removed this interception and rejoin the wire. My cruise control functions but I go back to kmh. It works in D/3 at speeds above 20kmh. I've not been able to test the maximum speed but it seems to be 110kmh.

Any advice on what might work to get both functioning.

 

[drew]

> Recently I discovered that the speedometer kmh to mph is disabling my cruise control.

Please send pics, it is really hard to determine what has been done.

Converting to from KPH to MPH should be a simple gauge face change

There is some strangeness with the AT that sends the cruise control through the cluster, whereas on the MT they send it along the main harness. On the surface makes little sense, but I have yet to delve deeply into this aspect.

 

[Old Guy]

All of the following is predicated on the info available in the LHD / North American service manual

Where abouts is the yel/red wire intercepted? Is it just on the yel/red wire connected directly to pin A21 which is the direct input to the odometer / speedometer drive circuit? As [MENTION=5430]drew[/MENTION] notes, the speed signal comes in on the yel/red wire and splits to the odometer/speedometer, ECU, cruise control, AT control unit and EPS control unit. If you intercepted the yel/red wire before it splits off to the other functions, you are sending a reduced pulse rate to those other devices which is going to interfere with their normal operation. If you intercept the yel/red wire that goes directly into pin A21 you should only alter the speedometer operation and not mess up the other stuff.

When I removed this interception and rejoin the wire. My cruise control functions but I go back to kmh. It works in D/3 at speeds above 20kmh. I've not been able to test the maximum speed but it seems to be 110kmh.​

I am not sure whether this is a question or just a statement of fact. According to the North American service manual the cruise control should operate with the shift lever in 2,3 and D. If yours does not operate in 2 you may have a selector switch problem or that might be a JDM specific problem. According to the North American service manual, the cruise control is blocked below 45 km/hr. Unless this is a JDM specific 'thing', the fact that your cruise control is operational down to around 20km/hr seems to indicate a problem. I am assuming that operation down to 20 km/hr is with the converter removed from the circuit so that indicates that this oddity is not related to the converter.

I assume your 'converter' is a pulse divider which divides the pulse frequency output by 1.61 so that actual 100 km/hr (62 mi/hr) now reads 62 km/hr on the speedo? There will always be a transition delay in the divider circuit so the speedo will not respond quite as quickly as it used to. 1.61 is a non integer number which makes pulse division complex unless you are using a micro controller. Without having done this, I expect that the circuit might be a 10x frequency 10x multiplier followed by a divide by 16 (which is easy to do) that gives a divide by 1.6. The divide by 16 will be very accurate the multiply by 10 circuit may not be so accurate so once you get this working perhaps do a test with a GPS unit to make sure that the speedo calibration is reasonable.

 

[Heineken]

All of the following is predicated on the info available in the LHD / North American service manual

Where abouts is the yel/red wire intercepted? Is it just on the yel/red wire connected directly to pin A21 which is the direct input to the odometer / speedometer drive circuit? As @drew notes, the speed signal comes in on the yel/red wire and splits to the odometer/speedometer, ECU, cruise control, AT control unit and EPS control unit. If you intercepted the yel/red wire before it splits off to the other functions, you are sending a reduced pulse rate to those other devices which is going to interfere with their normal operation. If you intercept the yel/red wire that goes directly into pin A21 you should only alter the speedometer operation and not mess up the other stuff.

Sounds very much like that could be the issue. The JDM cruise control is maximum 120 km/h (as required by law/agreement between manufacturers - the speed limit on Japanese highways).

 

[NSX9]

For info, my JDM 1996 manual has cruise limited to 109 kph, std Japan spec car. Frustrating when we like to sit on 112! Speed limit is 110 here.
I installed a HKS speed override and that works but cruise speed still at 109 with it either way.