Tachometer slow response / under reading in hot conditions?

[Benton]

This has happened a couple of times now.
First time  I noticed I was stuck in a queue of traffic / hot day for about 10 minutes. Rev needle dropped showing less than actual revs otherwise car was running fine. When car was subsequently moving tachometer was responding slowly and still under reading. When I got home idle speed showing at about 200rpm. Switched off, immediately  restarted engine needle now working fine!  Shift lights still appeared to be registering correctly, regardless of tachometer reading.  Since it was an easy thing to do I disconnected and reconnected the block connector on the back of the tachometer.
A couple of days later.same thing happened again, though I hadn't been stuck in traffic.
Been out a couple of times subsequently (albeit in cooler weather) and it's all appearing to be working perfectly and behaving itself.
I figured signal from the sensor to the ecu must be OK otherwise the engine wouldn't be running right.  Therefore must be either something in the connection between the ecu and the tachometer playing up, or the tachometer itself throwing a wobbly, though in either case it seems to be related to high ambient temperature.  For the record the cars a 620R.

Any similar experiences or thoughts?

Also has anyone got a wiring diagram for a 620?,  though I'm pretty sure the signal wire for the tachometer is white/black. 

 

 

 

 

[Jonathan Kay]

Private Message sent.

[aerobod - near CYYC]

Although it could be an internal component failing in the tach, there is likely an external pull-up resistor (typically 1K ohm) that could also be failing, that is across the ECU feed and 12V. The resistor itself may be badly crimped to a connector or have a broken lead, too.

[aerobod - near CYYC]

Jonathan, your mailbox is full, but as mentioned in the PMs, the tach signal should be Pin 34 on the ECU via WB72 (white with black stripe) on the engine/chassis connector then on to WB154 on the tach. If there is a pull-up, it would logically join WB154 to the green wire that ends up at the fuse block instruments fuse. An out of the box 9A4 ECU (from SBD Motorsport) I fitted in my car needed a pull-up resistor, whereas the stock old 992 fitted by Caterham didn't. I think all current Duratec cars now use the 9A4, as fitted by Caterham.

[Benton]

Many thanks for your information/suggestions. I'll certainly check out the resistor.