165 ECU no diagnostic mode

[pman92]

Hey all,
Have a 1987 corvette C4 with L98 engine in at work at the moment. Came in overheating in traffic. Have found the cooling fan is not operating. Have narrowed it down to the ECU not triggering the fan relay (I can power up the fan by earthing the control side of the relay with a test light).

Thought first thing I would do is check DTC's. Bridged the diagnostic request and earth wires in the ALDL connector, switched the key on and nothing. The Service Engine Soon light comes on with key on and doesn't flash. It goes off with engine running.
The information I've got says I should get 3 code 12's, followed by 3 lots of any present DTC's, followed by another 3 12's.

It has the 165 ECU. I remembered from reading here that its identical to the 808 with high speed Comms.

Is this a common problem? What do I do to get it to go into diagnostic mode? Is it running in limp home mode?
I noticed the ECU was very hot when I removed it. However it is pretty much mounted in the heater box and the heater was on before hand.

Thanks in advance.

[antus]

Yeah you just ground the diag request pin and you should get flashing codes. Check the earth pin on your connector is earth or run diag request to a better earth and if that makes no diff then it might not be running. When you click the key to on (but dont start the engine) do you hear the fuel pump run for about 2 seconds then switch off? That happening is an indicator it is running, if you dont get that then it almost certainly is not.

[yoda69]

http://www.corvettebuyers.com/c4vettes/codes.htm

Fan not working may not set a DTC if it is an ECU fault.
Maybe a faulty temperature sensor which would cause the vehicle to run rich as thinks vehicle is always cold and so fan won't come on either.

[Dylan]

If the Crovette doesn't run a BCM you coud plug in a known working 808 and fit the stock memcal and see if there's a change. Or fit a modded 808 and should be all the same.

[pman92]

Ok I've had a bit more of a look.
Fan relay control, Service Engine Soon, diagnostic request input, coolant temp circuits are all good.
Code is running. Fuel pump runs for 2 seconds on ignition on.

5v present at diagnostic request pin. Still does not enter diagnostic mode even earthing it at the ECU.

I've found the ECU is actually operating the fan. But it doesn't come on until 107 Celsius. Cut off temp is 99. Does this sound a bit warm?
Those figures are from the digital instrument cluster, which I believe gets the info over ALDL, as I can only find 2 coolant sensors, the ECU one and a 8 second cut off cold start injector switch. (It has a cold start injector, the switch part # is 0280130229). Also the fan comes on right as the cluster changes from 106 to 107 and cuts off right as it switches from 100 to 99.
So it's unlikely a faulty coolant sensor is telling the ECU it's colder than it is if the figures are confirmed on the cluster. A temp prob on the intake manifold near the top hose outlet confirms the temp is pretty well right.

I'm still not sure why I can't get it to flash codes.
Maybe this code only gives it over ALDL?

The ECU is 1227165 as said before, and memcal is ABTK

[Dylan]

Plug an 808 in and see if theres any change. Our V6's have a fan temp around that I think.
What's the thermostat rating?

[pman92]

Thermostat must be about 88 as it sits spot on 91 on the road.

Not sure an 808 would work as it has VATS (resistor in the ignition key), but I'm not sure if that works like ours over ALDL or just cuts a power wire or something. I don't have an 808 on hand anyway.

I've noticed on the ECU diagram it lists pins in the following words:
A8 - serial data
A9 - diagnostic test (ALDL)

A8 goes to the diagnostic connector and instrument cluster. A9 goes straight to the diagnostic connector only. A9 is what I had earthed. I would of thought A8 is the ALDL data line as it's labeled serial data.
Am I correct?
Does the ALDL in brackets mean DTC's come over ALDL when A9 is earthed or something?

[Jayme]

on an 808, A5 is the check engine light control. + to one side of the bulb and ecu earths it. when you go to diag mode the bulb will flash.
diag mode (pin earthed) on an 808 (or comms enabled 808 with late model code) makes no difference to the data line, but on an 808 with 808 code, if you put a 10k resistor on the diag line to earth, it will use the check engine lamp as a serial data line and flash all the data out at 160 baud (you will see the engine lamp flicker)

[MrWillys]

107.75C is the temp GM used to bring on the primary fan. The secondary fan is controlled by a fan switch in the head and the AC high pressure switch. Having someone burn you a custom chip can bring these temps down. For a stock T-Stat I bring them on at 99C and off at 94C. Shorting A and B should put it into diagnostic mode and kick the fans on. I would suspect an ECM fault if it won't go into this mode.