VikingTrad3r

Hi All,

I'm heading to attempt to help a fellow c4 owner (1990) with diagnosing why it keeps overheating after 15 minutes.

I have seen that others have posted with regards to the 1990 that the "analog temp gauge" should not be trusted.

Is there a digital as well? Do I need to press some buttons to get the temp to appear somewhere from a digital readout?

Thanks. VT.

WVZR-1

A '90 would have to have C68 AC to be able to see the CTS info. Does it?

antfarmer2

Mine is the gauge button on my 94 I think yours is on the climate control.

VikingTrad3r

not sure if he's got c68.

if he does, how do i make the coolant temp show up on the display?

antfarmer2

Sorry I do not know and cannot find any info for you. Start pushing buttons or hope for someone that knows. If he has the manual it should tell.

VikingTrad3r

turns out i have a 1990 in my pile of manuals for sale. ill see if i can find it there

WVZR-1

i did a quick review of this thread and it appears complete. I didn't check it against an FSM but a quick glance looks correct.

https://www.corvetteforum.com/forums...post1577429729

Check 1C1-3 thru 1C1-5 maybe

antfarmer2

VikingTrad3r

any idea why the 1990 manual would exlude parameter #16, coolant temp, as indicated on the post in the link provides upthread?

here is the 1990 blurb:







sry not sure why its rotated

MikeT 90 C4sixer

i have the same manual for my '90 C4. Might be just a typo for that parameter 16 got left off. Lots of errors in the FSM. sometimes you get the updates with the manual. its too bad you have to take the counts value and convert to a meaningful #. And the coolant temp is in C.....

WVZR-1

I've never noticed that - I guess because I've a Tech1 and never paid attention to it. I did use it once 13 or so years ago to confirm TACH operation to validate a rear gear ratio. I checked a later '91 FSM and it's in the '91. If your friend has C68 you'll be able to tell us if in fact it does appear.

VikingTrad3r

Update

All the parameters are available on the 90. Pretty amazing.

We were able to verify that his analog gauge was reading high.

C68 was reading 108-109 Celsius or 226.5 F when the driver side fan came on.

It took the heat down to 104 Celsius then shut off. Repeat.

We noticed coolant smell. It was rad cap. It says it's an 18. We swapped my 87's 16 on it. No more smell. Faulty seal in the cap.

Then we took it for a drive.

Cooled of to 90 Celsius 194 F.

His System is working fine. I think the two fan system doesn't cool as fast as my big single fans on my 87 and 85.

Thnx for the help guys.

VT

Churchkey

90 model: The right side "cooling fan" is not controlled by coolant temperature it is controlled by AC high side pressure.

Suggest connecting the 2 fan relay ground wires together with a jumper wire. Relays are mounted on the left side of the radiator shroud, ground wires are blue/black. Both fans will now run @ 228* or come on when the AC is in use.

VikingTrad3r

thanks you predicted my next question.

if it were mine, comparing to my 85/87 id do that simple mod in a second. it struggled to lower the temp at idle whereas my single fan systems hammer the temps down fast.

ill suggest to him.

Churchkey

My 90 relays are connected, with the AC on the coolant temp stays near thermostat opening temp, AC seems to blow a bit cooler in slow traffic.

Caution: The fan relays have 12V @ all times, grounding a relay wire will trigger fan operation @ any time, the ignition does not need to be in run position.

VT thanks for helping a fellow owner.

VikingTrad3r

Thanks Church.

just so i am sure, (and i will link him to your reply) the mod is to jumper the blue/black lines that are coming out of each relay. that way, when the coolant sensor triggers the ecu to turn on the fan at around 226.4 (in our case) then 12v the ground signal will travel to the cooling fan and then jump to the ac fan and turn them both on.

when completing this mod, i assume unconnecting the neg battery cable is the first step.

thanks Church.

MikeT 90 C4sixer

Sorry, but you are wrong about the fan control on the '90 model. The ECM controls both fan on/off temps. I do my own chips, and there are settings for both fans. Stock Fan1= on 225.95 off 220.55 Fan2= on 235.40 off 225.95. I have lowered both of mine, cuz i run a 180deg Tstat. The AC high side pressure will turn on Fan2, if you are under a certain speed.

I set my 2 fan turn-on temps about 2 deg apart, that way there is not a big in-rush current if they both come on at the same temp.
HTH.

Churchkey

Am NOT wrong about the secondary fan operation read the 90 8A electrical diagnosis service manual supplement. Vin 8 page 8A-31-0 shows the wiring diagram, page
8A-31-3 is the text explanation. The secondary fan is triggered on @ 190 psi AC high side pressure.

If you do not happen to have the 8A manual I have scanned the applicable pages & posted them on this forum a few years ago.

BTW the AC switch opens causing the ECU to ground the relay etc. Don't need to explain the E path the high side pressure switch is the trigger.

You have modified your ECU for a custom application God bless.

VikingTrad3r

is it possible you are both right? it looks like the ecu will turn on the passenger side fan on either from the ac psi trigger, or, if temps reach 235, ecu will "call in reinforcements" as the heat is gettung away from driverside fan?

if that is the case, even if it isnt, that a very good system.

c4sixer do you have the page in the manual where it backs up your statement?

WVZR-1

Actual cooling fan explanations are found in the 6E-C12 section of the FSM. I ain't gonna debate it or argue it BUT if it interests you check there before commenting.

I understand the "why" Churchkey has done and suggests doing what he's done, MikeT 90 has changed the programming to suite "himself" and that is way to involved for most C4 owners and many who do "for hire" tuning actually don't often accomplish much and screw up much more. I understand what he's explained. If you're into PROM modifications and want to do similar .. have at it. It is interesting. Perhaps he'll post back and mention what he found in the "TUNE" before he "TWEAKED" it to suite himself.