jerrysims71

i have an 87 iroc 5 Liter fans r not coming on with or without ac on changed fan relay didn't fix the problem. i jumped the A and B terminal in the obd 1 plug to scan code ( 34 and 36 btw) and the cooling fan turned on and ran only wen the terminals were jumped??im baffled anybody have thais problem???

Injuneer

Do you have the dual fans or the single fan?

Could be a problem with the coolant temp sensor that feeds the ECM.

Are you sure you read the codes right.... I don't see DTC 36 listed for an 87 5L.

jerrysims71

yes dual fansand i scanned three times and yes it gave a 34 then 36. gonna clean the maf sensor

Injuneer

Which 5.0L engine do you have? What is 8th digit of VIN? With the MAF sensor, it should be "F" injected.

Here's a good site for codes:

GM OBD Trouble Codes ?

The MAF code generally means the operating voltage is outside the normal limits... not so much a factor of dirt affecting the calibration, unless its a gross error. Using a volt meter, check for at least 0.50V at idle. If you can run test wires into the passenger compartment, you want no more than 4.7 Volts at max air flow (WOT, max RPM).

A faulty (open circuit or shorted circuit) on the coolant temp sensor would set DTC 14 or 15. But the sensor being out of calibration (resistance vs. degrees) would not set a code.

jerrysims71

thanx for your help il try some things and post progress

jerrysims71

pulled plug off temp sending unit grounded it and fan turned on when i change the tsu will coolant spill out? dumb question i know but have to ask to b pos..

Injuneer

Typically, the coolant temp sensor is immersed in coolant, and the coolant will leak out, unless the sensor is on top of the engine.

jerrysims71

thanx

jerrysims71

well i checked the fan switch wire grounded it and fan turned right on so i change d the switch with a napa one and fan still does not turn on at temp got real hot wen i checked it and i also removed the fan to make sure it worked gonna check to make sure there is power coming to fan next any suggestions anyone

jerrysims71

duh if it turned on wen i grounded wire i guess its got 12 volts to it (had to read it to think of that)

Injuneer

It's not a switch. It's a thermistor, a resistor that has a resistance that varies inversely with temperature - high resistance when the temperature is low, low resistance when the temperature is high. If you are looking at the correct device, when you grounded the wire (should have two wires - yellow is 5 volts from the ECM, black is ground, also provided by the ECM) the resistance is near "0", telling the ECM the temperature is extremely high. In response, that tells you that the system is working..... with one exception.

If you shorted the yellow wire to ground, the engine provided the ground. That may indicate the black wire is not providing the ground. Key on, connector off sensor, measure the voltage between the black and yellow wires..... should be 5 volts.

Were you looking at the sensor with two wires - yellow and black?

If not, you may have been looking at the "Fan AC Pressure Switch". That is a switch, and it switches 5 volts from the ECM to turn on the fan when the AC is running. That is a 2-wire device, with a grey and a black wire.

You have to be testing the sensor, not the switch - the one with yellow and black wires.