When I bought my 87 2 years ago the first items I purchased were the GM shop manuals for the car.

I had the misfortune of needing to use the electrical diagnosis manual to look at my electronic climate control - blower circuit. With the help of a knowledgeable friend I discovered that the instructions make no sense, eg. they seem to suggest that a circuit clearly labeled "blower feedback input signal" is actually feeding voltage to the blower to operate it - two paragraphs later they describe the same circuit as a feedback circuit. I'm thinking that it can't be both.

Has anyone else had similar experience with this manual? Hopefully this is an exception - the diagrams, component location, plug wiring diagrams are excellent. Are there updates available anywhere?

 

I can't speak to the GM wiring diagrams specifically since most of my experience is on brand X, maybe it's Y?
Anyway, in their system errors in the FSM were corrected by TSB's (Technical Service Bulletins) at one time mailed to dealers monthly, but for the last several years they are sent electronically. Unless someone in the dealership takes the time to print them and staple into the book they don't even get corrected at a dealer so you are unlikely to find updates for a 20 year old book.

 

I'll look at my 87's manual when I get home and comment.

 

The manual's explanation of the blower motor speed control looks fine to me, but I work with feedback controls and understand them. The explanation merely says that the control head gets motor speed info from the motor whose running voltage is from 4 volts to 12 volts and the signal sent from the control head to the motor control module goes from 0.5 volts to 5 volts.

 

Thanks for taking the time to look. I am trying to understand this stuff but I have no background. I hope that you will be willing to bear with me tell me where I have it wrong.

What confuses me is the "Circuit Operation" text for the C68 blower controls. The 3rd paragraph and 5th paragraph both talk about terminal C5 on the ECC. One describes it as an output and the other as feedback. Can it can do both at once?

Isn't terminal C4 the output from the ECC to the blower control module ( 1 - 5.5 v) which then feeds 5 - 12 v to the blower based on the voltage from the ECC? This seems to be what paragraph 4 is saying.

Test B in its 4th step also confuses me. It has you test A of C2 & ground with the blower control module disconnected. the correct result is supposed to be 12v but where is this coming from if the output from C5 on the ECC is only suppposed to be between 1 - 5.5 v? There doesn't seem to be any other source of voltage.

Thanks.

 

I'd have to go grab my manual to take a look at this. I went through all these tests last summer with my '87 and they did all work and diagnose the thing correctly in the end. It was the HELMS one, do you have that one?

 

Thanks for your interest. We were away for an extended weekend.

My manual is the actual GM shop manual (ST-3364-87 EDM). I am not familiar with HELMS - what is it?

 

Thanks for your interest. Sory for the slow response - we were away for a small vacation.

I am using the actual GM "Electrical Diagnosis service Manual supplement". I am not familiar with HELMS - what is it?

 

Helms is the company that published the factory shop manual, I don't have mine in front of me but there should be a li'l copyright notice somewhere on it indicating that. If it is the FSM then yeah it's the Helms manual everyone speaks about.

 

Terminal C5 is an input terminal on the control assembly. The control assembly uses the voltage on the power wire to the motor to detect motor speed as a feedback information source. The speed of permanent magnet dc motors is directly proportional to the voltage supplied to the motor. Paragraph 3 is incorrect, the blower control speed output is at C4!
The 4th test of B measures the blk/red wire voltage with the blower control module unplugged. I would expect no voltage on the blk/red wire, however, while not explained, apparently the control assembly has an output of 12v if the blower control module becomes open circuit. Otherwise there would not be a need for a 5 amp fuse on C5 since it is an input signal to the control assembly. My guess this is a fail safe circuit to allow windshield defogging if the blower control module fails while driving.

 

There are multiple mistakes in a lot of the Manuals which are corrected by the Electrical Supplement or Service Bulletin.

If you're Blower is funky, check the input on the Brown or Tan wire which should be 2.5 volts at 1, 6 volts at 10. Output is Black/Red or Purple - 4 volts at 1, 12 volts at 10. The controls monitor actual Blower Voltage or the output with a splice from the Output back to the Panel through a 5 amp fuse on the Firewall. If that fuse opens, you generally have nothing but high blower and it may run with the key off.

Module is powered up through a fusible link from the Starter Motor. Hot at all times. That's red.

Module ground is shared with the Blower Motor - another splice - terminating on the frame below it.

The other Module circuits for your Year control compressor ground. One wire from the compressor clutch which is powered up by the a/c fuse so it's always going to have voltage with the key in the run position. The other is from the Panel and it sends something like 6 volts to ground the circuit when a/c is requested. The Panel, however, won't send that signal if the low and/or high pressure switches are open (out of gas or plugged up) or if another circuit isn't completed to the ECM regardless of what the Panel may indicate.

If you need a new Module, they're used on all the GM's with Electronic Air for your year.

 

SunCr, on 87's there is no other wire from the starter large cable. Behind the battery is a bolt with 8 red wires all having fusible links one which supplies power to the blower motor.

 

Thanks SunCr and JFB!

I measure 1.3 v for both tests in "A.Blower motor test" .

I assume that this means there is no output from the blower control from TestA.

In "B.Blower control module test" I do have 12 v at C1 at all times.

I measure the output from c4 to the blower control module I get 10 v (brn at c2) regardless of the fan setting (3rd and 5th tests). Should this be between 1 - 6 v?

I get 1.3 v in the 4th and 6th tests (blk/red at c2) - no change with fan setting changes. This must be coming from c5

Sounds like both the blower control module and the heater A/C control assembly are shot?