Anti-Lock Brake Light Perplexing

[BP]

Anti-Lock Brake Light!! Mine lights periodically! Every Time I start up the car the question looms, WILL IT or WON"T it be on this trip?? Sometimes yes sometimes no!!<P>Doesn't seem to be any reason for it either! No codes read-out when you jumper the ADL (I think that's what its called - the little test box under the steering column) No BCM codes!! Sometimes it comes on and clears while your driving (did this once). But mostly comes on at startup and stays on for the ride!<BR>I have checked connections under hood and at the ECBM, all seem good & tight! I check for error codes whenever the light comes on but never get any! I'm hesitant to spend the $250.00 for the 32 pin test box the Service Manual calls for to check ECBM inputs, and yet I also hate to give up and let the Dealer have a whack it either the ANTI-LOCK BRAKE LITE WINS!<P>I read old posts and thought maybe it was the Main Relay on the fire wall playing tricks with me, (That seems to have plagued others in the past) I replaced it and thought I had won, But alas, ... NOT! About six light free starts later the Anti-Lock Brake Light !! was lit and Grinning at me while I drove! - <P>Any thoughts??!! <P>Your friend<BR>bp

[Howard]

There are two relays for the braking system. One is the main relay, the other is the pump relay. Try this. Open the hood and keep the driver door open so you can hear. Turn on the key. Do you hear the brake pump turn on and then off? The pump should not run more than 30 seconds. If not, the pump, pressure switch, or relay are bad. If the pump turns on, but will not turn off, it could be a bad acculator, relay (same one I mentioned) or pressure switch. Does both the red and yellow light turn on? If just the yellow light stays on, it means that the ABS system is disabled. Your brakes should work normally, but without ABS. If the red light is on, there is a presure problem. That is what the pump does. If the red light stays on, I would not drive the car till you find out what is wrong with the system. Frankly any brake problem, however minor should be corrected before the car is driven. Also check the connentions on the ABS computer in the trunk, left quarter. Howard<p>[ 02-17-2002: Message edited by: Howard ]

[Barney Eaton]

Read what Wally has to say about testing.<P>One rather simple test for the accumulator is to measure the fluid drop in the reservoir. Depressurize the system by pumping the pedal 25+ times with the key off.<BR>(I put a piece of masking tape on the reservoir to mark on) mark the level of fluid in the reservoir. Turn on the ignition and when the pump turns off, measure the fluid in the reservoir. A drop of 1/4-3/8 is good, anything over 1/2 inch your should be ordering a new accumulator.<P>The accumulator is your pressure reserve. As they age, they loose their nitrogen precharge and this can be measured by the amount of fluid they hold. Since you cannot do this directly, you can see the level in the reservoir.<P>This may not solve your ANTI-LOCK problem but it could save you from having a failure.<BR>The motor/pump are low % failure components, the pressure switch is probably close to the accumulator on failure rates, and I feel a lot of those are caused by contaminated brake fluid.

[BP]

Thanks <BR>Only the Amber Anti-Lock Light ever lights. <P>The Red Light only cycles initially when the car starts and then it goes out. But my friend the Amber Light seems to come & go as he pleases. <BR>I will Cycle through all the recommended tests outlined in the previous POSTS but braking functions and pump cycling appear to be OK. I'll confirm & conduct the tests to be sure. <P>I tried looking at the schematic again tonight but I get "cross-wired" quickly (pardon the pun) I'll get smarter in time (no place to go but up!!)<P>What would cause the Amber ANTI-Brake Light to light up & No Codes to Lock in? <P>Regardless ya gotta love this car .....<P>thanks<BR>bp

[Guest wally888]

I to, would be perplexed.<BR> I think I would try this:<BR> Be prepared to retrieve codes from the ALDL the next time yellow light is on. ( Of course there should be at least a History code there now)<BR> If there is no history code and no code when the light is on must be a problem other than a usual component of the antilock)<BR> I would think it is either a pressure problem ( Pressure is lurking just below the pressure that disables the Antilock and above the pressure that turns on Red light.)<BR> Or. An electrical malfunction of a connection to the Main Relay<BR> 3rd. Possibility. The pressure switch. <BR> The few interrmittant yellow lights reported here were Main Relay. I had one that always surprised me, unpredictable, just like the description at reatta.net.<BR> I tried successfully to engage antilock while the light was "ON". No antilock problem, just light problem!<p>[ 02-17-2002: Message edited by: wally888 ]

[BP]

OK – Friends Here are the results from my Brake Test. Please review the results and give me some feedback. Any additional insights, or suggestions would be appreciated. I’m kinda new in this arena, so any feedback from those of you with more experience is very welcomed. It would help me, if you could review & validated my findings or perhaps suggest some additional checks I can do.<P>Initial Conditions:<BR>RED BRAKE Light OFF Anti-Lock Light was LIT. NO ABS Codes present when ALDL was jumpered. NO BCM, ECM, or CRT codes active or in history. <P>Test Results are as follows:<BR>Test 1 – Conditions = System Charged,Car Running: Count the number of full pedal pumps to turn on Red & Yellow Lights (Results = 9 pumps)<BR>Test 2 – Conditions = System Charged,Car Off - Count the number of full pedal pumps till pedal gets HARD (Results = 29 pumps)<P>Note: Test Procedure has you check fluid level at this point – My Fluid Level was about .25inches above the fill line at this. I left it at this level for the first test then drained it to the Full line and re-ran all the tests a second time. – NO change was noted in any test results.<BR>Test 3 – Conditions = Charge System and "TIME" when lights go out and pump stops. (Results = Red Light out after 36 seconds. Yellow Light out after 38 seconds note the Yellow light stayed ON throughout the second test.)<BR>Test 4 – Pump Stops running. (Results = Pump stopped after 45 seconds)<BR>Test 5 – Conditions = System Charged,Car OFF How many pumps till pump starts (Results = 2 pumps)<BR>Test 6 – Conditions = System Pressurized Measure distance from fill Line (Results = .75 inches)<P>Conclusions:<BR>I feel test results lean toward accumulator replacement but are still somewhat inconclusive. <P>I did note some audible change in pump speed as system was charging, however, the change in pitch was much nearer the end of the pump run than the 2/3rds the notes suggests. This makes me lean toward the accumulator as the problem.<P>Other Notes:<BR>In Brakes Section from the Reatta.net pages it indicates – "No ABS Codes are stored when problems are caused by Pump/Motor, Accumulator, Pressure Switch, or the Fluid Level Switch.<P>Questions:<BR>Anyway to test the Pressure or Level Switches to eliminate them as the cause?<P>Thanks again for all the help you have already provided. I really am enjoying the maintenance, its fun peeling the onion to get to the problem. You all have been very helpful<P> - Sorry for such a long POST!<BR>Thanks<BR>bp

[tjenkins]

Short [first] recommendation - your accumulator. R/R with normal hand tools.<P>Btw, ENSURE you RELIEVE THE BRAKE PRESSURE FIRST as outlined in the manual!!!<P>Cost: ~$100 +/- 20%.<P>--Tom

[Guest Motu 56]

Get the accumulator from <A HREF="http://www.gmparts" TARGET=_blank>www.gmparts</A> direct.com. When you remove the accumlator<BR>cover it with a rag you still might have some pressure left in the system. I thought depressurized the system and got sprayed. Word to the wise.

[Guest wally888]

A 3/4 inch drop in reservoir fluid is almost a certain sign of Accumulator failure?<BR>Anyone had this much drop w/ a good Accumulator? I have not kept records re. this but would bet a new A. would solve your problems. A 1/4 inch drop seems little however is maybe an ounce. Where could it go except in a cavity normally occupied by the Nitrogen charge<BR>Once again: The ideal situation for troubleshooting is having a pressure guage attached . The GM procedure for testing includes an attachment w/ a pressure guage that goes between the Accumulator base and the Accumulator.<BR> I recently either posted here in another post or emailed someone the tests and timimg suggested in the Manual using the GM adapter and gage. W/ the Gage you can determine all cut on, cut off pressures ( pressure sensor operation) plus you can time the leakdown to specified pressures( check for internal leaks, especially back through the Pump)<BR> I'm sure w/ enough input (20 or more cars) we could devise a test to further determine a failed Accumulator similar to the leakdown time however we wouldn't know at what pressure we started or exactly what caused the pressure loss.<BR> In other words if we discovered a quick leakdown time to the Yellow Light coming on, we still wouldn't know the cause-bad A?- started at too low a pressure?-internal leak?